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Tunnel Interface MTU

Hi all,

I have a question that has boggled my mind and hopefully someone has come across this issue.

I am running a Cisco 3560 with Version 12.2(50)SE1, and using GRE tunnels to from field sites to the core....I have one of the sites showing the interface MTU to be 9108 bytes, irregardless that I try to change the MTU to 1500 it still shows on the interface as as 9108 bytes:

1#sh int tu1

Tunnel1 is up, line protocol is up

  Hardware is Tunnel

  Description:  Tunnel 1

  Internet address is

  MTU 9108 bytes, BW 1500 Kbit, DLY 500000 usec,

     reliability 255/255, txload 1/255, rxload 1/255

  Encapsulation TUNNEL, loopback not set

  Keepalive set (5 sec), retries 3

  Tunnel source, destination

  Tunnel protocol/transport GRE/IP

    Key disabled, sequencing disabled

    Checksumming of packets disabled

  Tunnel TTL 255, Fast tunneling enabled

  Tunnel transport MTU 1476 bytes

On my configuration for the interface:

interface Tunnel1

description  Tunnel 1

bandwidth 1500

ip address

ip ospf authentication message-digest

ip ospf message-digest-key

ip ospf mtu-ignore

delay 50000

keepalive 5

tunnel source

tunnel destination


Configuration on the system:

system mtu routing 1500

I have also tried to just have the system mtu 1500, but this does not change on the Tunnel interface.

Any help will be appreciated....

Hall of Fame Super Bronze

Tunnel Interface MTU

What's the MTU set for interface holding this IP address (

Re: Tunnel Interface MTU

That is a loopback interface...


it is a Fastethernet interface and is showing the MTU at 1500.

MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,

Tunnel Interface MTU

if I do

#sh system mtu

System MTU size is 1500 bytes

System Jumbo MTU size is 1500 bytes

Routing MTU size is 1500 bytes

This makes me want to pull out my

Tunnel Interface MTU

Any ideas?

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