09-20-2008 03:43 AM - last edited on 08-17-2023 04:27 AM by Translator
Hi Team,
We have 4 or 5 routers running on OSPF in MPLS LAN. Out of which only between two routers (one DR-OTHER and one DR) is flapping badly. The logs we are seeing in the router is:
Sep 20 15:52:17.304 IST: %OSPF-5-ADJCHG: Process 10, Nbr x.x.x.x on GigabitEthernet4/1 from FULL to DOWN, Neighbor Down: Too many retransmissions
Sep 20 15:53:17.302 IST: %OSPF-5-ADJCHG: Process 10, Nbr x.x.x.x on GigabitEthernet4/1 from DOWN to DOWN, Neighbor Down: Ignore timer expired
Sep 20 15:53:24.481 IST: %OSPF-5-ADJCHG: Process 10, Nbr x.x.x.x on GigabitEthernet4/1 from LOADING to FULL, Loading Done
Sep 20 16:12:41.810 IST: %OSPF-5-ADJCHG: Process 10, Nbr x.x.x.x on GigabitEthernet4/1 from FULL to DOWN, Neighbor Down: Too many retransmissions
Sep 20 16:13:41.864 IST: %OSPF-5-ADJCHG: Process 10, Nbr x.x.x.x on GigabitEthernet4/1 from DOWN to DOWN, Neighbor Down: Ignore timer expired
Sep 20 16:13:44.580 IST: %OSPF-5-ADJCHG: Process 10, Nbr x.x.x.x on GigabitEthernet4/1 from LOADING to FULL, Loading Done
The logs show the problem is due to "Too many retransmissions".
Kindly help me how do we troubleshoot this OSPF issue due to Too many retransmissions.
thanks in advance.
Solved! Go to Solution.
09-20-2008 06:49 AM - last edited on 08-17-2023 05:04 AM by Translator
Hello Arun,
most of the times under a "too many retransmissions" there is an
MTU
mismatch.
Verify with
sh interface x/y sh ip inteface x/y
you can use
debug ip ospf adj
to troubleshoot the issue.
if there's a
MTU
mismatch you need to fix it with
int x/y mtu
or you can use
int gx/y ip ospf mtu-ignore
I would recommend first solution
Database packets have the DF bit set in the ip header. For this reason an
MTU
mismatch can create problems.
Hope to help
Giuseppe
09-20-2008 06:49 AM - last edited on 08-17-2023 05:04 AM by Translator
Hello Arun,
most of the times under a "too many retransmissions" there is an
MTU
mismatch.
Verify with
sh interface x/y sh ip inteface x/y
you can use
debug ip ospf adj
to troubleshoot the issue.
if there's a
MTU
mismatch you need to fix it with
int x/y mtu
or you can use
int gx/y ip ospf mtu-ignore
I would recommend first solution
Database packets have the DF bit set in the ip header. For this reason an
MTU
mismatch can create problems.
Hope to help
Giuseppe
09-21-2008 10:14 PM - last edited on 08-17-2023 03:43 AM by Translator
Hi Giuseppe,
Thanks for your response. By configuring
ip ospf mtu-ignore
on other routers solved this issue. But I got confused and would like to know one thing.
If there is an MTU mismatch then the
adjacency
itself will not be formed. Database packets will be exchanged only when the
adjacency
is formed. And if MTU is different then the OSPF will be stuck in EXSTART state itself (as Database descriptor packet exchange process is elected).
Above is my understanding on
OSPF adjacency
formation and I would like to know is it correct.
09-22-2008 01:42 PM - last edited on 08-17-2023 04:54 AM by Translator
Hello Arun,
thanks for your kind remarks.
OSPF terminology makes a distinction between neighborship and
adjacency
.
to be neighbor two routers must see each other in the hello packets, where there is a list of
OSPF Router-ID
on routers on the link.
This state is the DRother, two routers that are not DR or BDR don't move on.
If one of the two routers is DR or BDR the exstart process is entered and the two routers start to exchange the whole OSPF database.
Actually at first only the
LSA headers are exchanged+ lsa sequence number+ lsa age.
At the end of the
LSA headers
sending the slave machine has built a list of LSA to be requested and makes these requests.
The other router answers with the full info of the requested LSAs.
Only at the end when this pair of routers have their DB in sync they become adjacent, Full adjacent so you see the FULL in the
sh ip ospf neigh.
So when you read the
OSPF adjacency
cannot form with MTU mismatch is meant that when comes the time to exchange the DB the packets having the DF bit set and being as big as possible for efficiency lead to problems.
So a successful DB exchange is required to reach a full
adjacency
and not the opposite.
Hope to help
Giuseppe
11-16-2017 11:25 AM
It's 2017 and This has taught me something i could not understand with OSPF thank you!!!
11-19-2017 03:01 PM
Hello lrizzay_j,
thanks for your kind remarks
>>It's 2017 and This has taught me something i could not understand with OSPF thank you!!!
until we deal with OSPFv2 for IPv4 there is no surprise that the behaviour should be this one as it is more in the nature of the protocol itself.
HTH
Giuseppe Larosa
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