Question on IS-IS router types

Spunkerspawn

Ok, so I am almost done studying for my BSCI exam and I think I have a good enough grasp on each topic however there's one topic that's been bugging me for a while. And that is (as mentioned in the topic) the IS-IS router types.

Basically I want to know the difference between a Level 1-2 router and a Level 2 router. Why would we need a Level 2 router (or even a Level 1 for that matter) if the default config for cisco routers is Level 1-2?

Or let's say if we have a Level 2 what makes it essential that we cannot replace it with a Level 1-2?

I really need clarity on this subject because it's been eating away at my brain for a while

Thanks for the help and wish me luck on my BSCI exam!

kpjungle

Spunkerspawn wrote:

Ok, so I am almost done studying for my BSCI exam and I think I have a good enough grasp on each topic however there's one topic that's been bugging me for a while. And that is (as mentioned in the topic) the IS-IS router types.

Basically I want to know the difference between a Level 1-2 router and a Level 2 router. Why would we need a Level 2 router (or even a Level 1 for that matter) if the default config for cisco routers is Level 1-2?

Or let's say if we have a Level 2 what makes it essential that we cannot replace it with a Level 1-2?

I really need clarity on this subject because it's been eating away at my brain for a while

Thanks for the help and wish me luck on my BSCI exam!

Good luck on the exam!

The reason why you dont want to have 1-2 routers everywhere is most likely because they will maintain two Link state databases, one for the level 1 and one for the backbone (level 2). Also, level 2 routers hand off default-information to level 1 routers in its area, so they can always "find their way out" of the level1.

I think all in all it comes down to effeciency, you dont necessarily want to have the entire backbone routing table on all your routers.