Multicast Anycast RP Configuration on Cisco IOS

Lessons Discussion
83

So it’s a little bit more readable :slight_smile:

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84

It’s me again…

asking the same thing today the answer is the same as yours…

Man still beats AI

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85

Hello Aronne

The short answer is you should keep separate MSDP mesh groups for each site’s internal Anycast-RP peering, and configure the inter-site peering between A1 and B1 as a standard MSDP peering that is NOT part of any mesh group.

The internal MSDP mesh-group peers (A1/A2/A3 and B1/B2/B3) serve a different purpose than the inter-site connection. Internal peers provide Anycast-RP redundancy within a single PIM domain, while the A1<–>B1 peering connects two separate administrative domains.

Remember that MSDP mesh-groups suppress SA re-flooding among mesh-group members to prevent loops. The critical forwarding rule is that an SA message received from a mesh-group peer is forwarded to all non-mesh-group MSDP peers. This behavior is important to understand the logic of the process:

  • When A2 generates an SA for a local source, it sends it to its mesh-group peers (A1 and A3)
  • When A1 receives this SA from mesh-group peer A2, it will NOT re-flood it to A3 which is also in the mesh-group.
  • However, A1 WILL forward this SA to B1 because B1 is a non-mesh-group peer
  • B1 then distributes this SA to its own internal mesh-group (B2, B3)

If you added B1 to Site A’s mesh-group, A1 would suppress forwarding SAs from A2/A3 to B1, breaking inter-domain source discovery.

Keep in mind that each site represents a separate PIM domain. Keeping mesh-groups scoped to each site maintains clean administrative control and allows you to apply different policies (SA filtering, limits) at the domain boundary. In this way, you also avoid unnecessary full-meshing across the WAN. Only designated border RPs (A1 and B1) need to peer inter-domain, not all six routers. Make sense?

I hope this has been helpful!

Laz

86

Hello Aronne

Ha, I’m glad that we humans are still useful! It means a lot to me to be able to beat out an AI… Thanks for your confidence in humanity :stuck_out_tongue: !

Laz

1 Like
87

Hi Laz,

many thanks for your explanation.

It works perfectly.

Wish you a great day.

Aronne

88

Hello Aronne

That’s great news, thanks for getting back to us and letting us know! I wish you a great day as well!

Laz