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RE: Network Lag on Cisco? [7:70648] posted 06/14/2003
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It's time to do some troubleshooting. Although the 3600 and 2600 routers
aren't considered high-end routers, they nonetheless shouldn't cause a
noticeable delay if they are correctly configured with a simple config. If
you're doing a bunch of access lists and NAT and other CPU-intensive stuff
on them, then maybe they would cause a delay, but, in general, they
shouldn't be noticeably slower than a Linux server acting as a router,
depending of course, on the CPU and RAM on the Linux server.

Check for errors on your routers and switches. Start at the lowest layer and
move up. I agree with Mark that you may very well have a duplex mismatch
somewhere in the network. Look for CRC errors and collisions.

Also check that you are using 100 Mbps, not 10 Mbps. If you are using 10
Mbps and utilization is high, move to 100.

Check for routing protocol issues. When problems occur, is BGP confused
about routes?

In your case (as opposed to your friend's case), it doesn't sound like
you've ruled out the server as being the problem. Put Ethereal on your
computer and watch what happens. Sometimes you can easily see proof that the
network isn't slow, but the server is. This proof comes in the form of quick
TCP ACKs, with no data in them. The network must be fast if you can quickly
receive an ACK, but the ACK is empty because the server application is slow.
TCP is fast, but the application is slow and doesn't have any data ready to
piggy-back onto the ACK.

I can recommend a good book on troubleshooting! :-) Try Troubleshooting
Campus Networks.

Good luck. Keep us posted! Thanks.


Priscilla Oppenheimer

Anil Gupte wrote:
> Our network is running a 3640 as a core router and several
> other smaller
> routers plus an HP Procurve for the Servers and LAN.  We run
> BGP across two
> upstream networks.  Now, for a long time we have had a slight
> lag on our
> network.  For example, whenever I am logged into our Linux
> servers, I will
> be typing something and I will lose the cursor, then suddenly a
> bunch of
> letters will appear at the cursor.  There are other examples. 
> We have
> hunted up and down and not found a problem/solution.
> Now comes the interesting part.  A friend of mine who has been
> running Linux
> for years used a Linux machine as a router for the last 3.5
> years.  At my
> urging he decided to try a Cisco because his server was getting
> old and
> needed an overhaul.  Yesterday he did and now has the same
> problem.  His
> config is very simple - he has a 2640 router running IOS 12.1 -
> one T-1 and
> one Ethernet port to which he has connected his Dialup
> equipment (he is a
> Dialup ISP).
> What gives?  He is now bad mouthing Cisco even more than he did
> before!
> Any ideas appreciated.
> Thanx,
> Anil Gupte

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