The image above shows 10 routers all running in OSPF area 0. They are each advertising their own loopback interface into the OSPF domain – ie. R<#> = #.#.#.#/24. They will each get Multicast and PIM Sparse mode enabled on their transit interfaces.

In addition, each router will point to R1 as it’s Rendezvous Point at 1.1.1.1.

On R10 when trying to ping the multicast group 224.1.1.1, I will not get a response, but now on R1 there should be some information gathered.

‘show ip mroute’ on R1 will display a few things.

(10.30.1.2, 224.1.1.1)
– Shows the sender (R10) and the group the sender was trying to reach (224.1.1.1).
– Shows incoming interface on Gig0/3, RPF neighbor 10.30.4.1 (neighboring router interface).
– Outgoing interface in regular Unicast table is Incoming interface in Multicast table.

(*, 224.1.1.1)
– Going to be used by the receivers. Right now it shows outgoing interface as NULL because there are no receivers.

On the reverse side, lets now have a receiver ‘join’ the multicast group of 224.1.1.1. This will be completed in R9, where an IGMP join will be setup.

On R9 there’s only one link towards the RP, which is gig 0/0.

Now on the RP when looking at the group 224.1.1.1, there is an interface in the outgoing interface list. Gig0/0 is the interface pointing to R6, which is the closest to the new IGMP group member.

Now if we move up to R5 and take a look at the mroute table, we’ll see the following:

Shared tree that is pointed towards the Rendezvous Point (RP).

  • (S, G) – (Source, Group) – (*, 224.1.1.1)

And the source tree (Shortest path tree) which is rooted at the group sender.

  • (S, G) – (Source, Group) – (10.30.2.1, 224.1.1.1)
  • Notice the incoming interface is traffic coming from the source, and outgoing interface is where traffic goes to reach the RP.
  • The same can be seen on R4, which is another hop up the chain closer to the Rendezvous Point.

Mroute on R1 is going to show the shared tree and the (S,G).

SPT SWITCHOVER:

  • The RP is ultimately about creating the control plane for traffic from sender to receiver(s). If the RP is not actually in the data path between (S, G), then there’s no point in sending traffic to RP just to have it redirect to somewhere else. To accomplish taking the RP out of the next hop, SPT Switchover is completed.

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