After a fun evening last night, I have decided not to trust the autosensing
on ethernet interfaces anymore.
I was at a branch office where the users could not access the corporate
network. The router, a 1720 setup as a bridge with the same IP address for
the FastEthernet as the Serial subinterface, both configured for
bridge-group 1. It was connected to a 2620 at the corporate office via a
Fractional Frame Relay connection.
I changed the switch out with an old spare hub I had lying around, and
connected only one workstation from the local network. After starting the
router up, I could ping the local workstation, and I could ping devices on
the corporate network, so both my FastEthernet and Serial interfaces were
working fine. However, I could not ping anything on the corporate network
from my workstation, nor could I from a telnet connection to my corporate
router ping the workstation, so traffic was not being passed through between
the interfaces.
That looked like a typical routing problem, but the only problem was that I
was not routing, I was bridging, so ?????
I did a "show bridge 1 group" and saw that the FastEthernet was in a
blocking state by the spanning tree, so something was wrong here. I cleared
the arp table on the router and on all other routers and switches. I tried
to assign a different mac address to the FE interface. I tried a different
workstation. No matter what I did, it kept being in a blocking state.
I went in and did a "bridge-group 1 spanning-disabled" on the interface, and
it changed to forwarding state, but I could still not pass traffic through.
This is when I called TAC, but after I guided them through to a telnet
connection to my routers, they decided after three hours that something
weird was going on with the router, and they did an RMA for a replacement
unit.
However, I decided to continue my troubleshooting, because I hate to give
up. I reconfigured everything, I tried to create a bridge-group 2 instead, I
forced it into IP routing, and back off it again, but no matter what, it
kept going into blocking mode (I had removed the spanning-disabled command
again at that time).
That's when it hit me to try and force the speed on the interface. It was in
AUTO, and my switch had been auto 10/100, but my hub was only 10. I changed
it from auto to 10 and power cycled the router. PLING!!! Now it started up
and after the listening and learning, it went in forwarding state, and I
could now ping through my router, and I could connect my workstation to the
corporate network.
What makes this strange is that I can apparently use my FastEthernet
interface from the router even though the speed is wrong, but the STP see's
this and blocks the interface for switched traffic. WEIRD!!!!!
Read the entire case study here:
http://www.RouterChief.com/CaseStudies/1.htm
Ole
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Ole Drews Jensen
Systems Network Manager
CCNP, MCSE, MCP+I
RWR Enterprises, Inc.
[EMAIL PROTECTED]
http://www.RouterChief.com
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NEED A JOB ???
http://www.oledrews.com/job
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