1.10.b - OSPF Neighbor relationship and authentication

For an OSPF neighbor adjacency to form, the following must match:

• The OSPF area must match between the two routers.
• The form of authentication if configured must match.
• The network subnets must match.
• The Hello / Dead timers must match on both routers.
• The stub flags eg. Stub area, Totally stubby areas, Not so stubby areas, etc must match between the two routers.
• The MTU (Maximum transmission unit) must match between the two routers.
  • If the MTUs are mismatched and you input the show ip ospf neighbors command, you will see the EXSTART/EXCHANGE state.

DR and BDR Election

• Carried in the Hello packet
• Configured in interface configuration mode:
  • Router(config-if) # ip ospf priority number
    • A priority of 0 excludes a router from participating in the election.

If a tiebreaker occurs, The following process determines which router becomes the Designated router.

• The router IDs are evaluated, and the highest router ID wins. The Router ID is a number that uniquely identifies a router within the OSPF domain. It looks the same as an IP Address.
  • Router(config-router) # router-id xxx.xxx.xxx.xxx
• If the router IDs are not configured, the highest IP address on a loopback interface wins.
• If there are no configured loopback interfaces, the highest IP address on an online interface wins.