ISIS when to and when not to advertise loopback0 into IGP?


I noticed when working through the INE full scale labs (The ISIS section) that the instructions specifically ask that the student advertise the loopback 0 of certain routers into IGP but for the routers that the instructions did not say advertise into IGP (whether L1 or L2)  the solution shows that it was done. My question……is it standard practice to always advertise the loopback0 into the IGP with  the “passive” statement unless instructed specifically not to? 




  • Hi Jesse,


    There are 3 parts to this question.


    1) Yes you generally want to advertise loopbacks into the IGP. Loopbacks are used for various troubleshooting and protocol level functions for example BGP and LDP peer addresses, RP addresses for multicast and GRE tunnel endpoints. If you flip the question around, if you gone to the effort of creating a loopback, why wouldn't you advertise it? For the SP lab I would 100% ensure Loopbacks are in the IGP.


    2) Never forget about the OSPF area and ISIS level when it comes to loopbacks. For example if the task said ensure all interfaces are enabled for ISIS level 1, it's very easy to focus on the physical interfaces and forget the loopback is running the default level 1-2, thus failing the task.


    3) It's a best practice to configure loopbacks as passive in OSPF and ISIS. It won't break anything protocol wise if you forget but it doesn't make sense to try and form neighbours on a loopback. Of course, if the task said ensure hellos are not sent on loopbacks you better make sure the loopback is passive if you want the points.



  • Thank you for taking time to answer my questions. I understand.

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