2 Replies Latest reply: Nov 8, 2018 3:09 PM by Steven Davidson RSS

    OSPF hello timer mismatch


      **** all,


      How does OSPF hello packet mismatch causes neighbor breakdown?


      The default value for hello packet is 10 seconds on broadcast Ethernet. The dead timer is 40 seconds.




                    R2 -----------------------R3



      Say I have R2 and R3 connected with Ethernet. I change R2's hello interval to 5 second. Its dead timer becomes 20 seconds. R3's dead timer would still be 40 seconds. And it sends hello every 10 seconds.


      Now when R2's dead timer reaches 10 (10 seconds passed), it would get a hello packet from R3. R2 would restart counting from 20.


      For R3, when its dead timer reaches 35 (5 seconds passed), it would get a hello packet from R2. It would restart counting from 40.


      This should keep OSPF in both routers alive. Right?


      In reality, I see the dead time counts down to 0 from 40 on R3, the neighbor relationship breaks. The dead timer doesn't get refreshed. Why?




      R3#show ipv6 ospf nei

                  OSPFv3 Router with ID ( (Process ID 3)

      Neighbor ID     Pri   State           Dead Time   Interface ID    Interface           1   FULL/BDR        00:00:15    4               GigabitEthernet1/0



      *Nov  8 13:55:28.265: %OSPFv3-5-ADJCHG: Process 3, IPv6, Nbr on GigabitEthernet1/0 from FULL to DOWN, Neighbor Down: Dead timer expired


        • 1. Re: OSPF hello timer mismatch




          By default, OSPF uses a 10-second hello timer and 40-second hold timer on broadcast and point-to-point links, and a 30-second hello timer and 120-second hold timer for all other network types. An interface's hello timer can be adjusted:

          Router(config-if)# ip ospf hello-interval 

          If we set Router1 to use a hello timer of 15 seconds and leave Router2 at its default setting of 10 seconds, they will not form an adjacency. Note that this behavior is different from EIGRP. If we enable OSPF hello packet debugging with debug ip ospf hello, the router reports mismatched hello parameters (specifically the hello and dead intervals):

          OSPF: Rcv hello from area 0 from FastEthernet0/0 OSPF: Mismatched hello parameters from OSPF: Dead R 40 C 60, Hello R 10 C 15  Mask R C

          Source: Hello timer behaviors - PacketLife.net


          Best regards!

          • 2. Re: OSPF hello timer mismatch
            Steven Davidson

            Simply put, this is a requirement of this standard-based protocol:


            RFC 5340 - OSPF for IPv6


            "A.3.2.  The Hello Packet



               Hello packets are OSPF packet type 1.  These packets are sent

               periodically on all interfaces (including virtual links) in order to

               establish and maintain neighbor relationships.  In addition, Hello

               packets are multicast on those links having a multicast or broadcast

               capability, enabling dynamic discovery of neighboring routers.



               All routers connected to a common link must agree on certain

               parameters (HelloInterval and RouterDeadInterval).  These parameters

               are included in Hello packets allowing differences to inhibit the

               forming of neighbor relationships.  The Hello packet also contains

               fields used in Designated Router election (Designated Router ID and

               Backup Designated Router ID), and fields used to detect bidirectional

               communication (the Router IDs of all neighbors whose Hellos have been

               recently received)."