From: Szarmach, Douglas (Douglas.Szarmach@cmegroup.com)
Date: Tue Jul 31 2007 - 13:53:37 ART
Be very careful especially if using other vendor hardware, as some
strange things can result. I ran into an issue before where the VLANs
became bridged in this manner when a user connected a hub back to the
wall on two ports, each in a different wiring closet and each on a
different VLAN. A CST was in use and the HP switch used the same MAC
address for each SVI. All the 500+ hosts on this segment would resolve
the MAC of their default gateway and send traffic toward it, but the
switch CAM table would randomly choose the alternating gigabit fiber or
10mb copper hub (with bonus duplex mismatch on uplink) options. Every
60 seconds or so they would cause a ton of collisions and start dropping
data as frames were switched toward the hub to get to the SVI gateway.
This was a pretty unique issue to troubleshoot and I finally figured it
out doing sniffer traces and noticed that data was moving in the wrong
direction though the switch uplinks about once a minute for a 1-3 second
period. I plugged my laptop into a switch in the closet in question and
then pulled the fiber uplink and *was still able to get to everything*
but very slow and dropping 2 of 5 pings. From there it was just a
little investigation of the MAC tables in the switches to find out where
and how the data was making it back to the core.
Douglas Szarmach
Senior Network Engineer
+1 312 648 3797
CME Group
A CME/Chicago Board of Trade Company
20 South Wacker Drive
Chicago, Illinois 60606
cmegroup.com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Ina&Laurean
Sent: Tuesday, July 31, 2007 11:32 AM
To: Toh Soon, Lim
Cc: ccielab@groupstudy.com
Subject: Re: Bridging VLANs by looping cable
I've done this type of configuration because I had to put an in-path
device
and I didn't have an extra switch. So I decided to go for two VLANs and
connect the in-path device between the two VLANs.
The in-path device looks like a layer 2 device and everything works
fine
for me, there is no problem with ARP but I have to check what the switch
is
doing with the CAM table and how it associates the MAC addresses to the
ports.
One more thing is STP, if you are running PVST there are no issues but
if
you running MST I believe it won't work because one port will be blocked
by
spanning tree.
Laurean
On 7/31/07, Toh Soon, Lim <tohsoon28@gmail.com> wrote:
>
> Hi All,
>
> This may sound simple and mundane but for curiosity sake, I need to
seek
> clarification.
>
> Has anyone tried "bridging" two VLANs by looping a cable to the same
> switch?
> E.g. in the following diagram:
>
> Gi0/1(VLAN359)----
> |
> |
> Gi0/2(VLAN360)----
>
> The same cable connects to interfaces Gi0/1 (VLAN359) and Gi0/2
(VLAN360).
> Can a host on VLAN359 now communicate with a host on VLAN360?
>
> What's the implication doing this, e.g. from STP point of view? Is it
> recommended at all?
>
> Any advice is appreciated.
>
>
> Thank you.
>
> B.Rgds,
> Lim TS
>
>
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