OSPF MTU ignore feature

In this article we are going to check how MTU can affect OSPF adjacency establishment

first let us write what are the conditions for two neighbor routers to become OSPF adjacent routers

1. subnet mask ( they have to be on same network)
2.same area
3.authentication
4.MTU
5.hello time
6.stub flags (one router in area x which is stub and its neighbor in same area must be also configured to be stub)

ok, so we have the MTU that has to match on both routers, I'm going to configure two routers with different MTU's
simple lab here:

I'm going to configure one router to be using the default MTU (1500), and change the other router to 1400 MTU

R2 Configs:
!
router osp 1
router-id 200.200.200.200
interface Ethernet0/0
 ip address 192.168.1.2 255.255.255.0
 ip mtu 1400
 ip ospf 1 area 0
end

R1 Configs:
!
router ospf 1
router-id 100.100.100.100
interface Ethernet0/0
ip address 192.168.1.1 255.255.255.0
ip mtu 1500
ip ospf 1 area 0
end

========================================


Ok, first thing you will notice that you will not get log for OSPF status changing to FULL
and if you use the command
#show ip ospf neighbor

you will see the process is stuck at Exchange
R2#
R2#show ip os nei

Neighbor ID     Pri   State           Dead Time   Address         Interface
100.100.100.100   1   EXSTART/BDR     00:00:36    192.168.1.1     Ethernet0/0
R2#

Ok, so now let's see some debug on R1

#debug ip ospf adj
R1>
*Nov 14 06:04:37.604: OSPF-1 HELLO Et0/0: Send hello to 224.0.0.5 area 0 from 192.168.1.1
*Nov 14 06:04:38.184: OSPF-1 ADJ   Et0/0: Rcv DBD from 200.200.200.200 seq 0x2A2 opt 0x52 flag 0x7 len 32  mtu 1400 state EXCHANGE
*Nov 14 06:04:38.184: OSPF-1 ADJ   Et0/0: Nbr 200.200.200.200 has smaller interface MTU
*Nov 14 06:04:38.184: OSPF-1 ADJ   Et0/0: Send DBD to 200.200.200.200 seq 0x2A2 opt 0x52 flag 0x2 len 52
R1>

the output is very clear, we have smaller MTU coming from R2
ok, so what is our options to solve this?
1. make both values equal (set both to 1500 or 1400)
2. ignore this value, ok how?

Note: this must be done on both routers and under interface level

#ip ospf mtu-ignore

now let us see the logs change on R1
R1(config-if)#

*Nov 14 06:07:05.456: OSPF-1 ADJ   Et0/0: Rcv DBD from 200.200.200.200 seq 0x1E02 opt 0x52 flag 0x1 len 52  mtu 1400 state EXCHANGE
*Nov 14 06:07:05.456: OSPF-1 ADJ   Et0/0: Exchange Done with 200.200.200.200
*Nov 14 06:07:05.456: OSPF-1 ADJ   Et0/0: Send LS REQ to 200.200.200.200 length 36 LSA count 1
*Nov 14 06:07:05.456: OSPF-1 ADJ   Et0/0: Send DBD to 200.200.200.200 seq 0x1E02 opt 0x52 flag 0x0 len 32
*Nov 14 06:07:05.457: OSPF-1 ADJ   Et0/0: Rcv LS UPD from 200.200.200.200 length 76 LSA count 1
*Nov 14 06:07:05.457: OSPF-1 ADJ   Et0/0: Synchronized with 200.200.200.200, state FULL
*Nov 14 06:07:05.457: %OSPF-5-ADJCHG: Process 1, Nbr 200.200.200.200 on Ethernet0/0 from LOADING to FULL, Loading Done




R1#show ip os nei

Neighbor ID         Pri   State                 Dead Time   Address         Interface
200.200.200.200   1   FULL/DR         00:00:38    192.168.1.2     Ethernet0/0
R1#


I hope this was helpful