[c-nsp] Multicast replication over GRE on 7600s

[Benjamin Lovell]

The same hardware(janus/metro) is responsible for the replication(no punt to CPU) but due to the GRE ecanp required the packet will have to go through a longer forwarding process(more lookups) and performance will be reduced. I don't have any solid numbers but my guess is that forwarding rate would be approx 1/2.

The part I am not sure about is if egress replication is still possible. In the mVPN scenario only ingress replication is possible due to the GRE encap/decap but I am not sure if this same limitation applies to P2P GRE tunnels. Let me know if this piece would be important to you and I can look into it. 

The one caveat to be careful of here(applies to unicast as well) is that each GRE tunnel must be sourced from a unique IP address on the box. Using the same source IP on more than one GRE tunnel will cause all traffic in GRE decap path to be punted to CPU and maybe multicast on encap path in some scenarios. 

-Ben



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          .            .          Benjamin Lovell
          |            |          AS Video Practice
         |||          |||         Cisco Customer Advocacy
       .|||||.      .|||||.       Research Triangle Park, NC
    .:|||||||||:..:|||||||||:.    Email:  belovell at cisco.com
             cisco            desk:919.392.8255 cell:203.509.1562
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On Sep 28, 2010, at 10:02 AM, John Neiberger wrote:

> I have an architectural question. As an example, let's say you have
> two 7600s directly connected via routed links running PIM and you have
> primarily multicast traffic. If you're running egress replication
> mode, you either have a Janus ASIC or Metropolis ASIC responsible for
> the multicast replication, which happens on the line card. But how
> would things change if you were not running multicast directly on the
> routed link, but instead used a GRE tunnel between the two routers?
> 
> I guess I have a couple of questions:
> 
> 1. How is GRE traffic processed in this scenario? Can it be forwarded
> at high rates on the line card or is it punted to the CPU?
> 
> 2. Which hardware is responsible for multicast replication over GRE tunnels?
> 
> Any ideas?
> 
> 
> Thanks!
> John
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