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Do multicast packets get forwarded using fast switching? Can they be configured to be sent using CEF?

Sam Brynes

A "show ip pim int" command shows multicast switching as "fast". Can we have it use CEF to reduce the load on our router?


RTR2(config-if)#do sh ip pim int et0/0 detail
Ethernet0/0 is up, line protocol is up
Internet address is
Multicast switching: fast
Multicast packets in/out: 0/0
Multicast TTL threshold: 0
PIM: enabled
PIM version: 2, mode: sparse
PIM DR: (this system)
PIM neighbor count: 1
PIM Hello/Query interval: 30 seconds
PIM Hello packets in/out: 15/16
PIM J/P interval: 60 seconds
PIM State-Refresh processing: enabled
PIM State-Refresh origination: disabled
PIM NBMA mode: disabled
PIM ATM multipoint signalling: disabled
PIM domain border: disabled
PIM neighbors rpf proxy capable: TRUE
PIM BFD: disabled
Multicast Tagswitching: disabled


Also, a related question. Routers perform the RPF check for multicast traffic. I'm not sure if that uses process switching, fast switching, or CEF, but would a large number of multicast packets cause a significant increase on the CPU load from all the RPF checks?


Joseph W. Doherty
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CEF for multicast? Not as far as I know. Keep in mind multicast routing is not like unicast routing, first you're not routing to the destination address, which isn't even really a destination address, second "set-up" for multicast flows is also quite different.

Regrading RPF checks, I suspect this handled somewhat like an ACL check. I.e. either a multicast packet is allowed to be accepted or not as it ingresses an interface. On a software based router, I'm sure RPF checks (in addition to multicast forwarding) add to CPU load, but likely very little per packet. That said, multicast processing might not be as well optimized as unicast processing.
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