MTU mismatch - ip ospf mtu-ignore on one side...

[javiercastro]

Hi,

I am not so sure about the behaviour of ospf mtu-ignore command.  I need to know if the following scenario would it work.

I have a link with one interface MTU set to 1500 and the second interface with MTU 1546.  I want to configure "ip ospf mtu-ignore" only on the MTU=1546 side. Would that work?

I am kind of confused if that only works configured on the low MTU side.

[Reza Sharifi]

Hi,

Yes, it does work

You need to set the "ip ospf mtu-ignore" on one side and that is the side that is different then default 1500.

in below example as soon as I changed the MTU to 1400 the OSPF goes to EXSTART/DR  and as soon as I add the command ip ospf mtu-ignore the ospf is back to FULL/DR       

C4#sh ip os ne

Neighbor ID     Pri   State           Dead Time   Address         Interface
130.130.0.1       1   EXSTART/DR      00:00:36    172.17.1.2      Ethernet0/0

C4#sh ip os ne

Neighbor ID     Pri   State           Dead Time   Address         Interface
130.130.0.1       1   FULL/DR         00:00:37    172.17.1.2      Ethernet0/0
C4#

interface Ethernet0/0
ip address 172.17.1.1 255.255.255.0
ip mtu 1400
ip ospf mtu-ignore
half-duplex
end

HTH

Reza

[javiercastro]

Actually, it is not the same scenario I am mentioning.

You test it configuring the command in the lower MTU interface, which is something I have seen every where that works.

What if you change that MTU to 1546?  Can you try it?  Does it work?

Many thanks for your support.

[Reza Sharifi]

I tested this with MTU 1546.  It it interesting since I raised it to higher then 1500, I did not even have to apply the ip ospf mtu ignore command.

The OSPF adjeceny stays up.   Have a look:

interface GigabitEthernet0/0
description $ETH-LAN$$ETH-SW-LAUNCH$$INTF-INFO-GE 0/0$
mtu 1546
ip address 172.17.1.2 255.255.255.0
ip pim sparse-mode
duplex auto
speed auto
end

T-1#sh ip os nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
120.120.0.1       1   FULL/BDR        00:00:36    172.17.1.1      GigabitEthernet0/0
T-1#

T-1#sh ip int gi0/0
GigabitEthernet0/0 is up, line protocol is up
  Internet address is 172.17.1.2/24
  Broadcast address is 255.255.255.255
  Address determined by non-volatile memory
  MTU is 1546 bytes

HTH

Reza

[pille1234]

I'd have a very bad feeling with this setup.

While the adjacency might ostensibly work, what is going to happen if Router A (mtu1546) sends ospf database updates with a packet size of 1546 to Router B (mtu 1500)?

Regards

Pille

[Bahaa J]

I have a diagram for segment of network where two router connected with switch, for the reason of different size of MTU, we are using ip ospf mtu-ignore

regards