Category: networks

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:

http://dtdccie.blogspot.com/2016/03/ospf-down-bit-set.html

https://mellowd.co.uk/ccie/ospf-as-the-pe-ce-routing-protocols-deep-dive-part-1-of-2/

https://mellowd.co.uk/ccie/ospf-as-the-pe-ce-routing-protocols-deep-dive-part-3-of-3-loop-prevention/

So with this background, I built a GNS3 lab to show OSPF Down-Bit in action:

https://github.com/thomarite/mpls-down-bit

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.

Cloudflare had an outage last week. And this time, I felt quite identify with that situation as it could happen to me:

https://blog.cloudflare.com/cloudflare-dashboard-and-api-outage-on-april-15-2020/

Conclusions

• 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…):

https://github.com/thomarite/mpls-basic

Install GNS3

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)

sudo aptitude install dynamips
sudo pip3 install gns3-server
sudo pip3 install gns3-gui
sudo pip3 install PyQt5==5.9.2
gns3

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.

How do you install Cisco IOS images:

https://www.computernetworkingnotes.com/ccna-study-guide/how-to-add-install-or-import-ios-in-gns3.html

After that, I tested if the image worked and found another issue. I needed something called “ubridge”. You can get it here:

https://github.com/GNS3/ubridge

And finally, you needd to reboot

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:

https://docs.gns3.com/appliances/alpine-linux.html

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.

SP2#show mpls ldp neighbor 
    Peer LDP Ident: 10.0.1.1:0; Local LDP Ident 10.0.2.1:0
	TCP connection: 10.0.1.1.646 - 10.0.2.1.61226
	State: Oper; Msgs sent/rcvd: 17/18; Downstream
	Up time: 00:08:34
	LDP discovery sources:
	  GigabitEthernet1/0, Src IP addr: 10.0.12.1
        Addresses bound to peer LDP Ident:
          10.0.12.1       10.0.1.1        
    Peer LDP Ident: 10.0.3.1:0; Local LDP Ident 10.0.2.1:0
	TCP connection: 10.0.3.1.31845 - 10.0.2.1.646
	State: Oper; Msgs sent/rcvd: 16/17; Downstream
	Up time: 00:08:04
	LDP discovery sources:
	  GigabitEthernet2/0, Src IP addr: 10.0.23.1
        Addresses bound to peer LDP Ident:
          10.0.23.1       10.0.3.1        
SP2#

4- Configure a L3VPN – VRF

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.

SP1
---
!
ip vrf CUST-A
 rd 100:1
 route-target export 1:100
 route-target import 1:100
!

SP3
---
!
ip vrf CUST-A
 rd 100:1
 route-target export 1:100
 route-target import 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
!
router bgp 100
 neighbor 10.0.3.1 remote-as 100
 neighbor 10.0.3.1 update-source Loopback0

SP3
!
router bgp 100
 neighbor 10.0.1.1 remote-as 100
 neighbor 10.0.1.1 update-source Loopback0

Check that BGP comes up between SP1-SP3

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.

SP1
!
router bgp 100
 address-family vpnv4
  neighbor 10.0.3.1 activate
  neighbor 10.0.3.1 send-community both
 exit-address-family

SP3
!
router bgp 100
 address-family vpnv4
  neighbor 10.0.1.1 activate
  neighbor 10.0.1.1 send-community both
 exit-address-family

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
!
router ospf 10 vrf CUST-A
 redistribute bgp 100 subnets
!
router bgp 100
 address-family ipv4 vrf CUST-A
  redistribute ospf 10 vrf CUST-A
 exit-address-family
!

SP3
!
router ospf 10 vrf CUST-A
 redistribute bgp 100 subnets
!
router bgp 100
 address-family ipv4 vrf CUST-A
  redistribute ospf 10 vrf CUST-A
 exit-address-family
!

Let’s check again our VPNv4 table:

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.

More to come hopefully.