This sounds like a particular problem I ran into several years ago with 3com
switches. In a nutshell, some 3com "workgroup" switches (don't remember the
model), only forward packets for unknown MAC addresses out a single
"backbone" port and not out every interface as is common with other
switches.
In your case, if you have the router on a non-backbone port and the router
is sending very little traffic, it could be that the MAC address is timing
out of the switches MAC table. If this happens on the particular switches
I'm thinking of, when packets would inter the switch destined for the router
MAC, they would get forwarded up the backbone port only and not to the
router port.
I saw this scenario with a Unix box. Unix boxen are generally pretty quiet
from a network perspective, so the box wasn't sending traffic very often and
the MAC entry would time out of the switch. This seems unlikely with a
router, but depending on _when_ your having these problems it could still be
possible if your not running a routing protocol on the router.
If the router is generating constant traffic such as from a routing
protocol, it should not time out of the switch so I would suspect something
else as the cause.
When you experience the problem, you need to look at the switches MAC tables
to see where it thinks the router and PC MAC addresses are located. You
could also do some sniffer traces and some router debugs. "debug arp" and
"debug ip packet" could be useful, but you need to be careful if you have a
busy router as "debug ip packet" could spike the CPU quite a bit.
Another possibility would be some sort of strange bridge loop. If your
switches are connected in a redundant fashion, spanning tree should be
blocking one path, if spanning tree is failing you could see unpredictable
behavior, up to an including a complete network meltdown. Check your
cabling and ensure that spanning tree is operating correctly.
Somewhere in there you should find somethign that indicates a problem. If
not, I'd suspect bad code on the router or switch as a last resort.
HTH,
Kent
-Original Message-
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
Jacek Malinowski
Sent: Friday, June 29, 2001 7:32 AM
To: [EMAIL PROTECTED]
Subject: ARP problem [7:10400]
I have very big problem in Ethernet on my 4500 Cisco router.
The problem is when some station are pinging my ethernet ip on the router.
In some case the station can't ping my ethernet ip.
After command clear arp cache on my Cisco router the station can ping my
ethernet ip.
I gave on the ethernet interface command arp timout 100 but it doesn't help.
My LAN is very big and have 5 3 Com switches.
I can't find the solution.
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=10412&t=10400
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