[c-nsp] 2950-48-SI arp problems ?

[Gert Doering]

Hi,

On Tue, Dec 21, 2004 at 09:09:09AM -0800, matthew zeier wrote:
> Hosts on the 2950 are able to connect to the Internet, however, if idle for 
> enough time (something between minutes and less than an hour), the host's 
> ARP table becomes empty and the gateway is unable to ping/reach the host (so 
> inbound connections are lost).
> 
> Running 'tcpdump' on one of the Linux hosts shows that there aren't any 
> packets from the gateway reaching the host.  If the host pings the gateway, 
> everything works - inbound connectivity is restored.

I've seen similar effects on a trunk "3750G<->5509" recently.  Broadcast
packets (especially ARP) have not been forwarded from the 3750G to the 
the 5509.

The effect was the same: if a host connected to the 3750G wanted to ping 
a host connected to the 5509, it didn't work.  Ping into the opposite
direction, and voila, both ways work.

Verifying with tcpdump on multiple machines (connected to various
switches in this cloud) showed that it was only this single link that
didn't forward broadcasts anymore - all other switches/links/machines 
in the VLAN affected saw the ARP requests, just machines connected to 
the 5509 didn't.

All *other* VLANs on that trunk did not have the effect.

Trying to diagnose it, I've removed and re-added the vlan in question to
the trunk, and that made the problem disappear.  So I can't say whether
it was the 5509 or the 3750.  I have a nagging suspicion that it's an
interaction issue between PVSTP (5509) and Rapid PVSTP (3750G), but
have no evidence.  STP seems to be well-behaving, and I can't imagine
why it should stop forwarding broadcasts, but then, I strongly distrust
STP..

To be on the safe side, we've also upgraded the 3750 from 12.1(something)EA 
to 12.2(20)SE3 - and now we're waiting whether it will happen again.

gert
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de