This year, in my employer, I completed the migration to a MPLS SR Arista core network from a Brocade MPLS LDP one. Our backbone is still pure IPv4 so anything IPv6 is not going to be added. But this week, via an APNIC blog post I read about SRv6. And it looks quite interesting. So I went to the first post to go a bit deeper about what SRv6 is. Based on the statements of the blog, really big networks are already using this technology and quite a lot of support from the open source community too. I missed Arista in that list though.
So I tried to find some “real” proof of this SRv6 is some pcap files to see the format and get a bit better view. I could find at lest a source with some. The examples are not like the ones mentioned in the APNIC blog post but just for taking a look, it is enough:
So I can see inside the IPv6 header, the SRv6 Header as defined in the rfc.
I dont really understand the second IPv6 header (Dst: b::2). From the first IPv6 header, the destination “f1::” has to be the first instruction SID1. I can see how it mentions it contains a SRH (Next Header: 43). And inside the routing header, we can see it is SR type (Type: 4). I assume that Address[0] and Address[1] are SID2 and SID3.
Today I have had “fun” troubleshooting an issue that looked easy at first sight. A colleague was trying to PXE boot some server from a network that we haven’t used for a while.
When the server boots up, asks for an IP via DHCP. As we have a centralized DHCP server infrastructure, we have configured DHCP relay in the firewall facing that server to send that request to the DHCP server.
First, let’s take a look at how DHCP relay works. This is a very good link. And this diagram from the mentioned link it is really useful:
One think I learned is the reply (DCHP Offer) doesnt have to use as destination IP the same IP it received as source in DHCP Discover. In the picture, it is packet 2a.
Checking in our environment, we confirm that:
Our server is in 10.94.240.x network. Our firewall is acting as DHCP relay, and send the DHCP Discovery (unicast) to our VIP DHCP Server IP.
The DHCP offer, uses as source the physical IP of the DHCP server and destination is the DHCP relay IP (so it is 10.94.240.1 – the firewall IP in 10.94.240.x network)
Ok, so everything looks fine? No really. The server receives the query, it answers… but we dont see a DCHP Request/ACK.
BTW, keep in mind that DHCP is UDP….
So, we need to see where the packets are lost.
This is a high level path flow between the client and server:
So we need to check this connection is three different firewall vendors….
The initial troubleshooting was just using the GUI tools from Palo/Fortigate. We couldn see anything…. but the server was constantly receiving DHCP Discover and sending DHCP Offer… I dont get it:
# tcpdump -i X udp port 67 or 66 -nn
14:58:06.969462 IP 10.81.25.1.67 > 10.81.251.47.67: BOOTP/DHCP, Request from 6c:2b:59:c1:32:73, length 300
14:58:06.969564 IP 10.81.251.201.67 > 10.94.240.1.67: BOOTP/DHCP, Reply, length 300
14:58:28.329048 IP 10.81.25.1.67 > 10.81.251.47.67: BOOTP/DHCP, Request from 6c:2b:59:c1:32:73, length 300
14:58:28.329157 IP 10.81.251.201.67 > 10.94.240.1.67: BOOTP/DHCP, Reply, length 300
Initially it took me a while to see the request/reply because I was assuming the dhcp request had source 10.94.240.1. So I was seeing only the Reply but not the Request. That was when I went to clarify my head about DHCP Relay and found the link.
So ok, we have the DHCP Request/Reply, but absolutely nothing in the Palo. Is the palo dropping the packets or is forwarding? No idea. The GUI says nothing, I took a packet capture and couldnt see that traffic neither…
Doesnt makes sense.
Let’s get back to basic.
Did I mention DHCP is UDP? So how a next generation firewall (like paloalto) with all the fancy features enable (we have nearly all of them enable…) treats a UDP connection? UDP is stateless… but the firewall is statefull… the firewall creates a flow with the first packet so it can track, any new packet is considered part of that flow. But why we dont see the flows? We actually have only one flow. The firewall has created that session and offloaded to hardware. So you dont see anything else in the control-plane / GUI. The GUI only shows the end of a connection/flow. And as our flow DHCP Relay hasnt’ terminated (it is UDP) and the firewall keeps receiving packets, it is considered life (the firewall doesnt really know what is going on). So for that reason we dont see the connection in the PaloUI. Ok, I got to that point after a while…. I need to proof that the packet from the server is reaching the firewall and it is leaving it too.
How can I do that? Well, I need to delete that flow so the firewall considers a new connection and the tcpdump can see the packets.
This is the a good link from paloalto to take captures. So I found my connection and the cleared it:
palo(active)> show session all filter destination 10.94.240.1
ID Application State Type Flag Src[Sport]/Zone/Proto (translated IP[Port])
Vsys Dst[Dport]/Zone (translated IP[Port])
135493 dhcp ACTIVE FLOW 10.81.251.201[67]/ZONE1/17 (10.81.251.201[67])
vsys1 10.94.240.1[67]/ZONE2 (10.94.240.1[67])
palo(active)>
palo(active)> clear session id 135493
And now, my packet capture in paloalto confirms that it is sending the packet to the next firewall (checking the destination MAC) !!!
Ok, so we confirm the first firewall in the return path was fine…. next one, it is fortigate.
BTW, we were checked and assumed that the routing is fine in all routers, firewalls, etc. Sometimes is not the case… so when things dont follow your thoughts, get back to the very basics….
We have exactly the same issue as in PaloAlto. I can’t see anything in the logs about receiving a dhcp offer from palo and forwarding it to the last firewall Cisco.
And again, we apply the same reasoning. We have an UDP connection, we have a next-generation firewall (with fancy ASIC). And one more thing, in this fortigate firewall, we allow intra-zone traffic, so it is not going to show anyway in the GUI monitor…
Fantastic, we have confirmation that the second firewall receives and forwards the DHCP Reply!!!
Ok, now the last stop, Cisco ASA. This is an old firewall, I think it could be my father or Darth Vader.
I dont have the fancy tools for packet capture like Palo/Fortigate…. so I went to the basic “debug” commands and “packet-tracer”.
First, this was the dhcp config in Cisco:
vader/pri/act# show run | i dhcp
dhcprelay server 10.81.251.47 EGRESS
dhcprelay enable SERVERS-ZONE
dhcprelay timeout 60
And, the ACL allows all IP traffic in those interfaces… and couldnt see any deny in the logs.
So, I enabled all debugging things I could find for dhcp:
vader/pri/act# show debug
debug dhcpc detail enabled at level 1
debug dhcpc error enabled at level 1
debug dhcpc packet enabled at level 1
debug dhcpd packet enabled at level 1
debug dhcpd event enabled at level 1
debug dhcpd ddns enabled at level 1
debug dhcprelay error enabled at level 1
debug dhcprelay packet enabled at level 1
debug dhcprelay event enabled at level 200
vader/pri/act# DHCPD: Relay msg received, fip=ANY, fport=0 on SERVERS-ZONE interface
DHCPRA: relay binding found for client f48e.38c7.1b6e.
DHCPD: setting giaddr to 10.94.240.1.
dhcpd_forward_request: request from f48e.38c7.1b6e forwarded to 10.81.251.47.
DHCPD: Relay msg received, fip=ANY, fport=0 on SERVERS-ZONE interface
DHCPRA: relay binding found for client 6c2b.59c1.3273.
DHCPD: setting giaddr to 10.94.240.1.
dhcpd_forward_request: request from 6c2b.59c1.3273 forwarded to 10.81.251.47.
vader/pri/act#
So, the debugging doesnt says anything regarding the packet coming back from Fortigate… Not looking good I am afraid. I wasnt running out of ideas about debug commands. I coudn’t increase an log level neither….
Let’s give a go to packet tracer… doesnt looks good:
vader/pri/act# packet-tracer input EGRESS udp 10.81.251.201 67 10.94.240.1 67
Phase: 1
Type: ACCESS-LIST
Subtype:
Result: ALLOW
Config:
Implicit Rule
Additional Information:
MAC Access list
Phase: 2
Type: ACCESS-LIST
Subtype:
Result: DROP
Config:
Implicit Rule
Additional Information:
Result:
input-interface: EGRESS
input-status: up
input-line-status: up
Action: drop
Drop-reason: (acl-drop) Flow is denied by configured rule
So, we are sure our ACL is totally open but the firewall is dropping the packet coming from fortigate. Why? How to fix it?
Ok, get back to basics. Focus in Cisco config. It uses as DHCP relay server, 10.81.251.47 (VIP). But the DHCP reply is coming from the physical IP 10.81.251.201….. maybe Cisco doesnt like that…. Let’s try to add the physical IPs as a new DHCP server:
vader/pri/act# sri dhcp
dhcprelay server 10.81.251.47 EGRESS
dhcprelay server 10.81.251.201 EGRESS
dhcprelay server 10.81.251.202 EGRESS
I think I nearly cried after seeing this in the dhcp logs in our server:
May 12 16:16:27 dhcp1 dhcpd[2561]: DHCPDISCOVER from f4:8e:38:c7:1b:6e via 10.94.240.1
May 12 16:16:28 dhcp1 dhcpd[2561]: DHCPOFFER on 10.94.240.50 to f4:8e:38:c7:1b:6e (cmc-111) via 10.94.240.1
May 12 16:16:28 dhcp1 dhcpd[2561]: Wrote 0 class decls to leases file.
May 12 16:16:28 dhcp1 dhcpd[2561]: Wrote 0 deleted host decls to leases file.
May 12 16:16:28 dhcp1 dhcpd[2561]: Wrote 0 new dynamic host decls to leases file.
May 12 16:16:28 dhcp1 dhcpd[2561]: Wrote 1 leases to leases file.
May 12 16:16:28 dhcp1 dhcpd[2561]: DHCPREQUEST for 10.94.240.50 (10.81.251.202) from f4:8e:38:c7:1b:6e (cmc-111) via 10.94.240.1
May 12 16:16:28 dhcp1 dhcpd[2561]: DHCPACK on 10.94.240.50 to f4:8e:38:c7:1b:6e (cmc-111) via 10.94.240.1
So at the end, finally fixed…. it took too many hours.
Notes:
DHCP Realy: It is not that obvious the flow regarding IPs.
UDP and firewalls, debugging it is a bit more challenging.
Cisco ASA dhcprelay server IPs…. VIPs and non-VIPs please.
I read once about how to do load-balancing when using Route-Reflectors (RR) in a MPLS L3VPN network. It is a insteresting topic because RRs only reflect the best prefixes to the its clients. So how we make the RR to send more than one?
We have one customer vrf “CUST-A” with three locations: TY, LD and NY.
We are using BGP for PE-CE routing. Each site will use a different private ASN. Our SP is ASN 100.
TY has two connection to our SP so we want to make use of both of them.
We have a RR SP2 that is in line. So we need a full-mesh iBGP from all PE to SP2.
Our SP IGP is OSFP.
The goal is to make all other PE connected to CUST-A sites to be able to load-balance to TY site prefixes 192.168.11.0/24 and 192.168.12.0/24 using TY-SP1 and TY-SP3.
We start building the whole network as standard. This is very similar as stated in our first lab:
This is RR SP2 config:
!
ip vrf CUST-A
rd 100:1
route-target export 1:100
route-target import 1:100
!
interface Loopback0
ip address 10.0.2.1 255.255.255.255
!
interface GigabitEthernet1/0
description to SP1-PE
ip address 10.0.12.2 255.255.255.0
negotiation auto
mpls ip
!
interface GigabitEthernet2/0
description to SP3-PE
ip address 10.0.23.2 255.255.255.0
negotiation auto
mpls ip
!
interface FastEthernet3/0
description TO-LD-SP4
ip address 10.0.24.2 255.255.255.0
duplex auto
speed auto
mpls ip
!
router ospf 1
log-adjacency-changes
network 10.0.2.0 0.0.0.255 area 0
network 10.0.12.0 0.0.0.255 area 0
network 10.0.23.0 0.0.0.255 area 0
network 10.0.24.0 0.0.0.255 area 0
!
router bgp 100
no synchronization
bgp log-neighbor-changes
neighbor 10.0.1.1 remote-as 100
neighbor 10.0.1.1 update-source Loopback0
neighbor 10.0.1.1 route-reflector-client
neighbor 10.0.3.1 remote-as 100
neighbor 10.0.3.1 update-source Loopback0
neighbor 10.0.3.1 route-reflector-client
neighbor 10.0.4.1 remote-as 100
neighbor 10.0.4.1 update-source Loopback0
neighbor 10.0.4.1 route-reflector-client
neighbor 10.0.5.1 remote-as 100
neighbor 10.0.5.1 update-source Loopback0
neighbor 10.0.5.1 route-reflector-client
no auto-summary
!
address-family vpnv4
neighbor 10.0.1.1 activate
neighbor 10.0.1.1 send-community both
neighbor 10.0.1.1 route-reflector-client
neighbor 10.0.3.1 activate
neighbor 10.0.3.1 send-community both
neighbor 10.0.3.1 route-reflector-client
neighbor 10.0.4.1 activate
neighbor 10.0.4.1 send-community both
neighbor 10.0.4.1 route-reflector-client
neighbor 10.0.5.1 activate
neighbor 10.0.5.1 send-community both
neighbor 10.0.5.1 route-reflector-client
exit-address-family
!
address-family ipv4 vrf CUST-A
no synchronization
exit-address-family
!
!
mpls ldp router-id Loopback0 force
The configs for the SP PE follow the same patern, this is TY-SP1:
!
ip vrf CUST-A
rd 100:1
route-target export 1:100
route-target import 1:100
!
interface Loopback0
ip address 10.0.1.1 255.255.255.255
!
interface FastEthernet0/0
description to HQ
ip vrf forwarding CUST-A
ip address 172.16.100.254 255.255.255.0
duplex half
!
interface GigabitEthernet1/0
description to SP2-P
ip address 10.0.12.1 255.255.255.0
negotiation auto
mpls ip
!
router ospf 1
log-adjacency-changes
network 10.0.1.0 0.0.0.255 area 0
network 10.0.12.0 0.0.0.255 area 0
!
router bgp 100
no synchronization
bgp log-neighbor-changes
neighbor 10.0.2.1 remote-as 100
neighbor 10.0.2.1 update-source Loopback0
no auto-summary
!
address-family vpnv4
neighbor 10.0.2.1 activate
neighbor 10.0.2.1 send-community both
exit-address-family
!
address-family ipv4 vrf CUST-A
neighbor 172.16.100.1 remote-as 65001
neighbor 172.16.100.1 activate
neighbor 172.16.100.1 soft-reconfiguration inbound
no synchronization
exit-address-family
!
mpls ldp router-id Loopback0 force
!
Let’ see if LD-CE1 can ping our TY-C1
LD-CE1#traceroute 192.168.12.1 source 172.16.30.1
Type escape sequence to abort.
Tracing the route to 192.168.12.1
1 172.16.101.254 8 msec 20 msec 8 msec
2 10.0.24.2 [MPLS: Labels 18/23 Exp 0] 40 msec 40 msec 36 msec
3 172.16.200.254 [MPLS: Label 23 Exp 0] 12 msec 32 msec 28 msec
4 172.16.200.1 60 msec 40 msec 40 msec
5 192.168.12.1 [AS 65001] 40 msec 60 msec 60 msec
LD-CE1#
LD-CE1#
LD-CE1#ping 192.168.11.1 source 172.16.30.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.11.1, timeout is 2 seconds:
Packet sent with a source address of 172.16.30.1
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 44/54/72 ms
LD-CE1#
LD-CE1#
LD-CE1#
LD-CE1#sh
LD-CE1#show ip rou
LD-CE1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
B 192.168.12.0/24 [20/0] via 172.16.101.254, 01:19:31
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
C 172.16.30.1/32 is directly connected, Loopback0
C 172.16.101.0/24 is directly connected, FastEthernet0/0
B 192.168.11.0/24 [20/0] via 172.16.101.254, 01:19:31
LD-CE1#
So, what do we see when everything is configured?
From SP2-RR, we see all BGP peers up to PEs and in the vpnv4 table we can see the TY prefixes 192.168.11.0/24 and 192.168.12.0/24. But only the path from TY-SP1 is preferred….
SP2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.0.4.1 1 FULL/DR 00:00:39 10.0.24.1 FastEthernet3/0
10.0.3.1 1 FULL/DR 00:00:39 10.0.23.1 GigabitEthernet2/0
10.0.1.1 1 FULL/BDR 00:00:37 10.0.12.1 GigabitEthernet1/0
SP2#
SP2#
SP2#show ip bgp summary
BGP router identifier 10.0.2.1, local AS number 100
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.0.1.1 4 100 98 111 1 0 0 01:25:16 0
10.0.3.1 4 100 93 108 1 0 0 01:25:05 0
10.0.4.1 4 100 96 114 1 0 0 00:55:06 0
10.0.5.1 4 100 29 32 1 0 0 00:28:02 0
SP2#
SP2#show ip bgp vpnv4 all
BGP table version is 9, local router ID is 10.0.2.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 100:1 (default for vrf CUST-A)
*>i172.16.30.1/32 10.0.4.1 0 100 0 65002 i
*>i192.168.11.0 10.0.1.1 0 100 0 65001 i
* i 10.0.3.1 0 100 0 65001 i
*>i192.168.12.0 10.0.1.1 0 100 0 65001 i
* i 10.0.3.1 0 100 0 65001 i
SP2#
Let confirm that the PE only receive the best prefix from the RR. So, from LD-SP4, we can see the paths to TY 192.168.11/12 via TY-SP1 only:
LD-SP4#show ip bgp vpnv4 all
BGP table version is 18, local router ID is 10.0.4.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 100:1 (default for vrf CUST-A)
*> 172.16.30.1/32 172.16.101.1 0 0 65002 i
*>i192.168.11.0 10.0.1.1 0 100 0 65001 i
*>i192.168.12.0 10.0.1.1 0 100 0 65001 i
LD-SP4#
How do we make RR-SP2 to learn and advertise TY-SP1 and TY-SP3 paths. We need to use different RD in TY-SP1/3 respectively.
We have RD 100:1 assigned to CUST-A in all PEs. We are going to change that in TY-SP1/3 so RR will see two different VPNv4 prefixes for the same destination.
Let’s change TY-SP1 RD 100:1 to 100:101 and TY-SP3 to 100:102. Watch out as all routing config related to VRF CUST-A will disappear.
And what about the RT config? Do we have to change anything? Actually, we need to keep it the same (we need to retype it), nothing changes here. Keep in mind that RT is used to import/export vpnv4 prefixes into the VRF. The RD is not used to import/export so for that reason (as we are going to see) we could actually use any RD for a VRF in a PE.
Let’s see the changes for TY-SP1:
TY-SP1(config)#ip vrf CUST-A
TY-SP1(config-vrf)#no rd 100:1
% "rd 100:1" for VRF CUST-A scheduled for deletion
TY-SP1(config-vrf)#
*Apr 27 22:28:48.347: %BGP-5-ADJCHANGE: neighbor 172.16.100.1 vpn vrf CUST-A Down Neighbor deleted
TY-SP1(config-vrf)#rd 100:101
% Deletion of "rd" in progress; wait for it to complete
TY-SP1(config-vrf)#
TY-SP1(config-vrf)#rd 100:101
TY-SP1(config-vrf)#route-target export 100:1
TY-SP1(config-vrf)#route-target import 100:1
TY-SP1(config-vrf)#exit
TY-SP1(config)#router bgp 100
TY-SP1(config-router)#address-family ipv4 vrf CUST-A
TY-SP1(config-router-af)# neighbor 172.16.100.1 remote-as 65001
TY-SP1(config-router-af)# neighbor 172.16.100.1 activate
TY-SP1(config-router-af)# neighbor 172.16.100.1 soft-reconfiguration inbound
TY-SP1(config-router-af)#
*Apr 27 22:33:50.571: %BGP-5-ADJCHANGE: neighbor 172.16.100.1 vpn vrf CUST-A Up
TY-SP1(config-router-af)#
So after repeating the same step in TY-SP3 (using RD 100:102), let’s see what happens in RR-SP2:
SP2#show ip bgp vpnv4 all
BGP table version is 51, local router ID is 10.0.2.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 100:1 (default for vrf CUST-A)
*>i172.16.30.1/32 10.0.4.1 0 100 0 65002 i
* i192.168.11.0 10.0.3.1 0 100 0 65001 i
*>i 10.0.1.1 0 100 0 65001 i
* i192.168.12.0 10.0.3.1 0 100 0 65001 i
*>i 10.0.1.1 0 100 0 65001 i
Route Distinguisher: 100:101
*>i192.168.11.0 10.0.1.1 0 100 0 65001 i
*>i192.168.12.0 10.0.1.1 0 100 0 65001 i
Route Distinguisher: 100:102
*>i192.168.11.0 10.0.3.1 0 100 0 65001 i
*>i192.168.12.0 10.0.3.1 0 100 0 65001 i
SP2#
Now we can see VPNv4 for 100:101 (TY-SP1) and 100:102 (TY-SP2)!!!
Ok, let’s what the other PE are seeing. In our case, let’s check LD-SP4:
LD-SP4#show ip bgp vpnv4 all
BGP table version is 18, local router ID is 10.0.4.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 100:1 (default for vrf CUST-A)
*> 172.16.30.1/32 172.16.101.1 0 0 65002 i
* i192.168.11.0 10.0.3.1 0 100 0 65001 i
*>i 10.0.1.1 0 100 0 65001 i
* i192.168.12.0 10.0.3.1 0 100 0 65001 i
*>i 10.0.1.1 0 100 0 65001 i
Route Distinguisher: 100:101
*>i192.168.11.0 10.0.1.1 0 100 0 65001 i
*>i192.168.12.0 10.0.1.1 0 100 0 65001 i
Route Distinguisher: 100:102
*>i192.168.11.0 10.0.3.1 0 100 0 65001 i
*>i192.168.12.0 10.0.3.1 0 100 0 65001 i
LD-SP4#
LD-SP4#
LD-SP4#show ip route vrf CUST-A
Routing Table: CUST-A
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
B 192.168.12.0/24 [200/0] via 10.0.1.1, 00:01:05
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
B 172.16.30.1/32 [20/0] via 172.16.101.1, 00:33:47
C 172.16.101.0/24 is directly connected, FastEthernet0/0
B 192.168.11.0/24 [200/0] via 10.0.1.1, 00:01:05
LD-SP4#
So, LD-SP4 is receiving the VPNv4 100:101 and 100:102 from RR-SP2!!! That’s good, but we are still seeing the path to TY 192.168.11/12 prefixes via TY-SP1 (10.0.1.1) only.
So why BGP ECMP is not working? Because we have to enable it.
LD-SP4(config)#router bgp 100
LD-SP4(config-router)#address-family ipv4 vrf CUST-A
LD-SP4(config-router-af)#maximum-paths eibgp 2
LD-SP4(config-router-af)#
*Apr 27 22:58:25.447: BGP: VPNv4 Unicast multipath configuration changed
*Apr 27 22:58:25.447: BGP-VPN(4): MPLS label changed for prefix 100:1:192.168.11.0/24
*Apr 27 22:58:25.447: BGP-VPN(4): multipath from neighbor 10.0.2.1 nexthop 10.0.3.1 new outlabel 24
*Apr 27 22:58:25.447: vpn: free local label 1048577 for remote prefix CUST-A:192.168.11.0/24
*Apr 27 22:58:25.447: vpn: get path labels: 100:1:192.168.11.0/255.255.255.0
*Apr 27 22:58:25.451: vpn(4): inlabel=nolabel, outlabel=22, outlabel owner=BGP
*Apr 27 22:58:25.451: vpn(4): Announce labels to IPRM CUST-A:192.168.11.0/24 gw 10.0.1.1 inlabel=nolabel, outlabel=22
*Apr 27 22:58:25.451: BGP-VPN(4): MPLS label changed for prefix 100:1:192.168.12.0/24
*Apr 27 22:58:25.451: BGP-VPN(4): multipath from neighbor 10.0.2.1 nexthop 10.0.3.1 new outlabel 23
*Apr 27 22:58:25.451: vpn: free local label 1048577 for remote prefix CUST-A:192.168.12.0/24
*Apr 27 22:58:25.451: vpn: get path labels: 100:1:192.168.12.0/255.255.255.0
*
LD-SP4(config-router-af)#endApr 27 22:58:25.451: vpn(4): inlabel=nolabel, outlabel=21, outlabel owner=BGP
*Apr 27 22:58:25.451: vpn(4): Announce labels to IPRM CUST-A:192.168.12.0/24 gw 10.0.1.1 inlabel=nolabel, outlabel=21
*Apr 27 22:58:25.455: vpn: get path labels: 100:1:192.168.11.0/255.255.255.0
*Apr 27 22:58:25.459: vpn(4): inlabel=nolabel, outlabel=24, outlabel owner=BGP
*Apr 27 22:58:25.459: vpn(4): Announce labels to IPRM CUST-A:192.168.11.0/24 gw 10.0.3.1 inlabel=nolabel, outlabel=24
*Apr 27 22:58:25.459: vpn(4): get path labels; 100:1:192.168.11.0/24 nexthop 10.0.3.1, not bestpath
*Apr 27 22:58:25.475: vpn: get path labels: 100:1:192.168.12.0/255.255.255.0
*Apr 27 22:58:25.475: vpn(4): inlabel=nolabel, outlabel=23, outlabel owner=BGP
*Apr 27 22:58:25.475: vpn(4): Announce labels to IPRM CUST-A:192.168.12.0/24 gw 10.0.3.1 inlabel=nolabel, outlabel=23
*Apr 27 22:58:25.479: vpn(4): get path labels; 100:1:192.168.12.0/24 nexthop 10.0.3.1, not bestpath
LD-SP4(config-router-af)#end
LD-SP4#
*Apr 27 22:58:27.411: %SYS-5-CONFIG_I: Configured from console by console
LD-SP4#
LD-SP4#
LD-SP4#show ip route vrf CUST-A
Routing Table: CUST-A
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
B 192.168.12.0/24 [200/0] via 10.0.3.1, 00:00:07
[200/0] via 10.0.1.1, 00:02:18
172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks
B 172.16.30.1/32 [20/0] via 172.16.101.1, 00:35:00
C 172.16.101.0/24 is directly connected, FastEthernet0/0
B 192.168.11.0/24 [200/0] via 10.0.3.1, 00:00:07
[200/0] via 10.0.1.1, 00:02:18
LD-SP4#
We finally got it! Our PE LD-SP4 is able to see two paths to TY prefixes!
In summary:
We need to change the VRF RD in the PE we want to be participant in load-balancing
This is a continuation of the other post abount installing and configuring a basic MPLS L3VPN network in GNS3.
Normally, we always have a routing protocol running between the customer CPE and the provider PE. OSPF was very common and I used to be give for granted the routing loop avoidance in a dual-home CPE, I knew the idea but never really hammered it in my head. Until a couple of months ago that I hit an issue during the migration of my employer MPLS network to a new vendor. The new vendor didnt implemented the OSPF Down bit. /o\
Summary: If an LSA arrives at a PE with the down bit set, that will never be redistributed into BGP. This prevents the route from leaking in from one PE back into another PE.
The RFC for using OSPF in PE-CE in MPLS VPNs is here:
Note: Down-Bit is only used in LSA3!
It was frustrating but it was a good excuse too because it pushed me (and I could justify) to move our PE-CE to BGP.
In general I always read these blogs when I want to refresh my OSPF Down Bit. So all merits are for them:
The big picture is: CE (HQ, BRANCH) routers are running OSPF with the PE (SP1/3/4) routers. The PE routers redistribute these OSPF routes into BGP and then converts them to VPNv4 NLRI. These VPNv4 NLRI are advetised to other PE routers via BGP. The PE also converts these VPNv4 routes back into OSPF and then off to the CE router.
Now in more detail, let’s see where we can have a routing loop:
1) HQ sends a LSA1 to SP1 with Lo:172.16.10.1/32 and the connected network to PE 172.16.100.0/24
HQ#show ip ospf database router internal self-originate
OSPF Router with ID (172.16.110.1) (Process ID 1)
Router Link States (Area 10)
Now in min table
Table index: 42 min 17 sec
LS age: 321
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 172.16.110.1
Advertising Router: 172.16.110.1
LS Seq Number: 80000003
Checksum: 0x7247
Length: 48
AS Boundary Router
Number of Links: 2
Link connected to: a Stub Network
(Link ID) Network/subnet number: 172.16.10.1
(Link Data) Network Mask: 255.255.255.255
Number of TOS metrics: 0
TOS 0 Metrics: 1
Link connected to: a Transit Network
(Link ID) Designated Router address: 172.16.100.1
(Link Data) Router Interface address: 172.16.100.1
Number of TOS metrics: 0
TOS 0 Metrics: 1
2) SP1 received the new OSPF route from HQ (172.16.10.1/32) and it is redistributed into BGP so other PEs can receive it (SP3 and SP4) as a VPNv4. The connected 172.16.100.0/24 is as well redistributed into BGP
SP1#show ip ospf database router internal adv-router 172.16.110.1
OSPF Router with ID (10.0.1.1) (Process ID 1)
OSPF Router with ID (172.16.100.254) (Process ID 10)
Router Link States (Area 10)
Routing Bit Set on this LSA
Now in min table
Table index: 45 min 42 sec
LS age: 648
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 172.16.110.1
Advertising Router: 172.16.110.1
LS Seq Number: 80000003
Checksum: 0x7247
Length: 48
AS Boundary Router
Number of Links: 2
Link connected to: a Stub Network
(Link ID) Network/subnet number: 172.16.10.1
(Link Data) Network Mask: 255.255.255.255
Number of TOS metrics: 0
TOS 0 Metrics: 1
Link connected to: a Transit Network
(Link ID) Designated Router address: 172.16.100.1
(Link Data) Router Interface address: 172.16.100.1
Number of TOS metrics: 0
TOS 0 Metrics: 1
SP1#
SP1#show ip route vrf CUST-A
Routing Table: CUST-A
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
B 172.16.200.0/24 [200/0] via 10.0.3.1, 00:41:47
B 172.16.201.0/24 [200/0] via 10.0.4.1, 00:41:47
B 172.16.20.1/32 [200/2] via 10.0.3.1, 00:41:47
O 172.16.10.1/32 [110/2] via 172.16.100.1, 00:43:58, FastEthernet0/0
O E1 172.16.110.1/32 [110/21] via 172.16.100.1, 00:43:58, FastEthernet0/0
C 172.16.100.0/24 is directly connected, FastEthernet0/0
SP1#
SP1#show ip bgp vpnv4 all
BGP table version is 14, local router ID is 10.0.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 100:1 (default for vrf CUST-A)
*> 172.16.10.1/32 172.16.100.1 2 32768 ?
* i172.16.20.1/32 10.0.4.1 2 100 0 ?
*>i 10.0.3.1 2 100 0 ?
*> 172.16.100.0/24 0.0.0.0 0 32768 ?
*> 172.16.110.1/32 172.16.100.1 21 32768 ?
* i172.16.200.0/24 10.0.4.1 2 100 0 ?
*>i 10.0.3.1 0 100 0 ?
*>i172.16.201.0/24 10.0.4.1 0 100 0 ?
* i 10.0.3.1 2 100 0 ?
SP1#
It is important to notice how the VPNv4 for 172.16.10.1/32 is built in SP1. Based on the rfc section 4.2.6 “Handling LSAs from the CE” we see the following:
When a PE router receives, from a CE router, any LSA with the DN bit [OSPF-DN] set, the information from that LSA MUST NOT be used by the route calculation. If a Type 5 LSA is received from the CE, and if it has an OSPF route tag value equal to the VPN Route Tag (see Section 4.2.5.2), then the information from that LSA MUST NOT be used by the route calculation.
Otherwise, the PE must examine the corresponding VRF.For every address prefix that was installed in the VRF by one of its associated OSPF instances, the PE must create a VPN-IPv4 route in BGP. Each such route will have some of the following Extended Communities attributes:
– The OSPF Domain Identifier Extended Communities attribute. If the OSPF instance that installed the route has a non-NULL primary Domain Identifier, this MUST be present; if that OSPF instance has only a NULL Domain Identifier, it MAY be omitted. This attribute is encoded with a two-byte type field, and its type is 0005, 0105, or 0205. For backward compatibility, the type 8005 MAY be used as well and is treated as if it were 0005. If the OSPF instance has a NULL Domain Identifier, and the OSPF Domain Identifier Extended Communities attribute is present, then the attribute’s value field must be all zeroes, and its type field may be any of 0005, 0105, 0205, or 8005.
– OSPF Route Type Extended Communities Attribute. This attribute MUST be present. It is encoded with a two-byte type field, and its type is 0306. To ensure backward compatibility, the type 8000 SHOULD be accepted as well and treated as if it were type 0306. The remaining six bytes of the Attribute are encoded as follows:
Area Number – Route Type – Options
So the very first paragraph is our answer when we reach SP3 (when dealing with a LSA3) and there is no loop. And the second paragrah is our answer when delaling with a LS5 and avoid a loop (more of this later). So this is our VPNv4 for 172.16.10.1/32
SP1#
SP1#show ip bgp vpnv4 rd 100:1 172.16.10.1/32
BGP routing table entry for 100:1:172.16.10.1/32, version 5
Paths: (1 available, best #1, table CUST-A)
Advertised to update-groups:
2
Local
172.16.100.1 from 0.0.0.0 (10.0.1.1)
Origin incomplete, metric 2, localpref 100, weight 32768, valid, sourced, best
Extended Community: RT:1:100 OSPF DOMAIN ID:0x0005:0x0000000A0200
OSPF RT:0.0.0.10:2:0 OSPF ROUTER ID:172.16.100.254:0
mpls labels in/out 21/nolabel
SP1#
So the extended communities generated from being a OSPF prefix are OSPF DOMAIN ID, OSPF Route Type (RT) and OSPF ROUTER ID.
I haven’t configured “ospf domain ID” in any router so Cisco IOS is generating one for itself (although it should be NULL) in OSPF DOMAIN ID.
For OSPF RT, we have are 10 (0.0.0.10) and LSA2 (although it should be LSA1). ROUTER ID is the expected one.
3) SP2 is just a P router so it is transparent here. Doesnt know anything about BGP, VPNv4, etc. It just does LDP and IGP.
SP2#show ip bgp summary
% BGP not active
SP2#show ip route ospf
10.0.0.0/8 is variably subnetted, 7 subnets, 2 masks
O 10.0.3.1/32 [110/2] via 10.0.23.1, 00:45:04, GigabitEthernet2/0
O 10.0.1.1/32 [110/2] via 10.0.12.1, 00:44:54, GigabitEthernet1/0
O 10.0.4.1/32 [110/3] via 10.0.23.1, 00:44:54, GigabitEthernet2/0
O 10.0.34.0/24 [110/2] via 10.0.23.1, 00:44:54, GigabitEthernet2/0
SP2#
4) SP3 received the new VPNv4, it is redistributed from BGP to OSPF as a LSA3 (The MPLS backbone is a super OSPF area 0). If we pay attention to the details of the LSA3 (Summary) from HQ prefix 172.16.10.1/32 “show ip ospf database summary 172.16.10.1” we can see two details. First, the two LSA are one from SP3 (advert router 172.16.200.254) and the other from SP4 (advert router 172.16.201.254). Second, both show “Downward” in the options field. As stated earlier, this is directed by the rfc for any PE sending a LSA3. So, if iBGP has AD of 200 and OSPF has AD of 110. How come we have installed the BGP prefix in the routing table for 172.16.10.1/32 instead of the OSPF prefix coming from SP4. As per the standard mentioned earlier, if a PE router receives an OSPF prefix with the down bit enabled (“Downward”), the PE router ignores that prefix. The “Downward” bit is saying the prefix is coming from another PE in the same area so if you accept it, you will trigger a routing loop. Keep in mind that SP4 is doing the same thing as we see below in the commands for SP3. If SP3 accepts the OSPF prefix from SP4 for reaching 172.16.10.1/32 (HQ), SP4 is doing the same thing, accepting the SP3 prefix for reaching 172.16.10.1/32 (HQ). So SP3 would send traffic to SP4, and SP4 would return it back to SP3. When both SP3/SP4 learn the OSPF prefix from each other, they will stop redistributing the BGP prefix (that is coming from SP1/HQ) into OSPF so we reach a point where there is no more LSA3 for 172.16.10.1! and the process starts again. As well SP3/4 will redistribute the OPSF prefix learned from the other SP into BGP. So we are back to the intial stage, SP3/SP4 only have the BGP prefix for 172.16.10.1 (from SP2 or SP3/4), as it is the best route, it is redistributed to OSPF, and you know what happens next.
SP3#show ip bgp vpnv4 all
BGP table version is 13, local router ID is 10.0.3.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 100:1 (default for vrf CUST-A)
*>i172.16.10.1/32 10.0.1.1 2 100 0 ?
* i172.16.20.1/32 10.0.4.1 2 100 0 ?
*> 172.16.200.1 2 32768 ?
*>i172.16.100.0/24 10.0.1.1 0 100 0 ?
*>i172.16.110.1/32 10.0.1.1 21 100 0 ?
* i172.16.200.0/24 10.0.4.1 2 100 0 ?
*> 0.0.0.0 0 32768 ?
* i172.16.201.0/24 10.0.4.1 0 100 0 ?
*> 172.16.200.1 2 32768 ?
SP3#
SP3#
SP3#show ip route vrf CUST-A
Routing Table: CUST-A
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
C 172.16.200.0/24 is directly connected, FastEthernet0/0
O 172.16.201.0/24 [110/2] via 172.16.200.1, 00:45:46, FastEthernet0/0
O 172.16.20.1/32 [110/2] via 172.16.200.1, 00:45:46, FastEthernet0/0
B 172.16.10.1/32 [200/2] via 10.0.1.1, 00:43:35
B 172.16.110.1/32 [200/21] via 10.0.1.1, 00:43:35
B 172.16.100.0/24 [200/0] via 10.0.1.1, 00:43:35
SP3#
SP3#show ip ospf database
OSPF Router with ID (10.0.3.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
10.0.1.1 10.0.1.1 1076 0x80000003 0x00D9F2 2
10.0.2.1 10.0.2.1 1132 0x80000004 0x00D79A 3
10.0.3.1 10.0.3.1 1105 0x80000004 0x0083C1 3
10.0.4.1 10.0.4.1 1095 0x80000003 0x00D0C5 2
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.0.12.2 10.0.2.1 1132 0x80000002 0x00FFFA
10.0.23.1 10.0.3.1 1105 0x80000002 0x009F4E
10.0.34.2 10.0.4.1 1095 0x80000002 0x002BB3
OSPF Router with ID (172.16.200.254) (Process ID 10)
Router Link States (Area 10)
Link ID ADV Router Age Seq# Checksum Link count
172.16.20.1 172.16.20.1 1105 0x80000004 0x00750C 3
172.16.200.254 172.16.200.254 1116 0x80000003 0x0059C2 1
172.16.201.254 172.16.201.254 1121 0x80000003 0x005DBA 1
Net Link States (Area 10)
Link ID ADV Router Age Seq# Checksum
172.16.200.254 172.16.200.254 1116 0x80000002 0x00F4E4
172.16.201.254 172.16.201.254 1121 0x80000002 0x00EBEA
Summary Net Link States (Area 10)
Link ID ADV Router Age Seq# Checksum
172.16.10.1 172.16.200.254 1116 0x80000002 0x000C61
172.16.10.1 172.16.201.254 1121 0x80000002 0x000567
172.16.100.0 172.16.200.254 1116 0x80000002 0x002AEA
172.16.100.0 172.16.201.254 1121 0x80000002 0x0023F0
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
172.16.110.1 172.16.200.254 1116 0x80000002 0x005FD9 3489661028
172.16.110.1 172.16.201.254 1121 0x80000002 0x0058DF 3489661028
SP3#
SP3#
SP3#
SP3#show ip ospf database summary 172.16.10.1
OSPF Router with ID (10.0.3.1) (Process ID 1)
OSPF Router with ID (172.16.200.254) (Process ID 10)
Summary Net Link States (Area 10)
LS age: 1127
Options: (No TOS-capability, DC, Downward)
LS Type: Summary Links(Network)
Link State ID: 172.16.10.1 (summary Network Number)
Advertising Router: 172.16.200.254
LS Seq Number: 80000002
Checksum: 0xC61
Length: 28
Network Mask: /32
TOS: 0 Metric: 2
LS age: 1132
Options: (No TOS-capability, DC, Downward)
LS Type: Summary Links(Network)
Link State ID: 172.16.10.1 (summary Network Number)
Advertising Router: 172.16.201.254
LS Seq Number: 80000002
Checksum: 0x567
Length: 28
Network Mask: /32
TOS: 0 Metric: 2
SP3#
Like we did in SP1, let’s see how SP3 deals with the VPNv4 for 172.16.10.1/32.
Based on th rfc “4.2.8” VPNv4 Routes received via BGP, we need to check “4.2.8.1 External Routes” (LSA5/7) and “4.2.8.2 Summary Routes” (LSA3) and the VPNv4 received:
SP3#show ip bgp vpnv4 rd 100:1 172.16.10.1/32
BGP routing table entry for 100:1:172.16.10.1/32, version 8
Paths: (1 available, best #1, table CUST-A)
Not advertised to any peer
Local
10.0.1.1 (metric 3) from 10.0.1.1 (10.0.1.1)
Origin incomplete, metric 2, localpref 100, valid, internal, best
Extended Community: RT:1:100 OSPF DOMAIN ID:0x0005:0x0000000A0200
OSPF RT:0.0.0.10:2:0 OSPF ROUTER ID:172.16.100.254:0
mpls labels in/out nolabel/21
SP3#
The DOMAIN ID has to match as we haven’t defined it. OSPF RT, is telling that is coming from OSPF area 10 and non-external. So SP3 can generate a LSA3 for 172.16.10.1/32 as we have OSPF area 10 defined too.
5) From SP4 perspective. Same view as SP3. SP4 ignores LSA3 with Down-bit.
SP4#show ip bgp vpnv4 all
BGP table version is 13, local router ID is 10.0.4.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 100:1 (default for vrf CUST-A)
*>i172.16.10.1/32 10.0.1.1 2 100 0 ?
* i172.16.20.1/32 10.0.3.1 2 100 0 ?
*> 172.16.201.1 2 32768 ?
*>i172.16.100.0/24 10.0.1.1 0 100 0 ?
*>i172.16.110.1/32 10.0.1.1 21 100 0 ?
* i172.16.200.0/24 10.0.3.1 0 100 0 ?
*> 172.16.201.1 2 32768 ?
* i172.16.201.0/24 10.0.3.1 2 100 0 ?
*> 0.0.0.0 0 32768 ?
SP4#
SP4#
SP4#show ip ospf database summary 172.16.10.1
OSPF Router with ID (10.0.4.1) (Process ID 1)
OSPF Router with ID (172.16.201.254) (Process ID 10)
Summary Net Link States (Area 10)
LS age: 1489
Options: (No TOS-capability, DC, Downward)
LS Type: Summary Links(Network)
Link State ID: 172.16.10.1 (summary Network Number)
Advertising Router: 172.16.200.254
LS Seq Number: 80000003
Checksum: 0xA62
Length: 28
Network Mask: /32
TOS: 0 Metric: 2
LS age: 1475
Options: (No TOS-capability, DC, Downward)
LS Type: Summary Links(Network)
Link State ID: 172.16.10.1 (summary Network Number)
Advertising Router: 172.16.201.254
LS Seq Number: 80000003
Checksum: 0x368
Length: 28
Network Mask: /32
TOS: 0 Metric: 2
SP4#
SP4#show ip route vrf CUST-A
Routing Table: CUST-A
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
O 172.16.200.0/24 [110/2] via 172.16.201.1, 01:31:12, FastEthernet3/0
C 172.16.201.0/24 is directly connected, FastEthernet3/0
O 172.16.20.1/32 [110/2] via 172.16.201.1, 01:31:12, FastEthernet3/0
B 172.16.10.1/32 [200/2] via 10.0.1.1, 01:28:57
B 172.16.110.1/32 [200/21] via 10.0.1.1, 01:28:57
B 172.16.100.0/24 [200/0] via 10.0.1.1, 01:28:57
SP4#
6) And Finally, BRANCH. It can see the prefix 172.16.10.1/32 (HQ) via two paths as we would expect. And without routing loops (the routes has been installed for over 1h 30minutes). BRANCH doesnt react to the Down-Bit so it accepts the LSA3 from SP2/3 and install the OSPF prefix.
BRANCH#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
C 172.16.200.0/24 is directly connected, FastEthernet0/0
C 172.16.201.0/24 is directly connected, FastEthernet3/0
C 172.16.20.0/24 is directly connected, Loopback0
O IA 172.16.10.1/32 [110/3] via 172.16.201.254, 01:30:38, FastEthernet3/0
[110/3] via 172.16.200.254, 01:30:39, FastEthernet0/0
O E1 172.16.110.1/32 [110/22] via 172.16.201.254, 01:30:34, FastEthernet3/0
[110/22] via 172.16.200.254, 01:30:34, FastEthernet0/0
O IA 172.16.100.0/24 [110/2] via 172.16.201.254, 01:30:38, FastEthernet3/0
[110/2] via 172.16.200.254, 01:30:39, FastEthernet0/0
BRANCH#
BRANCH#
BRANCH#
BRANCH#show ip ospf database summary 172.16.10.1
OSPF Router with ID (172.16.20.1) (Process ID 1)
Summary Net Link States (Area 10)
Routing Bit Set on this LSA
LS age: 1599
Options: (No TOS-capability, DC, Downward)
LS Type: Summary Links(Network)
Link State ID: 172.16.10.1 (summary Network Number)
Advertising Router: 172.16.200.254
LS Seq Number: 80000003
Checksum: 0xA62
Length: 28
Network Mask: /32
TOS: 0 Metric: 2
Routing Bit Set on this LSA
LS age: 1587
Options: (No TOS-capability, DC, Downward)
LS Type: Summary Links(Network)
Link State ID: 172.16.10.1 (summary Network Number)
Advertising Router: 172.16.201.254
LS Seq Number: 80000003
Checksum: 0x368
Length: 28
Network Mask: /32
TOS: 0 Metric: 2
BRANCH#
So, we have seen the Down-bit in action for LSA3. But what about the external LSA: LSA5 and LSA7? How we avoid routing loops for them?
In this case, we have the “tag” field. This is explained in the rfc too.
1) In the same scenario, we have HQ router advertising 172.16.110.1/32 as LSA5 External.
HQ#
HQ#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 172.16.100.1 YES NVRAM up up
GigabitEthernet1/0 unassigned YES NVRAM administratively down down
GigabitEthernet2/0 unassigned YES NVRAM administratively down down
FastEthernet3/0 unassigned YES NVRAM administratively down down
FastEthernet3/1 unassigned YES NVRAM administratively down down
Loopback0 172.16.10.1 YES NVRAM up up
Loopback1 172.16.110.1 YES NVRAM up up
HQ#
HQ#
HQ#
HQ#show ip ospf database
OSPF Router with ID (172.16.110.1) (Process ID 1)
Router Link States (Area 10)
Link ID ADV Router Age Seq# Checksum Link count
172.16.100.254 172.16.100.254 1270 0x80000005 0x00D7D1 1
172.16.110.1 172.16.110.1 1272 0x80000005 0x006E49 2
Net Link States (Area 10)
Link ID ADV Router Age Seq# Checksum
172.16.100.1 172.16.110.1 1272 0x80000004 0x007824
Summary Net Link States (Area 10)
Link ID ADV Router Age Seq# Checksum
172.16.20.1 172.16.100.254 1270 0x80000004 0x00586D
172.16.200.0 172.16.100.254 1270 0x80000004 0x00947E
172.16.201.0 172.16.100.254 1270 0x80000004 0x008988
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
172.16.110.1 172.16.110.1 1272 0x80000004 0x007253 0
HQ#
HQ#
HQ#show ip ospf database external
OSPF Router with ID (172.16.110.1) (Process ID 1)
Type-5 AS External Link States
LS age: 1276
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 172.16.110.1 (External Network Number )
Advertising Router: 172.16.110.1
LS Seq Number: 80000004
Checksum: 0x7253
Length: 36
Network Mask: /32
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0
HQ#
2) SP1 sees 172.16.110.1/32 as OSPF E1. And redistribute it into BGP and creates a VPNv4
SP1#
SP1#show ip route vrf CUST-A
Routing Table: CUST-A
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
B 172.16.200.0/24 [200/0] via 10.0.3.1, 02:00:18
B 172.16.201.0/24 [200/0] via 10.0.4.1, 02:00:18
B 172.16.20.1/32 [200/2] via 10.0.3.1, 02:00:18
O 172.16.10.1/32 [110/2] via 172.16.100.1, 02:02:29, FastEthernet0/0
O E1 172.16.110.1/32 [110/21] via 172.16.100.1, 02:02:29, FastEthernet0/0
C 172.16.100.0/24 is directly connected, FastEthernet0/0
SP1#
SP1#
SP1#
SP1#show ip ospf database
OSPF Router with ID (10.0.1.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
10.0.1.1 10.0.1.1 1303 0x80000005 0x00D5F4 2
10.0.2.1 10.0.2.1 1350 0x80000006 0x00D39C 3
10.0.3.1 10.0.3.1 1554 0x80000006 0x007FC3 3
10.0.4.1 10.0.4.1 1352 0x80000005 0x00CCC7 2
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.0.12.2 10.0.2.1 1350 0x80000004 0x00FBFC
10.0.23.1 10.0.3.1 1554 0x80000004 0x009B50
10.0.34.2 10.0.4.1 1352 0x80000004 0x0027B5
OSPF Router with ID (172.16.100.254) (Process ID 10)
Router Link States (Area 10)
Link ID ADV Router Age Seq# Checksum Link count
172.16.100.254 172.16.100.254 1400 0x80000005 0x00D7D1 1
172.16.110.1 172.16.110.1 1405 0x80000005 0x006E49 2
Net Link States (Area 10)
Link ID ADV Router Age Seq# Checksum
172.16.100.1 172.16.110.1 1405 0x80000004 0x007824
Summary Net Link States (Area 10)
Link ID ADV Router Age Seq# Checksum
172.16.20.1 172.16.100.254 1400 0x80000004 0x00586D
172.16.200.0 172.16.100.254 1400 0x80000004 0x00947E
172.16.201.0 172.16.100.254 1400 0x80000004 0x008988
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
172.16.110.1 172.16.110.1 1405 0x80000004 0x007253 0
SP1#
SP1#
SP1#
SP1#show ip ospf database external
OSPF Router with ID (10.0.1.1) (Process ID 1)
OSPF Router with ID (172.16.100.254) (Process ID 10)
Type-5 AS External Link States
Routing Bit Set on this LSA
LS age: 1409
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 172.16.110.1 (External Network Number )
Advertising Router: 172.16.110.1
LS Seq Number: 80000004
Checksum: 0x7253
Length: 36
Network Mask: /32
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 20
Forward Address: 0.0.0.0
External Route Tag: 0
SP1#
SP1#show ip bgp vpnv4 all
BGP table version is 14, local router ID is 10.0.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 100:1 (default for vrf CUST-A)
*> 172.16.10.1/32 172.16.100.1 2 32768 ?
* i172.16.20.1/32 10.0.4.1 2 100 0 ?
*>i 10.0.3.1 2 100 0 ?
*> 172.16.100.0/24 0.0.0.0 0 32768 ?
*> 172.16.110.1/32 172.16.100.1 21 32768 ?
* i172.16.200.0/24 10.0.4.1 2 100 0 ?
*>i 10.0.3.1 0 100 0 ?
*>i172.16.201.0/24 10.0.4.1 0 100 0 ?
* i 10.0.3.1 2 100 0 ?
SP1#
SP1#show ip bgp vpnv4 rd 100:1 172.16.110.1/32
BGP routing table entry for 100:1:172.16.110.1/32, version 7
Paths: (1 available, best #1, table CUST-A)
Advertised to update-groups:
2
Local
172.16.100.1 from 0.0.0.0 (10.0.1.1)
Origin incomplete, metric 21, localpref 100, weight 32768, valid, sourced, best
Extended Community: RT:1:100 OSPF DOMAIN ID:0x0005:0x0000000A0200
OSPF RT:0.0.0.0:5:0 OSPF ROUTER ID:172.16.100.254:0
mpls labels in/out 23/nolabel
SP1#
3) Again SP2, is transparent.
4) SP3 receives the VPNv4 for 172.16.110.1/32 from SP1. Installs it into BGP and then redistribute to OSPF. If we compare the ospf database output of SP1 with SP3. We see that SP3 has a different value for “tag” in 172.16.110.1/32. So that tags is created by SP3 when redistributing the BGP prefix to OSPF (based on the extended communities in the VPNv4 prefix). As per the rfc, the tag is generated based on the ASN (100). As are all our SPs are in the same ASN, the tag will be the same in all of PE generating the LSA from the VPNv4.
SP3#show ip bgp vpnv4 all
BGP table version is 13, local router ID is 10.0.3.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 100:1 (default for vrf CUST-A)
*>i172.16.10.1/32 10.0.1.1 2 100 0 ?
* i172.16.20.1/32 10.0.4.1 2 100 0 ?
*> 172.16.200.1 2 32768 ?
*>i172.16.100.0/24 10.0.1.1 0 100 0 ?
*>i172.16.110.1/32 10.0.1.1 21 100 0 ?
* i172.16.200.0/24 10.0.4.1 2 100 0 ?
*> 0.0.0.0 0 32768 ?
* i172.16.201.0/24 10.0.4.1 0 100 0 ?
*> 172.16.200.1 2 32768 ?
SP3#
SP3#
SP3#show ip route vrf CUST-A
Routing Table: CUST-A
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
C 172.16.200.0/24 is directly connected, FastEthernet0/0
O 172.16.201.0/24 [110/2] via 172.16.200.1, 02:06:43, FastEthernet0/0
O 172.16.20.1/32 [110/2] via 172.16.200.1, 02:06:43, FastEthernet0/0
B 172.16.10.1/32 [200/2] via 10.0.1.1, 02:04:33
B 172.16.110.1/32 [200/21] via 10.0.1.1, 02:04:33
B 172.16.100.0/24 [200/0] via 10.0.1.1, 02:04:33
SP3#
SP3#
SP3#show ip ospf database
OSPF Router with ID (10.0.3.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
10.0.1.1 10.0.1.1 1556 0x80000005 0x00D5F4 2
10.0.2.1 10.0.2.1 1602 0x80000006 0x00D39C 3
10.0.3.1 10.0.3.1 1804 0x80000006 0x007FC3 3
10.0.4.1 10.0.4.1 1602 0x80000005 0x00CCC7 2
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.0.12.2 10.0.2.1 1602 0x80000004 0x00FBFC
10.0.23.1 10.0.3.1 1804 0x80000004 0x009B50
10.0.34.2 10.0.4.1 1602 0x80000004 0x0027B5
OSPF Router with ID (172.16.200.254) (Process ID 10)
Router Link States (Area 10)
Link ID ADV Router Age Seq# Checksum Link count
172.16.20.1 172.16.20.1 1640 0x80000006 0x00710E 3
172.16.200.254 172.16.200.254 1625 0x80000005 0x0055C4 1
172.16.201.254 172.16.201.254 1626 0x80000005 0x0059BC 1
Net Link States (Area 10)
Link ID ADV Router Age Seq# Checksum
172.16.200.254 172.16.200.254 1625 0x80000004 0x00F0E6
172.16.201.254 172.16.201.254 1626 0x80000004 0x00E7EC
Summary Net Link States (Area 10)
Link ID ADV Router Age Seq# Checksum
172.16.10.1 172.16.200.254 1625 0x80000004 0x000863
172.16.10.1 172.16.201.254 1626 0x80000004 0x000169
172.16.100.0 172.16.200.254 1625 0x80000004 0x0026EC
172.16.100.0 172.16.201.254 1626 0x80000004 0x001FF2
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
172.16.110.1 172.16.200.254 1625 0x80000004 0x005BDB 3489661028
172.16.110.1 172.16.201.254 1626 0x80000004 0x0054E1 3489661028
SP3#
5) So let’s see with details the VPNv4 prefix for 172.16.10.1/32 (OSPF LSA3) and 172.16.110.1/32 (OSPF LSA5). Both originated by HQ.
SP3#show ip bgp vpnv4 rd 100:1 172.16.10.1/32
BGP routing table entry for 100:1:172.16.10.1/32, version 8
Paths: (1 available, best #1, table CUST-A)
Not advertised to any peer
Local
10.0.1.1 (metric 3) from 10.0.1.1 (10.0.1.1)
Origin incomplete, metric 2, localpref 100, valid, internal, best
Extended Community: RT:1:100 OSPF DOMAIN ID:0x0005:0x0000000A0200
OSPF RT:0.0.0.10:2:0 OSPF ROUTER ID:172.16.100.254:0
mpls labels in/out nolabel/21
SP3#
SP3#show ip bgp vpnv4 rd 100:1 172.16.110.1/32
BGP routing table entry for 100:1:172.16.110.1/32, version 11
Paths: (1 available, best #1, table CUST-A)
Not advertised to any peer
Local
10.0.1.1 (metric 3) from 10.0.1.1 (10.0.1.1)
Origin incomplete, metric 21, localpref 100, valid, internal, best
Extended Community: RT:1:100 OSPF DOMAIN ID:0x0005:0x0000000A0200
OSPF RT:0.0.0.0:5:0 OSPF ROUTER ID:172.16.100.254:0
mpls labels in/out nolabel/23
SP3#
6) So SP3, based on the Extended communities, knows the VPNv4 prefix 172.16.110.1/32 was an OSPF LSA5 and it creates a tag. Keep in mind that SP4 is doing exactly the same thing as SP3:
SP4#
SP4#show ip route vrf CUST-A
Routing Table: CUST-A
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 6 subnets, 2 masks
O 172.16.200.0/24 [110/2] via 172.16.201.1, 02:18:34, FastEthernet3/0
C 172.16.201.0/24 is directly connected, FastEthernet3/0
O 172.16.20.1/32 [110/2] via 172.16.201.1, 02:18:34, FastEthernet3/0
B 172.16.10.1/32 [200/2] via 10.0.1.1, 02:16:19
B 172.16.110.1/32 [200/21] via 10.0.1.1, 02:16:19
B 172.16.100.0/24 [200/0] via 10.0.1.1, 02:16:19
SP4#
SP4#
SP4#
SP4#show ip ospf database
OSPF Router with ID (10.0.4.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
10.0.1.1 10.0.1.1 253 0x80000006 0x00D3F5 2
10.0.2.1 10.0.2.1 310 0x80000007 0x00D19D 3
10.0.3.1 10.0.3.1 504 0x80000007 0x007DC4 3
10.0.4.1 10.0.4.1 301 0x80000006 0x00CAC8 2
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.0.12.2 10.0.2.1 310 0x80000005 0x00F9FD
10.0.23.1 10.0.3.1 504 0x80000005 0x009951
10.0.34.2 10.0.4.1 301 0x80000005 0x0025B6
OSPF Router with ID (172.16.201.254) (Process ID 10)
Router Link States (Area 10)
Link ID ADV Router Age Seq# Checksum Link count
172.16.20.1 172.16.20.1 315 0x80000007 0x006F0F 3
172.16.200.254 172.16.200.254 347 0x80000006 0x0053C5 1
172.16.201.254 172.16.201.254 315 0x80000006 0x0057BD 1
Net Link States (Area 10)
Link ID ADV Router Age Seq# Checksum
172.16.200.254 172.16.200.254 347 0x80000005 0x00EEE7
172.16.201.254 172.16.201.254 315 0x80000005 0x00E5ED
Summary Net Link States (Area 10)
Link ID ADV Router Age Seq# Checksum
172.16.10.1 172.16.200.254 347 0x80000005 0x000664
172.16.10.1 172.16.201.254 315 0x80000005 0x00FE6A
172.16.100.0 172.16.200.254 347 0x80000005 0x0024ED
172.16.100.0 172.16.201.254 315 0x80000005 0x001DF3
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
172.16.110.1 172.16.200.254 347 0x80000005 0x0059DC 3489661028
172.16.110.1 172.16.201.254 315 0x80000005 0x0052E2 3489661028
SP4#
SP4#
SP4#
SP4#show ip ospf database external 172.16.110.1
OSPF Router with ID (10.0.4.1) (Process ID 1)
OSPF Router with ID (172.16.201.254) (Process ID 10)
Type-5 AS External Link States
LS age: 350
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 172.16.110.1 (External Network Number )
Advertising Router: 172.16.200.254
LS Seq Number: 80000005
Checksum: 0x59DC
Length: 36
Network Mask: /32
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 21
Forward Address: 0.0.0.0
External Route Tag: 3489661028
LS age: 319
Options: (No TOS-capability, DC)
LS Type: AS External Link
Link State ID: 172.16.110.1 (External Network Number )
Advertising Router: 172.16.201.254
LS Seq Number: 80000005
Checksum: 0x52E2
Length: 36
Network Mask: /32
Metric Type: 1 (Comparable directly to link state metric)
TOS: 0
Metric: 21
Forward Address: 0.0.0.0
External Route Tag: 3489661028
SP4#
SP4#
SP4#
SP4#show ip bgp vpnv4 rd 100:1 172.16.10.1/32
BGP routing table entry for 100:1:172.16.10.1/32, version 8
Paths: (1 available, best #1, table CUST-A)
Not advertised to any peer
Local
10.0.1.1 (metric 4) from 10.0.1.1 (10.0.1.1)
Origin incomplete, metric 2, localpref 100, valid, internal, best
Extended Community: RT:1:100 OSPF DOMAIN ID:0x0005:0x0000000A0200
OSPF RT:0.0.0.10:2:0 OSPF ROUTER ID:172.16.100.254:0
mpls labels in/out nolabel/21
SP4#
SP4#
SP4#show ip bgp vpnv4 rd 100:1 172.16.110.1/32
BGP routing table entry for 100:1:172.16.110.1/32, version 11
Paths: (1 available, best #1, table CUST-A)
Not advertised to any peer
Local
10.0.1.1 (metric 4) from 10.0.1.1 (10.0.1.1)
Origin incomplete, metric 21, localpref 100, valid, internal, best
Extended Community: RT:1:100 OSPF DOMAIN ID:0x0005:0x0000000A0200
OSPF RT:0.0.0.0:5:0 OSPF ROUTER ID:172.16.100.254:0
mpls labels in/out nolabel/23
SP4#
7) As you can see, SP3 and SP4 are generating the same “tag” 3489661028 for the LSA5 172.16.110.1/32 (because they are in the same ASN 100). So as the receiving LSA for the other SP in the same Area 10 has the same tag, SP3/SP4 ignore the LSA. And again, the BGP prefix is installed in the routing table instead of the OSPF AD110 172.16.110.1/32 and we dont have a routing loop.
Design: When you aim for HA, even a single patch panel is a SPOF no matther how much redundancy you have in your transit providers, routers, switches, firewalls, etc etc. So, look for SPOF!
Documentation: For DC stuff, in my current employer we use patchmanager. It is supper handy for remote locations and it is our source of truth. Keep in mind that tool is as good as you keep it updated…. For example, for the PoPs we visit more often and we make more changes, we find more failures that we would like… For remote PoPs, as we know we are not going to come back for a couple of years, we are much more throrough. For network kit, we have RANCID+Git so we know always the lattest config and when changes where introduced (in 30m intervals at least).
Process: We follow a risk assesment for any change we plan to introduce. Then on Thursday we have a CAB metting to schedule what changes are going to happen during the weekend. The aim is to have several people from different teams to understand and have a say in what is going to happen. This has proobed very useful. Four pairs of eyes are better than half 🙂 Still you need to be regirous in this process
Even having all this into account, you will have an outage. Have a retrospective, learn from it (no finger pointing) and apply it. Trully agile 😛
When I was studying CCNP back in 2009, I found a lab for MPLS that was fantastic. It showed how to buid a MPLS L3VPN network from scratch. I managed to build that in my laptop with GNS3 at that time.
Now I want to review some MPLS features so I decided to install GNS3 and build that lab again. You can find it in my github account (that was gathering dust…):
I searched several pages to find out how to do it now a days. It seems it is mainly managed by python. This is what I had to do for Debian 10 (Testing)
Once I managed to run the program, I found some other issues.
First, you need to get the software to simulate the routers. I searched for recommended images for running MPLS and it seems c7200-adventerprisek9-mz.124-24.T2 was a good one. If you search for that, it will not be difficult to find somewhere to download it.
It took me a while but at the end I could create a lab with several routers, power them up and login to them.
As well, I modified GNS3 to use “terminator” as default terminal when connecting to the devices. That was handy.
There are many things you can configure with GNS3. Like a basic linux host to test (Alpine). I installed it as I think it will be usefull in the future:
There are many more things you can configure but for what I want, this is enough.
MPLS L3VPN
So once we have aworking GNS3 environment, we can get our hands dirty and create our MPLS L3VPN lab.
This is the diagram:
We are going to simulate a Service Provider (SP) network that is formed by SP1, SP2 and SP3. The customer network CUST-A is formed by HQ and Branch:
SP1 and SP3 will be PE routers (they will manage the L3VPN) (PE = Provider Edge)
SP2 will be just a P router (doesnt have visibility of any L3VPN, just handle labels) (P = Provider)
HQ and BRANCH are CPE routers (Customer Provider Edge). They interact with PE.
The SP network uses OSPF (area0 – backbone area) as IGP to build the iBGP full mesh (as100)
CUST-A is connected to our SP network in different locations. Internally is running its own routing and both locations are in the same OSPF area 10. So the prefixes learned in HQ and Branch should be seen as Inter-Area (IA). This is quite important.
So, lets get step by step:
1- IP addressing
We need to configure the IP connectivity in all links
SP1
---
!
interface Loopback0
ip address 10.0.1.1 255.255.255.255
!
interface GigabitEthernet1/0
description to SP2-P
ip address 10.0.12.1 255.255.255.0
!
interface FastEthernet0/0
description to HQ
ip address 172.16.100.254 255.255.255.0
!
SP2
---
!
interface Loopback0
ip address 10.0.2.1 255.255.255.255
!
interface GigabitEthernet1/0
description to SP1-PE
ip address 10.0.12.2 255.255.255.0
!
interface GigabitEthernet2/0
description to SP3-PE
ip address 10.0.23.2 255.255.255.0
!
SP3
---
!
interface Loopback0
ip address 10.0.3.1 255.255.255.255
!
interface GigabitEthernet1/0
description to SP2-P
ip address 10.0.23.1 255.255.255.0
!
interface FastEthernet0/0
description to BRANCH
ip address 172.16.200.254 255.255.255.0
!
HQ
---
!
interface Loopback0
ip address 172.16.10.1 255.255.255.0
!
interface FastEthernet0/0
description to SP1-PE
ip address 172.16.100.1 255.255.255.0
!
BRANCH
---
!
interface Loopback0
ip address 172.16.20.1 255.255.255.0
!
interface FastEthernet0/0
description to SP3-PE
ip address 172.16.200.1 255.255.255.0
!
Verify you can ping each directly connected router:
HQ#ping 172.16.100.254
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.100.254, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/16/20 ms
HQ#
2- Routing in SP
We are going to configure OSPF (area 0 – backbone) as IGP in our SP network. We only want the SP loopbacks and backbone links in OSPF.
SP1
---
router ospf 1
network 10.0.1.0 0.0.0.255 area 0
network 10.0.12.0 0.0.0.255 area 0
SP2
---
router ospf 1
network 10.0.2.0 0.0.0.255 area 0
network 10.0.12.0 0.0.0.255 area 0
network 10.0.23.0 0.0.0.255 area 0
SP3
---
router ospf 1
network 10.0.3.0 0.0.0.255 area 0
network 10.0.23.0 0.0.0.255 area 0
Verify that OSFP comes up in all expected links. If SP2 has two neighbors, all good:
SP2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.0.3.1 1 FULL/DR 00:00:36 10.0.23.1 GigabitEthernet2/0
10.0.1.1 1 FULL/DR 00:00:34 10.0.12.1 GigabitEthernet1/0
SP2#
3- MPLS in SP
Now we are going to configure MPLS in SP. We are going to use LDP for label distribution. The configuration is pretty easy, just enable LDP using Lo0 as router ID and configure mpls only in backbone links.
SP1
---
mpls ldp router-id Loopback0 force
!
interface GigabitEthernet1/0
description to SP2-P
mpls ip
!
SP2
---
mpls ldp router-id Loopback0 force
!
interface GigabitEthernet1/0
description to SP1-PE
mpls ip
!
interface GigabitEthernet2/0
description to SP3-PE
mpls ip
!
SP3
---
mpls ldp router-id Loopback0 force
!
interface GigabitEthernet1/0
description to SP2-P
mpls ip
!
Check that LDP neighbors come up. If SP2 has two, all good.
For SP, you need each customer in a VRF so you can isolate them and the customer can use any IP addressing schema. You need to make those IP prefixes unique inside SP if you want to exchange them via a routing protocol. For doing that, you need to create VPNV4 addresses that are a combination of the customer IP prefix and a RD (Router Distinguisher – 8 bytes). Each VRF has a RD and is locally significant, you could configure each PE with CUST-A using a different RD, but as best practive we keep the same RD per VRF. Having each VRF with a different RD, eachc customer cand use the same private IP prefix but to the SP eyes, after building the VPNV4, the customer prefixes will be diffierent and there will no be leaking (if you dont configure it). For exporting/importing prefixes in a VRF, we use RT (Route Target). And that is defined during the VRF creating too. Keep in mind that we only define VRFs in PE routers (SP1 and SP3). The P routers (SP2) dont need to know. In our case we are going to use RD 100:1 and RT 1:100.
The config above says that for each VPNv4 prefix we export from CUST-A VRF we add RT 1:100. And for any VPNv4 prefix with RT:1:00 learned by the router (more about this later in point 6) we will import it in CUST-A VRF.
Now we can configure the links to customers in their own VRF:
SP1
---
!
interface FastEthernet0/0
description to HQ
ip vrf forwarding CUST-A
ip address 172.16.100.254 255.255.255.0
!
SP3
---
!
interface FastEthernet0/0
description to BRANCH
ip vrf forwarding CUST-A
ip address 172.16.200.254 255.255.255.0
!
Now, let’s check if we keep the IP connectivity with CUST-A from our PE routers.
SP1#ping vrf CUST-A 172.16.100.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.100.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 16/21/32 ms
SP1#
Keep in mind, that for CUST-A, all this is transparent.
5- Customer Routing
So now, we want our customer devices to exchange routes because they want reach each other (if not, why do you want a network 🙂 We are going to use OSPF in area 10 as routing protocol between CUST-A and SP. But SP is already using OSFP??? Yes, but keep in mind that we are using VRFs, and the OSPF implementation will be in the customer VRF. It will not interact with the SP OSPF Area 0. So we need to configure OSPF in the interfaces connecting to SP and interfaces we want to advertise (Lo0). And again, our SP2 (P) doesnt need to know anything about this.
HQ
!
router ospf 1
log-adjacency-changes
network 172.16.10.0 0.0.0.255 area 10
network 172.16.100.0 0.0.0.255 area 10
!
BRANCH
!
router ospf 1
log-adjacency-changes
network 172.16.20.0 0.0.0.255 area 10
network 172.16.200.0 0.0.0.255 area 10
SP1
!
router ospf 10 vrf CUST-A
network 172.16.100.0 0.0.0.255 area 10
!
SP3
!
router ospf 10 vrf CUST-A
network 172.16.200.0 0.0.0.255 area 10
!
Check that OSPF between CUST-A devices and SP comes up, and you are learning the CUST-A prefixes:
SP1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.0.2.1 1 FULL/BDR 00:00:36 10.0.12.2 GigabitEthernet1/0
172.16.10.1 1 FULL/BDR 00:00:38 172.16.100.1 FastEthernet0/0
SP1#
SP1#show ip route vrf CUST-A
Routing Table: CUST-A
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
O 172.16.10.1/32 [110/2] via 172.16.100.1, 00:38:48, FastEthernet0/0
C 172.16.100.0/24 is directly connected, FastEthernet0/0
SP1#
In the output above, you will see that SP1 has two OSPF neighbors, one to SP2 (OSPF area 0 – backbone) and one to HQ (CUST-A VRF). As well, you will see that SP1 is learning Lo0 prefix from HQ via OSPF.
6- BGP
Now we have routing between SP1-HQ and SP3-BRANCH. But we dont have communication between HQ-BRANCH yet. And this is the goal at the end of the day.
So now, we need our PE routers to exchange the customer prefixes. We are going to use BGP/MP-BGP.
As we are in the same AS100 (Autonomous System) we are going to use iBGP (internal BGP). Following best practices, we are going to build our full mesh iBGP on loopbacks. iBGP relays on a IGP, and that is already configured via OSPF in our network.
SP1#show ip bgp summary
BGP router identifier 10.0.1.1, local AS number 100
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
10.0.3.1 4 100 53 53 1 0 0 00:50:08 0
SP1#
So we have BGP between our PE routers. Now we need to configure the exchange of those VPNv4 routes so CUST-A devices can learn prefixes from its own network.
The above part is the MP-BGP part (MultiProtocol-BGP). Inside our BGP connection between SP1-SP3 we have enabled a type of exchange of prefixes for vpnv4.
But, we dont have VPNv4 prefixes yet.
SP1#show ip bgp vpnv4 all
BGP table version is 7, local router ID is 10.0.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
SP1#
Because they need to be in the BGP table first. We have routing between PE-CE (ospf area 10) but we dont have any kind of redistribution between OSPF-BGP. Let’s do that:
SP1#show ip bgp vpnv4 all
BGP table version is 9, local router ID is 10.0.1.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 100:1 (default for vrf CUST-A)
*> 172.16.10.1/32 172.16.100.1 2 32768 ?
*>i172.16.20.1/32 10.0.3.1 2 100 0 ?
*> 172.16.100.0/24 0.0.0.0 0 32768 ?
*>i172.16.200.0/24 10.0.3.1 0 100 0 ?
SP1#
So now, we can see in SP1 the prefixes from HQ and BRANCH routers!
Now, let’s check the CUST-A routing table from SP1 and HQ:
SP1#show ip route vrf CUST-A
Routing Table: CUST-A
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks
B 172.16.200.0/24 [200/0] via 10.0.3.1, 01:08:19
B 172.16.20.1/32 [200/2] via 10.0.3.1, 00:01:33
O 172.16.10.1/32 [110/2] via 172.16.100.1, 01:15:49, FastEthernet0/0
C 172.16.100.0/24 is directly connected, FastEthernet0/0
SP1#
HQ#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
172.16.0.0/16 is variably subnetted, 4 subnets, 2 masks
O IA 172.16.200.0/24 [110/2] via 172.16.100.254, 01:07:09, FastEthernet0/0
O IA 172.16.20.1/32 [110/3] via 172.16.100.254, 00:00:22, FastEthernet0/0
C 172.16.10.0/24 is directly connected, Loopback0
C 172.16.100.0/24 is directly connected, FastEthernet0/0
HQ#
HQ#
So from SP1, we can see that is learning BRANCH 172.16.200.0/24 and 172.16.20.1/32 via iBGP (from SP3 loopback 10.0.3.1). The 172.16.10.1/32 (HQ loopback) via OSPF.
From HQ, we see it is learning BRANCH Prefixes too and they come up as O IA. This is very important, and it is material for another post about OSPF Down-Bit.
7- Conclusion
We have IP connectivity between our CUST-A devices across a MPLS L3VPN
HQ#ping 172.16.20.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.20.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 44/76/156 ms
HQ#
HQ#traceroute 172.16.20.1
Type escape sequence to abort.
Tracing the route to 172.16.20.1
1 172.16.100.254 16 msec 8 msec 12 msec
2 10.0.12.2 [MPLS: Labels 17/20 Exp 0] 72 msec 40 msec 60 msec
3 172.16.200.254 [MPLS: Label 20 Exp 0] 52 msec 8 msec 60 msec
4 172.16.200.1 40 msec 52 msec 40 msec
HQ#
We have built a MPLS L3VPN from the bottom up. There are many points that can be explained with much more detail but that wasnt the goal. Just wanted to build this MPLS network so I can do some hands-on troubleshooting and review a couple of concepts.
Ideally, at some point, I would like to build a MPLS Segment Routing GNS3 networks.
Today I had to troubleshoot a websocket issue. I had never dealt with this before. I was told that HAproxy config was fine that it was to be our NGFW doing something nasty at L7.
The connection directly to the server doing websocket was fine from my PC but for some requirement we need to put that server behing a HAproxy. From my PC to the haproxy that is doing “proxy” fore the websocket service failed…
Funny enough HAproxy and the websocket service were running in the same host.
As usual I took a look at the firewall logs. Nothing wrong there at first sight. I took a tcpdump from my pc when connecting to the websocket service and to the haproxy.
The service is very verbose and it is difficult to follow in the capture files as it spawns several connections. I went to the easy part, the capture to the haproxy was showing a lot of TCP retransmissions… The other trace to the websocket service was pretty clean.
Taking into account that the path from my PC to the haproxy server is the always the same (and I was going through a VPN) I could think it was a NGFW issue or something between HAproxy and the websocket service (that is a localhost connection).
As well, I was seeing weird things latency wise. Some TCP resets were taking more than 200ms to arrive to the server when the average RTT was 3ms.
I tried to take a tcpdump between the haproxy service and the websocket service just in case that packet loss was caused locally. The capture was chaos to follow. I had to understand better the sessions in HAproxy.
I changed direction and I went to the NGFW and created a rule that disabled any fancy security check for me to the haproxy server. I wanted to be sure the firewall was innocent.
It was. Same issue. I tried different browsers and always the same.
So I was nearly sure the problem was in HAproxy but I had to prove it. I kind of failed checking the backend connection (haproxy to websockt proxy) so I took again a look to the trace from my pc to haproxy. I was quite frustrated because there was so many connetions openned and then retransmissions started happening that I couldnt really see any problem.
By luck, I noticed that in the good trace (the one going directly to the websocket service) I could see a HTTP GET request for “socket” from my PC. Keep in mind that I have no idea how websocket works. I tried to find a similar request in the haproxy trace, and I saw the problem….
Rejected HTTP GET socket request
and this is a good connection:
Successful HTTP GET socket request
So at the end, HAproxy was at fault (we dont know how to fix it though yet) and my firewall (for once) it is innocent.
The summary, I got overwhelmed by the TCP retransmissions. I was lucky that I saw the GET socket and I assumed that had to be the way to get the websocket connection established. So I should have started investigating how a websocket connections is stablished. As well, I didnt manage to find the HAproxy logs, I am pretty sure I should have found the same answer. So I need to learn to check that.
I learned something new. As usual, it didnt come easy neither quick 🙂
I use gkrellm as my linux monitoring app. I have used it since I started but something I miss is I would like to know what app and destination IPs are causing a traffic spike in my laptop.
Searching a bit a come up with this page with several tools:
I am subscribed to Cloudflare blog as they are in general really good. And definitely, you always learn something new (and want to cry because you have so much to learn from these guys).
This time was a dissection of conntrack in iptables to improve their firewall performance.
