OSPF Router ID Selection Trivia

True or false: If you have an (enabled) loopback interface configured on the router, its IP address will always be used as the OSPF router ID.

Answer: False. Each OSPF process running on the router needs a unique Router ID. If you have a single loopback interface and two OSPF processes, the second OSPF process is forced to select another interface address as its router ID.

Here is a sample router printout:

Test#show running | inc ^interface|^ ip address
interface Loopback0
 ip address 10.0.0.1 255.255.255.255
interface FastEthernet0/0
 ip address 10.1.0.1 255.255.255.0
Test#show ip ospf | inc Routing Process
 Routing Process "ospf 2" with ID 10.1.0.1
 Routing Process "ospf 1" with ID 10.0.0.1

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8 comments:

  1. Arturo Servin 25 April 2009 11:22
    You also have a nice explanation here:

    http://blog.ioshints.info/2007/10/ospf-router-id-does-not-change-when.html


    -as
  2. Anonymous 25 April 2009 15:23
    In case of single OSPF process when you configure Loopback after OSPF, OSPF will use IP from any "live" interface.
  3. Inevitable 25 April 2009 16:32
    now i got ur point , thanks Ivan -:)
  4. shivlu jain 27 April 2009 10:38
    thanks ivan for great input.

    regards
    shivlu jain
  5. V.H. 27 April 2009 11:21
    selection of router-id for single ospf process:
    1) router-id command
    2) ip address of loopback interface (higher address in case of more that one loopback)
    3) ip address of interface wiht higher ip address
  6. Jeff 17 November 2009 13:34
    Does the router ID have to be an IP that's assigned to an interface? If we used 1.1.1.1 does there have to be an interface with that IP assigned to it?
  7. Ivan Pepelnjak 17 November 2009 13:37
    No, the OSPF router-ID is a 32-bit quantity that has to be unique throughout the OSPF domain. Cisco has long tied it to an active interface, but you can configure it manually with the "router-id" configuration command.
  8. sajid 11 November 2011 15:27
    how to hint sopf router properly plea hint aboute
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