Integrated IS-IS Configuration on Cisco IOS

This topic is to discuss the following lesson:

Hi Rene,
Thx for your very nice article as always :slight_smile:
If there is multiple Level1-2 Router on a area then due to default Route asymmetric Routing will be occurred. I saw your topic about Route leaking but I think its not scalable more . So is there any other scaleble option here ??

Hi @Zaman.rubd,

Since IS-IS routers only have a default route to get to other areas, sub-optimal routing can be an issue yes. It will depend on your network design. There’s not really an alternative to route leaking as far as I know…somehow you have to get those more specific prefixes in the routing table of your L1 routers.

For anyone else that is reading this and wondering what this is about, here is an example for IS-IS route leaking and sub-optimal routing:

Hello René,
Thank you a lot for this lesson. But I don’t see this command circuit-type level anywhere in the course. Normal?

Hello Djan,

I haven’t added this command indeed. It is pretty straight-forward though, you can use it to tell the router which hello packets to send on the interface: L1, L2 or L1 and L2. You can do this per interface so on Gi0/1 you could send L1 hello packets and on Gi0/2 only L2 hello packets.


Hello Rene,
I’ve a question about the is-type in the R2, before it had the is-type level-1 because we had one area only, after we added another area and R2 became a L1-L2 router, right. You have typed the is-type level-1-2 within router isis. My question is, Could I omit the is-type in router isis configuration because the router cisco has a L1-L2 behavior as default?

Hello Eduardo

Since the default is-type is level 1-2, then you don’t actually have to explicitly configure the router as a level 1-2 router. This command can indeed be deleted. However, as is the case with many such commands, it’s a good idea to verify that the configuration is correct by implementing the command. This is all the more important when you configure devices that have been previously configured and may have some other configuration already set up.

I hope this has been helpful!


Excellent explaining. We learned a lot

1 Like


I have a question, just to be sure: the 2 backbone routers become l2 neighbors because they’ve exchanged l2 LSP and found the directly connected net between them in the other router LSP and the other router has another area. Is this correct?

Thank you,

Hello Staut

Yes, this is correct.


1 Like

Thank you! One more question: if R1 would be set to L1-L2 he would see the R2 neighbor in the same area and would not establish a L2 neighborship thus not processing L2 LSP from R2. I guess R2 sends L2 LSP to R1 too, but since there is a L1 neighborship, R1 would not process them. Is this also correct?

Hello Stuat

Yes, if two routers are set to be L1-L2, and they are in the same area, they will only establish an L1 neighborship because they detect that they are in the same area.

I hope this has been helpful!


1 Like

Thank you! Just wanted to make sure that it’s not necessarily to set is-type level1 command if routers are in the same area.

1 Like