multicast egress replication

Unanswered Question
Aug 12th, 2008

Hi Experts,

I want to understand how multicast egress replication works on catalyst 6500. Say, the ingress module wants to send a mcast packet to six egress modules. The question I have is that - if the Crossbar Switching Fabric is able to replicate the packet itself and the ingress RE queues only one original packet to Crossbar input, or it is a job of the ingress RE to replicated six copies of the packet and queue them to Crossbar?



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Giuseppe Larosa Tue, 08/12/2008 - 10:39

Hello Alex,

a good starting point for your interesting question can be the following link:

It says:

You can prevent redundant replication of multicast packets across the switch-fabric connection by entering a command that instructs the two replication engines on these modules to forward packets only to local interfaces which are associated with the switch-fabric connection that the replication engine supports.

The requirements are:

Local egress replication is supported with the following software configuration and hardware:

•IPv4 egress replication mode

•Dual fabric-connection DFC-equipped modules

•Layer 3-routed interfaces and subinterfaces that are not part of a port channel

Hope to help


whoknowswhat86 Tue, 08/12/2008 - 11:09

Giuseppe, thank you for the clarification. My question is rather to understand if the ingress RE sends six copies of the packet to each egress slot or it sends only one packet and the Crossbar Arbiter is responsible for activating crossbar cross-points correspondent to the output slots, effectively replicating the multicast packet inside the Crossbar. To rephrase it - Is Crossbar involved in the packet replication?



Giuseppe Larosa Tue, 08/12/2008 - 12:08

Hello Alex,

because ingress replication is the older method I think its description is that the ingress linecard makes 6 copies and send them to the interfaces in the oilist for the multicast group.

With egress replication the replication should happen in more scalable manner (this is the advantage) without using 6 times for each packet the crossbar.

In some way the packet takes an internal tag and is taken from the cross bar by all interested modules that look for packets with that tag (and may be for several other ones)

More like a shared memory model.

Note that the requirements are:

sup >= supp720


all modules in chassis must support egress multicast replication.

I had seen a presentation of C7600 architecture but I couldn'f find it

Hope to help



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