[c-nsp] IP multicast traffic overwhelms switches

[Jay Ford]

On Fri, 10 Jul 2009, victor wrote:
> On Fri, 10 Jul 2009 20:30:19 +0400, Jay Ford <jay-ford at uiowa.edu> wrote:
>> I don't think you want “ip mroute-cache”, at least not on 7600/6500 boxes.
>> My guess is that by configuring that you're disabling the hardware-based
>> forwarding & forcing it to software-based forwarding.  Get rid of the “ip
>> mroute-cache” & see if things get better on the 7600.
>> 
> ip mroute-cache is the default mode for interfaces. Are you suggesting to do 
> "no ip mroute-cache" to disable cef completely.

In your original message you said:
    I explicitly entered "ip mroute-cache" under every interface
which I took to mean that you were changing the default.

In my experience on 6500 boxes running various 12.2SX versions "ip 
mroute-cache" does not show up by default.

If you do "show ip interface" for your edge-facing interfaces, does
    IP multicast multilayer switching is enabled
appear near the end of the output?

Also, does "show mls ip multicast" show your multicast traffic being hardware 
switched?

>> Are the 4900 boxes doing L3 or just L2?  I suspect they'd do much better at
>> L2 fan-out of multicast than at L3 fan-out.  You're probably hitting a pps 
>> or
>> packet replication limit before hitting the bps limit.
> I agree that this switch will probably perform better doing L2 exchange but 
> then there is another problem: C7604 carry QinQ vlans and C4924 terminates 
> them giving each tunnel's payload out of a deferent dot1q-tunnel access port. 
> If I don't do multicast routing I will need to carry the same multicast 
> traffic on every configured outer vlan. This will eat up all the bandwidth.

Bummer, dude.  I don't have anything to offer about that, other than to 
speculate that the QinQ tunnel stuff might be undermining the ability of 1 or 
both boxes to efficiently deal with multicast traffic.  You might have to 
get your Cisco support people in on this one.

________________________________________________________________________
Jay Ford, Network Engineering Group, Information Technology Services
University of Iowa, Iowa City, IA 52242
email: jay-ford at uiowa.edu, phone: 319-335-5555, fax: 319-335-2951