%Warning: MTU set 1420 is greater than default 1396, fragments will happen

[Paul Morgan]

I have configured a GRE tunnel and set the IP MTU to 1420 then I received this message.

%Warning: MTU set 1420 is greater than default 1396, fragments will happen

What and where is this default MTU? Ive never set one and it doesnt look like any values Ive encountered for the MTU before. (its an 1841)

Even a link to a document would be fantastic here as I cant find any info so far.

many thanks.

Paul

[Richard Michael]

Hello Paul,

The Router would assume that the default value of the MTU to be either 1500 or 1476, on some cases we have even seen that the tunnel comes up with MTU 1514.

you can check it with the command  sh ip  interface command output.

The Normal average overhead we consider is 80

that is,

1476 - 80= 1396

thats where the number came from . The above is just an warning message and just indicates that it is above the 1396 (default MTU) and would be fragmented.

Thanks,

Ricky Micky

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[Paul Morgan]

Serial0/0/0 is up, line protocol is up

  Internet address is x.x.x.x/29

  Broadcast address is 255.255.255.255

  Address determined by non-volatile memory

  MTU is 1420 bytes

Tunnel31 is up, line protocol is up

  Internet address is 172.21.70.1/24

  Broadcast address is 255.255.255.255

  Address determined by setup command

  MTU is 1420 bytes

Both interfaces show same MTU as its set by us. Is this default something I can change?

[Richard Michael · Accepted solution]

Hello Paul,

Yes you can tweak in the way you need by using ip mtu command. As i said earlier the above message is just a warning and i have explained you on how cisco have come up with 1396 size.

Thanks,

Ricky Micky