Does isis loopback interface need to config hellointerval&holdtimer&cost value with 0?

[Ma Juner]

Hello Experts,

      Recently, I have some misunderstand on the isis protocol issue.

      As we know, the loopback interface is configured in isis protocol for the route stability and adjacencies connection protection.

     The vender set some default value for the hello interval and hold times.

     In IS-IS, the hello times and hold times must not match for the adjacency to be accepted, it's very variable.

     So, if I config the loopback interface isis hello interval and hold times with  0, is that a bad idea for the whole network?

     What about to config the loopback inferface isis cost with 0 ?

     I want to make above in the edge routers, because I think that

     1) the IS-IS use the real physical interfaces to form an IS-IS adjacency, the loopback interface is just used to detect failed neighbors

        through these real physical interfaces status, So the loopback interface hello interval and hold times can function when there's no

        value in the real physical interfaces configuration.

     2) the edge routers are usually the end leafs of the isis SPF, so isis the loopback inferface with a cost of 0 may be some beneficial

         for the IS-IS database synchronization or shortest-path tree building , I just guess that!!!!

Thanks for all the help

[Vaibhava Varma · Accepted solution]

Hi Ma Juner

The Loopback interface is used for iBGP/MP-iBGP peerings and MPLS-TE setups and need not send out IS-IS hellos at all and in general ( Hellos are not needed on Loopbacks for any IGP) and we can use passive-interface to disable Hello-Advertisement. But the loopback has to be made reachable. ISIS adjaceny is formed over physical interface only.

We can not set the Timers to be 0 for ISIS neither we can set the metric to be 0. I think the best option is to make the loopbacks as passive.

Regards

Varma

[Ma Juner]

hi Vaibhava Varma,

    Thank you for explaination.

    I  still want to know why we can not set the times adn the metric to be 0 for ISIS,

    Is that suggested from some document as to ISIS protocol?

    Please give me some theory explain about the mechanism!

[Vaibhava Varma]

Hi Ma Juner

Keeping the timers to be 0 what we want to achive is that neighbourship is not formed over the interface. Passive Interface is there for that purpose. Keeping the cost 0 means we want to exclude the link from ISIS SPF and that means we do not need to enable ISIS at all on the link.

The cisco IOS does not allows the timers and metric to be set as 0. The minimum value is 1.

Hope this helps to understand you the reason. Do let me know for any more clarifications.

Regards

Varma

[Ma Juner]

hi Vaibhava Varma,

Thank you it make sense to me. 

Appreciate your prompt respond.