[c-nsp] 6500 & broadcast-storm control

[Phil Mayers]

All,

We're running an (otherwise excellent) non-Cisco stackable switch at the 
edge. We're having some stability problems, resulting in individual 
units crashing. When this happens, it seems to cause a broadcast storm.

Out architecture is:

coreA === coreB
  |          |
  \- switch -/

The problem is that the broadcast storm seems to flood the coreA->coreB 
link too, causing STP drop-outs and flapping.

Obviously one thing to look at is broadcast storm control on the 6500s. 
However, from what I can make it it's rather primitive; the rate of 
broadcast traffic is capped only in 1-second windows and doesn't take 
account of packet-size? Does anyone have any experience of it? Does it 
work well.

The second thing I'm a bit confused about is how the flood interrupts 
STP packets. My understanding was that the box generally prioritised 
control plane traffic for transmission over data-plane. Is that not the 
case for STP? In any event, the coreA<->coreB links is 2x10G whereas the 
core->switch links are only 1G, so it's hard to see how 1G could swamp 20G.

Is it more subtle, and the SP is being overwhelmed by the punt? We run 
CoPP but obviously that's layer3. I don't have any layer2 MLS 
rate-limiters enabled, and since they're per-box rather than per-port I 
doubt they'd help.

Advice appreciated.