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Basic IS-IS question

I know to most of you this is a basic question so please excuse me for asking it. But I am reading about IS-IS and am not positive if Level-1-2 routers would be considered backbone routers right along with Level-2 routers. It appears on the face that only Level-2 routers are backbone routers but it is not conclusive in my mind.

Cisco Employee

Re: Basic IS-IS question

All L2 routers can be considered backbone routers including the L1-L2 routers. The L1-L2 router maintains both a L1 and a L2 database. I'm not sure this answer your question. If not please clarify.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
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New Member

Re: Basic IS-IS question

The main reason I ask is on the BSCI exam there were two questions on this so I want to be positive when I retake it. (I failed it obviously). I said that level-1-2 routers are also part of the backbone in my response but the more I read I get a very ambiguous read on how Cisco interprets the backbone. On one hand Level-2 is the only one mentioned as being in the backbone but since Level-1-2 are both ONE and TWO at the same time then maybe in the twisted Cisco sense Level-1-2 would be considered part of the backbone as well.

Thanks for the fast response.

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