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IS-IS Design considerations on MPLS backbone

IS-IS Design considerations on MPLS backbone

Using IS-IS with MPLS require some important design considerations. IS-IS as a scalable link state routing protocol has been used in the Service Provider networks for decades.

In fact, eight of the largest nine Service Providers use IS-IS routing protocol on their network as of today. Read more

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IS-IS level 1, IS-IS Routing Protocol Levels

What is IS-IS Level 1 ? Why IS-IS level 1 is used ? What are the IS-IS levels ? What is the corresponding Area type in OSPF ?

IS-IS Level 1 is also called as IS-IS Level 1 sub domain. IS-IS is a link state routing protocol, similar to OSPF. You can read detail comparison of OSPF vs. IS-IS from here.

There is an Area concept in IS-IS as well but the purpose of IS-IS Areas are different than OSPF Areas.

In IS-IS Areas are used to create  IS-IS Levels and IS-IS Levels are the similar logical concepts with OSPF Areas.


Two create IS-IS Level 1 adjacency between two routers, same IS-IS area number should be used. Otherwise only IS-IS Level 2 adjacency can be created. This is the first rule of IS-IS adjacency relationship.


Which OSPF Area type is similar to IS-IS Level 1 Sub domain ?

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IS-IS Suboptimal Routing Design

If you design multi level IS-IS network and if you have more than one exit (L1-L2 routers) from the Level 1 domain, you will likely create a suboptimal routing. Multi-level IS-IS design is for large-scale network. What’s more, most of the real life networks use only flat Level 2 IS-IS as their interior gateway protocol (IGP).


is-is routing protocol


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Orhan Ergun 9 Comments

IS-IS Routing Protocol FAQ

IS-IS is a link state routing protocol. Commonly used in Service Provider networks.

Back in old days, IS-IS routing protocol software was more stable and robust compare to OSPF,  thus many service provider choose IS-IS as their interior routing protocol.

I collected the questions which I received from my students and readers related with IS-IS routing protocol and my answers in this post. Read more