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Community Member

MTU problem

I have a problem with accessing a lot of web pages from my office. I strongly believe that problem is in MTU size.

Is it possible to resolve MTU problem on CPE router that has

NAT configured on it?

I don't have permission to reconfigure core equipment and I wouldn’t like to reconfigure every station in my LAN.

If you could recommend me some actions, please do.

1 ACCEPTED SOLUTION

Accepted Solutions
Cisco Employee

Re: MTU problem

First, to make sure it is mtu related, you should do a "ping -f -l 1472 " from a workstation for one of the websites you have issues with. This will tell you if you can pass 1500 bytes (1472 payload + 20 bytes IP header + 8 bytes ICMP header) from the workstation to the destination website.

If the mtu is the issue then you could use the "ip tcp adjust-mss" command on the CPE to force down the mss that will be negotiated between the workstation and the destination website.

for more information on this command, refer to the following URL:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124tcr/tbba_r/bb_a1ht.htm#wp1126741

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
2 REPLIES

Re: MTU problem

You can certainly got to the interface of the CPE router and change the MTU size as desired.

regards,

-amit singh

Cisco Employee

Re: MTU problem

First, to make sure it is mtu related, you should do a "ping -f -l 1472 " from a workstation for one of the websites you have issues with. This will tell you if you can pass 1500 bytes (1472 payload + 20 bytes IP header + 8 bytes ICMP header) from the workstation to the destination website.

If the mtu is the issue then you could use the "ip tcp adjust-mss" command on the CPE to force down the mss that will be negotiated between the workstation and the destination website.

for more information on this command, refer to the following URL:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124tcr/tbba_r/bb_a1ht.htm#wp1126741

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
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