ISDN dialer interface MTU change

Unanswered Question
Aug 28th, 2008

I have a problem whereby the dialer interface is configured to be of MTU 1600 but changes to 1524 when a call is made.

This is causing the OSPF adjacencies not to form in between. I could use the ip ospf mtu-ignore command, but I would like to know why the above problem is happening

The device is a Cisco2811




I have this problem too.
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Giuseppe Larosa Thu, 08/28/2008 - 00:46

Hello Mayamba,

post the output of

sh int bri0/1/0

sh ip int bri0/1/0

sh int dialer1

sh ip int dialer1

you set the MTU to 1600 to dialer interface and what do you do on the BRI port ?

Hope to help


Giuseppe Larosa Thu, 08/28/2008 - 02:01

Hello Mayamba,

the BRI has MTU 1500 B

now the dialer has MTU 1500 B too.

To be able to send a 1600 byte IP packet with DF set (like OSPF database packets) you would need to increase both MTU values

If this is not possible I would suggest to stay with the default values of MTU on both interfaces and OSPF will be fine too.

Hope to help



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