[c-nsp] CEF and multicast on a 3750

[Phil Mayers]

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?

> 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?