OSPF issue

[tanmoymm91]

I have changed the MTU on R2 interface (Gi0/0/0) from default (1500) to 1300 but its neighborship still showing in Full state it should be EXSTART state .

what happened wrong , kindly share your views .

i am sharing all related configs on both routers 

R2#show run | s ospf

router ospf 10

router-id 1.1.1.1

log-adjacency-changes

network 10.1.1.0 0.0.0.255 area 0

R2#

R2#show int gi0/0/0

GigabitEthernet0/0/0 is up, line protocol is up (connected)

Hardware is Lance, address is 0030.a3c4.aa01 (bia 0030.a3c4.aa01)

Internet address is 10.1.1.2/24

MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,

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

Encapsulation ARPA, loopback not set

Full-duplex, 100Mb/s, media type is RJ45

ARP type: ARPA, ARP Timeout 04:00:00,

Last input 00:00:08, output 00:00:05, output hang never

Last clearing of "show interface" counters never

Input queue: 0/75/0 (size/max/drops); Total output drops: 0

Queueing strategy: fifo

Output queue :0/40 (size/max)

5 minute input rate 61 bits/sec, 0 packets/sec

5 minute output rate 59 bits/sec, 0 packets/sec

215 packets input, 14056 bytes, 0 no buffer

Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

0 input packets with dribble condition detected

196 packets output, 12764 bytes, 0 underruns

0 output errors, 0 collisions, 2 interface resets

0 babbles, 0 late collision, 0 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

R1#show run | s ospf

router ospf 10

router-id 2.2.2.2

log-adjacency-changes

network 10.1.1.0 0.0.0.255 area 0

R1#show int gi0/0/0

GigabitEthernet0/0/0 is up, line protocol is up (connected)

Hardware is Lance, address is 0000.0cc5.5c01 (bia 0000.0cc5.5c01)

Internet address is 10.1.1.1/24

MTU 1300 bytes, BW 1000000 Kbit, DLY 10 usec,

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

Encapsulation ARPA, loopback not set

Full-duplex, 100Mb/s, media type is RJ45

ARP type: ARPA, ARP Timeout 04:00:00,

Last input 00:00:08, output 00:00:05, output hang never

Last clearing of "show interface" counters never

Input queue: 0/75/0 (size/max/drops); Total output drops: 0

Queueing strategy: fifo

Output queue :0/40 (size/max)

5 minute input rate 61 bits/sec, 0 packets/sec

5 minute output rate 60 bits/sec, 0 packets/sec

253 packets input, 16804 bytes, 0 no buffer

Received 0 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

0 input packets with dribble condition detected

236 packets output, 15448 bytes, 0 underruns

0 output errors, 0 collisions, 1 interface resets

0 babbles, 0 late collision, 0 deferred

0 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out

R2#show ip ospf nei

Neighbor ID Pri State Dead Time Address Interface

2.2.2.2 1 FULL/DR 00:00:32 10.1.1.1 GigabitEthernet0/0/0

R1#show ip osp nei

Neighbor ID Pri State Dead Time Address Interface

1.1.1.1 1 FULL/BDR 00:00:34 10.1.1.2 GigabitEthernet0/0/0

[M02@rt37]

Hello @tanmoymm91,

OSPF adjacencies can remain stable even when there are changes to underlying parameters like MTU. In such cases, the routers continue using the existing adjacency without renegotiating. To ensure that the MTU change is recognized, you can try clearing the OSPF adjacency on both routers using the "clear ip ospf process" command.

[tanmoymm91]

I think the issue with the Packet tracer tool , although i am using the latest one (8.2.1) .

but yes , the concept wise MTU must be same on both side otherwise it will stuck in exstart state 

[MHM Cisco World]

the OSPF not detect MTU change until change UPDATE. 
so either you wait until the prefix aging or clear OPSF process 

[tanmoymm91]

I think the issue with the Packet tracer tool , although i am using the latest one (8.2.1) .

but yes , the concept wise it should be in exstart state .

[MHM Cisco World]

I will check using GNS3 

[MHM Cisco World]

change the MTU of R1 interface, the OSPF still UP 
later I clear ospf process and debug show you in which steps the OSPF stop EXSTART and reason of that the MTU mismatch