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

[Benjamin Lovell]

You would not have to force the box back to ingress. These packet would take the ingress forwarding path instead of egress. Other groups would still function in egress. 

I agree. it's hard to see how this would work in egress as the idea of replication is all packets are getting the same rewrite(on ingress) and egress card just needs to make copies.  I suppose you could replicate in the normal fashion to each egress LC plus one more copy for the GRE tunnel would would then loop through lookup process again for GRE encap but this is purely conjecture on my part.

-Ben



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          .            .          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 12:34 PM, John Neiberger wrote:

> Now that I think about it, I bet that egress mode isn't allowed in
> this scenario. It would make sense that only ingress mode would work,
> that way the ingress Janus/Metro would take care of replicating out to
> all the receivers, including the GRE tunnel. I'm having trouble
> visualizing how that would work in egress mode.
> 
> It was worth checking into, though. We have a situation where this
> might be useful temporarily. But since we're running egress on our
> 7600s, moving back to ingress is just not an option.
> 
> Once again, thanks for your help!
> John
> 
> On Tue, Sep 28, 2010 at 10:25 AM, Benjamin Lovell <belovell at cisco.com> wrote:
>> 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
>> 
>> 
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>           .            .          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
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> 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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