OECG – Chapter 20

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In local area networks (i.e. multiaccess networks versus point-to-point links), there are three areas the book touches on that are notable.

Prune Override

  • In a LAN scenario, it’s possible to have several routers sharing a network segment. So what happens if one router sends a prune out his RPF interface for a specific multicast group, but one or more other downstream routers on that segment still want the multicast traffic? Remember that multicast traffic has a destination of the multicast group address. So (forgetting about RGMP for a moment), it’s all or nothing for that segment, right? A prune from one router could cause other downstream routers on that segment to not receive multicast traffic they should be receiving.
  • Prune override helps us with this. When a Prune message goes out, it is sent to the All-PIM router address of So not only the upstream router sees the Prune, but also the other PIM routers on the segment. When the upstream router detects that it is on a multiaccess segment, it will wait 3 seconds before pruning the S, G. If within those 3 seconds, another router sends a join request, the upstream router will continue forwarding for that S, G uninterrupted. But if the 3 seconds expires with no Join requests received, then the router is pruned.

Assert Message

  • The assert message is used to determine which router will be responsible to forward multicast group traffic onto a LAN segment, when multiple routers are capable of performing the task. (I.E., one of the router will “assert” themselves over the others.)
  • Routers on a segment will know this situation has arisen if they see each other’s forwarded multicast traffic on the same segment. When that happens, they send assert messages to one another.
  • The router will win based on the administrative distance and metric of the unicast route covering the multicast source.
    • The router with the lowest administrative distance wins.
    • If tied, the router with the lowest advertised metric wins.
    • If still tied, the router with the highest IP address on that LAN segment wins.

Designated Router

  • The PIM-DM (or PIM sparse mode) router with the highest IP address becomes a designated router.
  • The DR’s job is to determine who will send IGMPv1 queries onto the segment. Remember that IGMPv2 handles this on its own by electing a querier, but that IGMPv1 relies on the multicast routing protocol to figure this detail out.
  • Note that you can have one router be the DR, and a different router become the forwarder for a multicast group via the assert process. They are not inter-dependent processes.

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