From: Wes Stevens (wrsteve33-gsccie@yahoo.com)
Date: Sun Aug 07 2005 - 12:01:28 GMT-3
Finger pointing is a way of life where I work.
Microsoft has so many error messages that can kinda
point to the network that our server group always
points to the network first. They don't like it when I
get the call because I have more Microsoft experience
then they do.
I have seen mtu break some apps run on windows. They
sent packets larger then the allowed through a tunnel
and the df bit was set. No mtu discovery either. So
was it a network problem?? Of course, according to the
server folks. Of course they don't have a clue what
the df bit is or mtu discovery.
Wes
--- Kirk Graham <kgraham@instructors.net> wrote:
> 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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