Troubleshooting MTU Issues

[jecker]

Hey Guys,

I'm wondering if anyone out there has any good troubleshooting commands to find out if MTU issues are going on.

I know I can use patth MTU, tcp adjust-mss, ip mtu reset the df bit, but i'm just wondering if there are any good show commands or debug output that would help me identify that MTU issues are going on.

Thanks.

[andrew.prince]

Wireshark

[Ivan Krimmel]

Hi Jecker,

it depends on what do you mean under 'MTU issue'. The MTU size by itself is not a problem, the fun begins with the protocols going over it. With ping sweep and DF bit set you can find out the MTU size on a given network segment. Also a 'show interface' will tell you the MTU of the interface. In a Wireshark capture you could have seen fragmented packets, which might be due to low(compared to packet) size etc.

Hope that clarifies,

Ivan.