[AG-TECH] FYI: New Cisco Catalyst Feature

David Mitchell mitchell at ucar.edu
Mon Jun 2 15:26:53 CDT 2003

It was recommended that I pass along a new feature of CatOS which has
caused us a fair amount of headache. Newer versions have implemented a
feature which rate-limits PIM, IGMP, MOSPF and DVMRP packets headed to
the supervisor. By default, it allows 100 packets per 30 second
interval. If the traffic rate goes over this amount, the switch will
begin forgetting about router ports, as shown in the output of "show
multicast routers". The end result is IGMP state timing out on the
router with an expected loss of multicast connectivity. If you are using
dual MSFC's as routers, things will begin breaking at about 50 subnets.

This wouldn't be a bad thing except that the switch produces absolutly 
_no_ trap or syslog message indicating that it has started dropping the
traffic. The feature also doesn't seem to be very well documented, to
the point of not being in the release notes and not being known by the
TAC engineer on the case nor his escalation team. So, I thought I would
pass word of this on in case other people are/will be bitten by it.

The official bug ID of the feature is CSCdx09717. It can be summarised
by the fact that there are two new commands "show igmp ratelimit-info"
and "set igmp ratelimit" which do pretty much what you might expect. As
I mentioned, the main symptom of this is "show multicast routers"
failing to show complete lists of which routers are present on which

-David Mitchell

| David Mitchell (mitchell at ucar.edu)       Network Engineer IV  |
| Tel: (303) 497-1845                      National Center for  |
| FAX: (303) 497-1818                      Atmospheric Research |

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