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How do you change the MTU size in a Cisco 871?

This 871 is at a remote site and is an ezvpn IPsec client (network extension mode) back to a 3030 headend.

We're having problems with a PC trying to connect through the IPsec tunnel and we think it may be an MTU size problem.

Int F4 is the outside interface.

We are using a virtual-template associated with the crypto ipsec client ezvpn statement.

When I go into any of the 871 interfaces and type 'mtu 1370' it errors out with 'The F4 (or whatever interface) does not allow manual MTU size configuration.

If I type 'ip mtu 1370' on F4 (or vlan1 or virtual-template 1) this is accepted, but when I do a 'show int f 4', it still shows MTU of 1514 - even after a reload.

What is the correct way to set the MTU size in the 871 router - and is it best set on the F4 interface, the vlan, or the virtual-template interface?


Re: How do you change the MTU size in a Cisco 871?


As per the supporting doc Cisco 871 has one want ethernet interface and 2 switch ports.

I feel you are trying to change the mtu under the switch port which may not be possible.

You can refer the below link for more info..


New Member

Re: How do you change the MTU size in a Cisco 871?

I figured out how to set the MTU. Under the vlan interface that the 4 'private' fastethernet ports are part of, I added this line:

'ip tcp adjust-mss 1330'

...this gives us an MTU of 1370 (adding the 40 byte header to the MSS) and now all is well.

Thanks for responding!

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