Thursday, December 22, 2011

OSPF over Unnumbered Link

OSPF over Unnumbered Link

The only time that OSPF forms adjacency between neighbors that are no reside on the same subnet is when the neighbors are connected through an unnumbered point-to-point link. When enabling OSPF on an unnumbered interface, use the address-wildcard-mask pair of the primary physical interface which the unnumbered interface relies upon.
Note: OSPF does not work if one side unnumbered while the other side numbered on a link, as this would create discrepancy in the OSPF database and prevents routes from being installed into the routing table.

Below shows the OSPF neighbors, IP routing table, and OSPF database on RT1:
RT1#sh ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface           0   FULL/  -        00:00:33      Serial0/0
RT1#sh ip route

Gateway of last resort is not set is subnetted, 1 subnets
C is directly connected, Loopback0 is subnetted, 1 subnets
O [110/65] via, 00:00:34, Serial0/0 is subnetted, 2 subnets
C is directly connected, FastEthernet1/0
O [110/65] via, 00:00:34, Serial0/0
RT1#sh ip ospf database

            OSPF Router with ID ( (Process ID 100)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count         58          0x80000002 0x00C835 3         61          0x80000001 0x00B73E 3

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