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

[Tim Stevenson]

With a tunnel you don't know which is the egress card until the encap 
is done. That's why tunnel OIFs are always ingress replicated.

Tim

At 09:43 AM 9/28/2010, Ben Lovell (belovell) declared:

>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
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>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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>
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Tim Stevenson, tstevens at cisco.com
Routing & Switching CCIE #5561
Distinguished Technical Marketing Engineer, Cisco Nexus 7000
Cisco - http://www.cisco.com
IP Phone: 408-526-6759
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