From: Joe Freeman (joe.freeman@xxxxxxxxxxxxxx)
Date: Thu Aug 22 2002 - 11:28:43 GMT-3
Based on the STP process, as I've seen it in action with HSRP, if the same
mac address is seen on two ports at the same time, the switch will bounce
one of the ports...
I saw this most recently in a situation where I had two 7200's running HSRP
on the Ethernet interfaces, connected to a Cat3548XL. Portfast was NOT
enabled on either port, so STP was running on both ports.
Since both routers were in the same HSRP group, they were both using the
same HSRP virtual MAC address; when the switch saw the first instance of
this virtual MAC, it entered it into the CAM table, no problem. However,
when the other router joined the bridge group, the switch would see a second
instance of the same virtual MAC. At that point, the switch would
immediately expire the first entry from the CAM table, install the second
entry, and bounce the first port out of the bridge group to restart STP on
it. Once the first port rejoined the bridge group and made it through the
STP process, the switch would again see the virtual MAC, expire the CAM
entry for the second port, make the entry for the first port, and force the
second port out of the bridge group to restart STP. This process repeated
continuously.
How this presented itself was most noticeable in the many dropped packets
going through these routers (very poor throughput/performance).
Investigation revealed that the HSRP standby groups (using MHSRP on these
routers' Ethernet interfaces to effect some load balancing) were constantly
switching from Standby to Active and back again on approximately 75 second
intervals. Watching the switch showed the routers' ports joining and leaving
the bridge group in similar intervals.
The simplest fix for it was to enable portfast on the routers' ports, with
bpduguard, to suppress the STP process.
Cisco has a known issue with this on CCO, in the bug tool, I think. I'd post
the link, but the boss is demanding I get this DR pipe turned up RIGHT NOW!
Can't understand why ;)
HTH someone -
Joe Freeman, CCNP-VA/CCDP
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com]On Behalf Of
Brian
Sent: Thursday, August 22, 2002 6:34 AM
To: ccielab@groupstudy.com
Subject: Challenge Question
I know this group likes these challenge questions, so I have one for you
and hopefully it has not been put to this group recently.
You have two hosts each with identical MAC addresses on an ethernet LAN.
They also have identical IP addresses. Why or why not would this be a
problem for the client communicating (assuming each of the dupe machines
doesnt need to communicate with eachother only to other hosts on the LAN
and through the gateway)?
Ok, similar to above, same MAC addresses but different IP addresses. Why
or why not would this create communications on the LAN or through the
gateway?
good luck!
Brian
-----------------------------------------------
Brian Feeny, CCIE #8036 e: signal@shreve.net
Network Engineer p: 318.222.2638x109
ShreveNet Inc. f: 318.221.6612
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