Day: 29 June 2020

Route Reflectors – Notes

Reflect:
client -> clients and non-clients
non-client -> clients
No Reflect:
non-client -> no-client (normal ibgp)

always advertises to eBGP peers (normal ebgp)
eBGP learned prefixes, advertised to client and non-clients (normal ebgp)

Full Mesh iBGP:

  • between RRs
  • RRs and non-clients
  • clients just need iBGP to RRs.

Reflects ONLY the best route

RRs dont modify on reflected routes: NEXT_HOP, AS_PATH, LP and MED.

Prevent routing information loops: ORIGINATOR_ID and CLUSTER_LIST

Clustering:

  • ORIGINATOR_ID: The first RR creates the Originator_ID and sets it to the BGP router ID of the router that originates the route. So when a client receives a route with its own Originator ID, it is dropped.
  • CLUSTER_LIST: if the local CLUSTER_ID is found in the list, the route is discarded.
    *This is done ONLY in RRs.
    This is ONLY created or updated on a RR during Reflection.
Hierarchical Route Reflection:
2 levels:
- level1 RRs are clients of level2 RR -> level1 RR dont need full mesh between them
- level2 RRs are full mesh between them.

Consider:
size of top-level mesh
number of alternative paths

Hierarchical Route Reflection:
2 levels:

– level1 RRs are clients of level2 RR -> level1 RR dont need full mesh between them
– level2 RRs are full mesh between them.

Consider:

  • size of top-level mesh
  • number of alternative paths

RR Design Priciples:

  • keep logical and physical topologies congruent to increase redundancy and path optimization and prevent loops
    — follow physical topology
    — change physical topology
    — modify logical topology
    — follow physical topology
    — session between RR and non-client shouldnt traverse a client
    — session between RR and client shouldnt traverse a non-client
  • use comparable metrics in route selection to avoid convergence oscillations
    (ie MED – only used between prefixes from same neighbor AS – not used for different AS)
    — full ibgp mess –> no
    — always-compare-med –> no
    — deterministic-med -> ok
    — med=0 (via RM in) -> ok
    — bgp communities ->
  • set proper intra and inter cluster IGP metrics to avoid convergence oscillations
    — multicluster RR architecture: intracluster metrics lower than intercluster.
  • modify next-hop with care. Do so only to bring the RRs into forwarding path.
  • use peer groups with RR to reduce convergence time.