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Suspected MTU Issue


I believe I have problems with MTU sizes and would like to know if anyone else has come across similar.

I have users on 2 sites trying to access a web server which is hosted on site 1, on a dmz. All users go through a proxy server.

When I connect to the web site on site 1 the HTTP TCP packet sizes go up to 1314 bytes. However, on site 2, which is connected via a 100mb LES, the largest HTTP TCP packets that I see is 590 bytes.

I believe that this is partially responsible for the slowdown.

The 100mb LES termination at site 1 is connected to a copper SFP in a 3750 switch. The termination at site 2 is connected to f0/1 in a 3550 switch. The link is running a trunk, one vlan on which is carrying routing traffic. There is QoS over the link, but only to give telephony priority.

Both switches have the ethernet MTU's set to 1500 bytes.

In the trace site 2 traffic to site 1 is not all at a max of only 590, this is just HTTP TCP traffic coming from a Windows proxy server.

When on site 1, this is not a problem.

Has anyone seen this before? Comments/suggestions welcome.

Best Regards,


New Member

Re: Suspected MTU Issue

Packet size is determined by the sending device. I am assuming in this case it is a workstation. Routers or firewalls do not determine the MTU size used in a conversation. Packet size is determined above layer 3.

This is more than likely a PC issue. It could be a outdated NIC driver, bad NIC, or I have seen where making sure it has the latest OS patches has helped.

I have worked on a lot of these issues due to poor performance on a web application. Seeing small packet size is the most common cause of poor performance.

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