CCIE Pursuit Blog

December 8, 2008

Question Of The Day: 08 December, 2008

***Update 09 December***

Assume that OSPF was configured on the router after any changes were made to the interfaces on R1.

Given the following information:

R1#sh ip int br | e ass
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            155.1.146.1     YES NVRAM  up                    up
Serial0/1                  155.1.13.1      YES NVRAM  up                    up
Loopback0                  150.1.1.1       YES NVRAM  administratively down down

R1#sh run | sec router ospf
router ospf 100
log-adjacency-changes
network 155.1.13.1 0.0.0.0 area 0

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6 Comments »

  1. i didn’t know you could shutdown loopbacks.

    Comment by jason — December 8, 2008 @ 6:51 pm | Reply

  2. it is not possible to give exact answer, because OSPF will not change router id after elected interface shutdown.

    Comment by NTllect — December 8, 2008 @ 9:23 pm | Reply

  3. was the loopback admin downed before or after the ospf process started?

    Comment by nullrouter — December 9, 2008 @ 4:08 am | Reply

  4. @Jason – I didn’t realize that a loopback could be shut down either…until it came up in a job interview. 🙂

    @NTllect and @nullrouter – I should have written the question to state that the OSPF configuration was added after the interfaces were configured as they are in the “show ip int br” output. I will edit the question to reflect this.

    Comment by cciepursuit — December 9, 2008 @ 9:51 am | Reply

  5. A 32-bit number that uniquely identifies the router. The highest IP address on
    an active interface is chosen by default unless a loopback interface exists or the router ID is
    manually configured

    so the router id would be 150.1.1.1

    Comment by vhandsom — December 12, 2008 @ 10:23 pm | Reply

  6. Be careful guys, almost always the real proof of the pudding is actually labbing it up yourself 🙂

    R0 using IOS 12.4:

    R0(config)#do sh ip int b | e una
    Interface IP-Address OK? Method Status Protocol
    FastEthernet1/0 155.1.146.1 YES manual up up
    Serial2/0 155.1.13.1 YES manual up up
    Loopback0 150.1.1.1 YES manual administratively down down
    R0(config)#do sh ip o

    R0(config)#router ospf 100
    R0(config-router)#net
    R0(config-router)#net 155.1.13.1 0.0.0.0 area 0
    R0(config-router)#do sh ip o
    Routing Process “ospf 100” with ID 155.1.146.1 <=================

    Always a good idea to test what really happens in these situations.

    Thanks CCIE Pursuit for the post.
    I’ve created a post of my own out of this own to understand the rid selection process better @
    http://roy.ccieblog.com/2009/01/04/understanding-the-ospf-rid-selection-process/

    Comment by rwaterman — January 4, 2009 @ 10:44 am | Reply


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