Changing MTU Size

Unanswered Question
Mar 29th, 2009

Dear All,

I want to change the MTU Size on the interface to overcome DOS attacks. In my 7609 on gigabit ethernet interface i get only 9216 MTU size. How do i enable MTU size of 1556 for an example. Please assist me on this to overcome the issue ASAP.

Many Thanks,


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Giuseppe Larosa Sun, 03/29/2009 - 05:02

Hello Saran,

you can override mtu settings on a per interface basis

L2 interface


mtu 1576

L3 interface

you can use

ip mtu 1556

or again the mtu command that should take in account Ethernet overhead

notice that mtu command applies to all upper layer protocols not only ipv4 but also ipv6 or clns packets for isis.

if using a routing protocol over it like OSPF you need to modify on both sides of the link.

Hope to help


saran_chn Sun, 03/29/2009 - 05:34

In interface configuration mode i am getting only 9216 MTU size its not allowing to set defined MTU size. Please suggest.

Many Thanks,


Giuseppe Larosa Sun, 03/29/2009 - 05:47

Hello Saran,

try on the SVI interfaces for the vlans permitted on the link if it is a L2 trunk.

By the way, the port is part of what linecard ?

What IOS version are you using ?

if you like you can post

sh module

sh ver

and sh run interface type x/y

Hope to help



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