MTU size

[Warren]

Quick question for you....over the weekend we did a hot cut from one location to another it went ok but later found out that if we send anything over 1475 the packets will not be sent, we double checked the routers and mine are set to 1500 the providers are set to 1700 but if we send anything more than 1475 it fails any ideas...also if I set my router to 1400 what is the possibilities of locking myself out of the router?

[lgijssel]

You probably have a (GRE)tunnel in the path that reduces the mtu. It should not be too hard to find out where it resides, actually I find it hard to imagine how you can be unaware of it's existence.

You could issue a traceroute and then use extended ping to locate the hop where the packets start to be dropped.

Regards,

Leo

[Collin Clark]

If you change the MTU on the router, it changes it for packets originated by the router. You should set it with

RTR(config-if)#ip tcp adjust-mss ?

<500-1460> Maximum segment size in bytes

When a client sends a packet that is larger than (example 1400) 1400 bytes, the router will repsond to the client and tell them to send a smaller packet.

[jdevoll]

I've also experienced problems with the IOS prefragmentation not correctly dynamically fragmenting packets utilizing the MSS as part of the decision. I highly recommend setting the TCP MSS to 40 bytes your MTU.

The problems I've had manifest themselves as application issues. Users complain of slowness, and pieces of some applications simply don't work. I had a web page which failed to load all of the images. Once I did a bit of research and set the MSS everything worked.