CCIE Pursuit Blog

March 11, 2009

Core Knowledge Question of the Day: 11 March 2009

R1#ping FE80::2
Output Interface: Serial1/0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to FE80::2, timeout is 2 seconds:
Packet sent with a source address of FE80::5
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/8/12 ms

Why must you specify an output interface when pinging FE80::2?

Highlight for answer: FE80::2 is an IPv6 link-local address.  IPv6 link-local address do not need to be unique on a router, but must be unique on an interface.  Because the same IPv6 link-local address can be present on multiple interfaces on a device, IOS needs to know the output interface.



  1. I really like these questions.. Force you to put in the thought process 🙂
    Keep up the good work!

    Comment by Kim Pedersen — March 11, 2009 @ 11:38 am | Reply

  2. Great question. Funny thing, when doing a lab I never knew, but I knew before answering by just stopping for a second and thinking, why DO you have to put a output interface for link-local addre-…. Oh! Haha, keep ’em coming.

    Comment by Luis Garcia — March 11, 2009 @ 12:39 pm | Reply

  3. Yepiie, got this correct. But my answer was “because its a link-local address”. Just kept it brief as cisco likes, but was nice to read the entire answer that you provided with.

    Keep them coming, I hope you will continue even after you done with your #. 😀

    Comment by Andy Hogard — March 12, 2009 @ 12:51 am | Reply

  4. You need to add the interface type and number because link local is only locat to the particular interface other interface within the network might have thesame link local address

    Comment by OMOOLOFIN — March 13, 2009 @ 5:40 pm | Reply

    • It’s actually local to the router’s interface as well, you can have the same link-local on multiple interfaces.

      Comment by ryan — April 2, 2009 @ 6:51 pm | Reply

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