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Changing MTU on Cisco 1841 Router


hi all,

a client was having some email issue and was requested to change the 1841's LAN and WAN interface MTU to 1400 bytes. i've used 'mtu' command but was rejected and got an error like to one attached. so i used 'ip mtu' instead to make the change.

could someone advise what's the difference between the 2 commands and if this would achieve the said change. i've checked using the show interface it's still showing MTU of 1500 bytes. thanks in advance!

6 Replies 6

ajay chauhan
Rising star
Rising star

MTU is the Maximum Transmission Unit of general frames entering/exiting

the interface.

IP MTU is the MTU for IP packets entering/exiting the interface.

The difference is there because of the different features you can add

(ie. run IPX etc.)

If you have MTU-problems, try setting "ip tcp adjust-mss 536" on your

inside interface... you could always adjust to a bigger value.. use a

ping with different sizes to see which size gets through..

If you use the microsoft-ping, remember that the "-l size" parameter

(-s for unix-version) is the payload of the ICMP-packets, so you should

subtract 28 bytes (20 bytes IP header + 8 bytes ICMP header) from the

total.. If you ping with a size of 1472, this would equal a IP-packet

with MTU of 1500.

The MSS is the Maximum Segment Size, ie the data payload of

TCP-packets.. this should be (normally in a ethernet-environment) 1500

- 20 bytes IP header - 20 bytes TCP header = 1460 bytes. The minimum IP

MTU which must be supported on the 'net is 576 bytes, which gives an

MSS of 536.

Hope this help.


thanks for your inputs! so which one is would meet the this requirement? ip mtu or ip tcp adjust-mss command?

Router(config-if)#ip tcp ad

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

  <500-1460>  Maximum segment size in bytes

Router(config-if)#ip tcp adjust-mss 1400


I agree to Ajay. Here is the command reference link, thought might be useful.



Regards, Sunil Khanna

tcp adjust-mss 1360  << For LAN

set ip mtu 1400  <<< For WAN


tcp adjust-mss --> is at layet 4

set ip mtu --> is setting the fragment size.

Regards, Sunil Khanna

Joseph W. Doherty
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Normally, you shouldn't need to modify both LAN and WAN interfaces; often just the WAN (or tunnel) interface needs the configuration change.

The difference between the two commands, MTU should correspond to the interface's physical capabilities, IP MTU impacts just IP packets.  Normally both are equal, but IP MTU might be set when something on an interface is going to reduce the size available to IP such as tunneling and/or PPoE.  In such cases, you would configure the IP MTU to the reduced space.  For example, if using a GRE tunnel, you'll lose 24 bytes to GRE, so if the MTU is 1500 you could set the IP MTU to 1476 (on the tunnel interface).


tcp adjust-mss is normally set to 40 bytes less than IP MTU.  It would be set where you set IP MTU.  It only affects TCP connection setup.


Before reducing IP MTU to 1400, I would first suggest trying to determine why there's an email issue. If it is related to MTU, and if you're not using tunnels, it's often worthwhile to identify why.  Is this PPoE?  Might there be mis-configured MPLS by the Service Provider?  (I've seen a Service Provider "forget" to properly adjust for MPLS tag.)

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