[c-nsp] CEF and multicast on a 3750

[Chuck Church]

Spanning tree maybe?  TCNs will cause IGMP memberships to clear, leading to
temporarily flooding if you don't have portfast enabled.

Chuck

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of John Neiberger
Sent: Tuesday, June 26, 2012 8:56 AM
To: Phil Mayers
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] CEF and multicast on a 3750

On Jun 26, 2012 1:38 AM, "Phil Mayers" <p.mayers at imperial.ac.uk> wrote:
>
> On 06/26/2012 04:20 AM, John Neiberger wrote:
>>
>> I'm trying to help troubleshoot a really odd multicast issue on a 
>> 3750 that involves intermittent dropped packets. The problem seems to 
>> be
>
>
> Odd. How intermittent? What's the traffic rate?
>

We seem to start losing packets at around 250 Mbps of total multicast
traffic, as I recall. I'll double check when I get in to work.

>
>> questioning my assumptions. If the layer three interface isn't being 
>> used for routing, would disabling CEF on it have any effect on 
>> traffic within the Vlan?
>
>
> In theory, no. On that 3750, the multicast should all be layer 2,
forwarded based on the FDB and entries created with IGMP snooping. Does the
output of "sh mac address-table multicast" look right, with the correct
layer2-mapped addresses on the correct ports?
>
> Having said that - "disabling CEF" is seldom a good idea, particularly 
> on
the Catalyst boxes which forward there CEF-a-like in hardware. Maybe
disabling CEF is interfering with IGMP, or causing the IGMP join refreshes
to be programmed into the layer2 CAM in a "slow" way. Is there any reason
not to just remove that command?

That's a very interesting thought. I have no idea why CEF is disabled on
that vlan. I'll enable it and see if things improve.

We've been troubleshooting this for a long time with Cisco and lots of
engineers from our company and the cause is still eluding us. It would be
interesting if this turned out to be the cause. We've already done a major
IOS upgrade and replaced the equipment.

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