CCIE Pursuit Blog

January 10, 2008

Humbled By The Humble Loopback

I had a technical interview yesterday.  Not to brag, but I was on my game.  To be fair, the interview questions were not CCIE level, they were more CCNA/CCNP level – but still, it was great to be able to easily and confidently answer all of the questions.  Not only was I answering all of the questions, but I was doing it in detail as well as expounding on each technology.  I was absolutely crushing this interview.  Until……

In response to a question about the usefulness of loopback interfaces (I had already rattled off router-id, monitoring traps, BGP peering, OSPF virtual-links,  etc…) I got hit with this:

“Can you configure a loopback interface to be down via the shutdown command?”

Screeeeeeech……what?….let’s see…..ummmmmm…….FUCK!!!

I laughed and answered honestly:

“You know, I’m not sure.  I would think that you should be able to do this.  I can see how it would be a great feature to simulate a router that you’re pinging going down – without actually dropping the router….but since I’ve never actually configured ‘shutdown’ on a loopback interface, I can’t say with 100% certainty that it is possible.”

He didn’t let me off the hook by answering the question.  I added that I would definitely be checking this out the second that I got on a device.  For the first time in the interview, I was worried.

I wasn’t worried about flubbing the interview based on this question.  “He was doing so well, but we can’t give him the position because he can’t tell me whether or not you can manually shut a loopback interface.”  No, I was more worried about the next question.  In my mind, he had just put me in check (with a pawn no less) and was getting ready for checkmate.  In my mind I could hear him dismantling my OSPF knowledge with this question:

“Okay, you told me earlier about how OSPF chooses a router-id if you do not explicitly configure one.  Let’s say that you CAN shutdown a loopback interface.  You have an up and up serial interface with the IP address 1.1.1.1 and a loopback interface of 2.2.2.2 – BUT the loopback interface is down.  What would the OSPF router-id be in this case?”

I have learned that OSPF will use the highest configured loopback address as the OSPF router-id if a router-id is not explicitly configured.  But would it use a loopback that was admin down, or would it use the serial interface’s IP address?

Thankfully, that question never came.

For those of you playing at home:
1) Yes, you can shutdown a loopback interface.
2) A loopback interface will not be considered for the OSPF router-id if it is shutdown.

r1#sh ip int br | e ass
Interface                  IP-Address  OK? Method Status        Protocol
FastEthernet0/0    1.1.1.1       YES manual up                    up
Loopback0            2.2.2.2      YES manual administratively down down

r1#sh run | sec router ospf
router ospf 100
 network 0.0.0.0 255.255.255.255 area 0

r6#sh ip os data

            OSPF Router with ID (1.1.1.1) (Process ID 100)

                Router Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         50          0x80000001 0x00C660 1

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2 Comments »

  1. You have no idea how big the smile is on my face right now. I’ve interviewed a lot of people, but I haven’t been interviewed for a while. I forget how stressful a technical interview can be.

    Comment by Ethan Banks — January 10, 2008 @ 4:26 pm | Reply

  2. sounds like you had a fun interview,.., I knew that you can shut the interface down but didn’t know the next question, what would have to opsf peering that was using the interface as a router id. your blog explains it well!

    Comment by Jedi Cisco — January 12, 2008 @ 4:50 am | Reply


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