Multicast Lab Tips

  • On each device do a show ip pim interface and show ip pim neighbour , highlight the multicast links and note down the DR and the mode e.g. SD, S or D (Dense). Also enable ip multicast-routing on all participating devices.
  • Use BSR instead of Autorp if you can as BSR allows you to filter multicast with the ip pim bsr-border command easily at the edge routers, it is tricky to so this in autorp. Also no need for ip pim autorp listener on each router when using autorp (alternatively you can put it into sparse-dense mode if the listener command is restricted on the lab – needed for Sparse mode though).
  • Use ip pim nbma-mode to effectively disable split horizon. Alternatively use a GRE tunnel between the spokes if that command is not an option or if it dense mode running across a NBMA network as dense mode can not bypass split horizon rules.
  • Set the ip pim dr-priority to 0 on all spokes and a high value, say 255, on the hub.
  • Anything to do with multicast can typically be found under ip pim, ip multicast and ip igmp. e.g. ip multicast boundary under the interface (alternative to using scope).
  • If mapping agent is required to associate a particular group to an RP, you will need to use rp-announce-filter which references access-lists that contains both the RP address rp-list and the groups group-list. You will also have to use the exact same group-list on the RP itself after the send-rp-announce command so that the RP can volunteer for those groups.
  • Deny statements do not work in a RP group-list / access-list. as it treats a deny as flooding in dense mode
  • To deny a PIM neighbourship form forming, use ip pim neighbor-filter # under the interface.
  • ip pim spt-threshold is to be used if you need to deny changing to shortest path source tree. RP is a shared tree where devices sends registers and join requests too. Once everyone figures out the stream, they will bypass the RP and use shortest path tree (S,G). Set spt-threshold to infinity so that devices always uses the RP and not their own shortest path. Do show ip mroute and look at the flags to verify (J in flags means shortest path).
  • Use ip igmp static-group instead of join-group if an application is unable to use IGMP to respond to a group or it needs to be fast switched. Verify this with show ip igmp membership=
  • Can filter / restrict specific multicast group under the interface using ip igmp access-group 1
  • 224.0.1.40 is the mapping agent and 224.0.1.39 is the auto-rp
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