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Binoj Baby
Cisco Employee

how an ospf router knows , whether the hello packet is packet for establishing adjacency or hello to maintain the adjacency

hi

two routers are connected  and ospf is enabled , after creating adjacency , how router is knowing that hello packet is a just a hello packet  to keep the adjacency up  .

Regards

Binoj

5 REPLIES 5
Giuseppe Larosa
Hall of Fame Master

Hello Binoj,

OSPF keeps a finite state machine (FSM)  for each neighbor, after having reached the adjacency FULL state receiving timely hellos makes this neighbor status to be stable.

Each received hello resets the dead interval timer for that neighbor.

To be noted from Two way state the received hello packet contains the OSPF router-ids of the OSPF neighbors existing on that specific interface, so the local router can see its own OSPF RID in each of the received Hello packets.

Edit:

see

http://www.ietf.org/rfc/rfc2328.txt

section 10.3 Neighbor state machine

Hope to help

Giuseppe

Thanks for the explanation and the link you provide

Regards

Binoj

John Blakley
Advisor

Binoj,

During the initial adjacency, the routers will send hello packets with their router-id listed in that packet. If the router-id exists in the neighbor table, then it's considered a keep-alive and the dead interval is incremented. If the router-id is new and doesn't exist in the neighbor table, then the adjacency process begins.

Neighbor ID     Pri   State           Dead Time   Address         Interface

192.168.12.1      1   FULL/BDR        00:00:37    192.168.12.1    FastEthernet0/0

If I brought up another interface on another router with router-id 192.168.12.2, the adjacency process would start. Otherwise, if I'm receiving hello packets from the above router with router-id 192.168.12.1 then it's considered a hello packet and the dead time increases back to 4 times the hello period. (10 second hellos and 40 second dead interval).

HTH,

John

HTH, John *** Please rate all useful posts ***

Thanks for the detailed explanation

Regards

Binoj

good explanation guiseppe and john