From: Kirk Graham (kgraham@instructors.net)
Date: Sun Aug 07 2005 - 10:12:25 GMT-3
Ian has a good suggestion there for troubleshooting the problem. I would
also suspect the new Exchange install over the network.
But FYI, for TCP packets Microsoft does set the DF bit. It should then
receive a Fragmentation Needed message with the proper packet size from the
router if the packets are too large. As long as the router's MTU size is
properly configured, that shouldn't be a problem.
I have seen a real-world network with an improperly configured MTU that did
cause problems with larger email packets (such as emails with attachments).
The server was sending 4K packets and the router had a physical 1500 byte
MTU, but was misconfigured with a 4K MTU. So the router basically said, "I
can't fit 4K packets, send me a 4K packet." We saw this with a sniffer, and
that told us which router was the problem. Fixing the MTU size corrected
the problem.
All devices on the same physical network should always agree on MTU.
As Ian said, you can do extended pings with DF bit set, and different MTU
sizes to find if there is a problem.
Good luck,
--kg
At 07:40 AM 8/7/2005, Ian Stong wrote:
>Have you tested a client locally connected to the same network as the
>servers. If that worked then I would investigate the microwave link more.
>If it doesn't work then it's a Microsuck problem.
>
>If it works locally but not over the link the for your ping tests you should
>run extended pings over the link and use various packet sizes as well as
>different data patterns, with the DF bit not set and then set, etc.
>
>
>
>Ian
>www.ccie4u.com
>Rack Rentals starting at only $12 and discounted lab scenarios
>
>
>
>-----Original Message-----
>From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
>chon_mon@nym.hush.com
>Sent: Sunday, August 07, 2005 5:01 AM
>To: ccielab@groupstudy.com
>Subject: OT: Real world finger pointing at network
>
>I have a simple issue with email. My exchange 5.5 clients and
>servers communicate fine over a microwave link (building to
>building) without issue. Everyone is happy with that. Recently,
>the exchange server was upgraded to an AD 2003 clustered solution,
>with updated clients as well. The upgrade did not go as planned,
>and the finger is being pointed at the network as the problem.
>
>Typically, I choose to go through the 7 layer model for
>troubleshooting, however I am stuck with people who believe that
>the MTU is the issue across the microwave link. So now, I am to
>SPAN ports on each side to see the traffic coming and going with
>large and small emails sent by the new outlook client.
>
>I don't see the point to this, because what difference does it make
>if its the old clients sending large and small emails, versus the
>new test client sending large and small emails if they all have to
>travel the same link between two routers, which don't distinguish
>between different versions of Exchange (assuming, of course, no
>access-lists or restrictions on traffic, etc.)? And if MTU was an
>issue between buidlings, wouldn't that lead to other problems in
>general? Don't Cisco routers fragment packets by default if they
>are too big, and queue them?
>
>There was a ping test, and it was successful in reachability to the
>new clustered AD 2003 exchange IP address from across the microwave
>link.
>
>Now I can understand if packets are being sent across a link with
>the DF bit set, and are dropped because they are larger than the
>MTU size. However, I don't think Exchange sends packets with the
>DF bit set.
>
>Any input on this would be of help.
>
>
>Server<----->Cisco router<----microwave---->Cisco router<-----
> >Client
>
>Thanks in advance.
>
>-Sean
>
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