cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
22949
Views
46
Helpful
5
Replies

OSPF flap due to "Too many retransmissions"

arun kumar
Level 1
Level 1

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.

1 Accepted Solution

Accepted Solutions

Giuseppe Larosa
Hall of Fame
Hall of Fame

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

 

View solution in original post

5 Replies 5

Giuseppe Larosa
Hall of Fame
Hall of Fame

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

 

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.

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

 

It's 2017 and This has taught me something i could not understand with OSPF thank you!!!

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

Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: