Monday, March 2, 2009

Things that should match to build OSPF neighborship

Here is a small but list of  interesting things those should match to build and establish OSPF neighborship. So you must watch it next time if neighborship is not coming up between two ospf  routers :)

  • OSPF must be enabled on the interfaces on each router that are connected to the same layer 2 network (w/ the network command).
  • The neighboring primary IP addresses and masks must be in the same subnet.
  • Authentication must pass
  • Interfaces must be in the same area
  • Areas must have the same area type (stubby, NSSA, etc)
  • Must NOT have duplicate RIDs
  • OSPF Hello and Dead timers must match on the two routers
  • I am not sure if some references has mentioned this but " MTU size " does creates problems as well some times and this can be avoided by running following command under ospf process  -"ip ospf mtu-ignore"

    Actually OSPF checks whether neighbors are using the same MTU on a common interface. This check is performed when neighbors exchange Database Descriptor (DBD) packets. If the receiving MTU in the DBD packet is higher than the IP MTU configured on the incoming interface, OSPF adjacency will not be established.

    Additionally, there are a couple of items that appear to be an issue, but are not. In particular:

    • The OSPF process ID (on the router ospf command) do not have to match.
    • Must use the same reference bandwidth (ip ospf reference-bandwidth command)
    Best Regards,
    Deepak Arora

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