[isalist] Re: OT: Diagnosing a network problem
- From: "Mayo, Bill" <bemayo@xxxxxxxxxxxxxxxx>
- To: <isalist@xxxxxxxxxxxxx>
- Date: Tue, 1 Apr 2008 15:24:52 -0400
The only downside to spanning-tree is that while it is in discovery
mode, the ports block. This amounts to about 30 seconds where no
traffic will go through a port while it discovers loops and disables
ports that would cause them. Again on Cisco switches, you can set
portfast on ports that you KNOW will only have client computers to
eliminate this blocking period. Those are the ports where you would
also want to BDPUguard.
________________________________
From: isalist-bounce@xxxxxxxxxxxxx [mailto:isalist-bounce@xxxxxxxxxxxxx]
On Behalf Of Rob Moore
Sent: Tuesday, April 01, 2008 3:21 PM
To: isalist@xxxxxxxxxxxxx
Subject: [isalist] Re: OT: Diagnosing a network problem
This particular switch was just a dime store desktop switch. But I guess
you're talking about the switch it was plugged into, right? It's an HP.
I will look at it to see if spanning tree is on or off.
I'm not much of a switch guy. What are the implications of turning
Spanning Tree on (assuming it is off)?
Thanks,
Rob
From: isalist-bounce@xxxxxxxxxxxxx [mailto:isalist-bounce@xxxxxxxxxxxxx]
On Behalf Of Mayo, Bill
Sent: Tuesday, April 01, 2008 2:01 PM
To: isalist@xxxxxxxxxxxxx
Subject: [isalist] Re: OT: Diagnosing a network problem
It sounds like he created a loop, but I am surprised that spanning-tree
didn't kick in and solve the problem (after a minute or so, at least).
I would make sure than spanning-tree has not been disabled on the
switch. On Cisco switches at least, you can also turn on BDPUguard to
prevent any switches from being connected to a port that you don't
intend. When that is enabled on a port, it will listen for BDPUs, which
are indicative of a switch (with spanning-tree) being attached. It will
then shut down the switch port, preventing potential loops as well as
sneaky end-users.
Bill Mayo
________________________________
From: isalist-bounce@xxxxxxxxxxxxx [mailto:isalist-bounce@xxxxxxxxxxxxx]
On Behalf Of Rob Moore
Sent: Tuesday, April 01, 2008 1:48 PM
To: isalist@xxxxxxxxxxxxx
Subject: [isalist] OT: Diagnosing a network problem
Hello all-
This morning I had a weird experience. Everything was going along
normally, when suddenly network traffic came to crawl or stopped all
together. I did some frantic work in the server room and localized the
problem in some ways, but still couldn't figure out what was going on.
Then I thought, "Well, the only thing I know that has changed in the
last few minutes is that another IT guy added a switch down the hall."
(We are in the middle of a building renovation and switches are
occasionally being added or subtracted in various places to accommodate
swing space while the construction is going on.) I went and checked the
switch. Turns out amongst the cables that were coiled up ready for the
swing space desks there was also an identical-looking patch cable coiled
up with them. (Because this is temporary swing space, there is no patch
panel.) When the guy plugged all the workstation cables into the
temporary switch, he also plugged in both ends of the patch cable to the
same switch. (As I said, the patch cable was coiled up with the other
cables so it was not obvious that he had done something wrong.)
Judging by the activity lights on the switch, this loop caused a lot of
traffic to go out on the network.
Anyway, that got me to thinking, what if someone did this with a little
desktop switch and we didn't know about it? Is there a way to figure out
what and where the problem is?
Sorry for the off-topic post, but it is vaguely related (network stuff).
And this group is never shy about giving out opinions!
Meanwhile, I've been telling folks the outage was just my idea of a good
April Fool's Day joke. J
Thanks,
Rob
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Rob Moore
Network Manager
215-241-7870
Help Desk: 800-500-AFSC
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