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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 ?

IS-IS Level 1 Sub domain specifically similar to the OSPF Totally NSSA Area.

Let’s remember what was allowed and what was denied in OSPF Totally NSSA Area.

  • Type 3, Type 4 and Type 5 LSA is not allowed in Totally NSSA Area
  • Redistribution is possible and redistributed prefixes are presented as Type 7 LSA

Similar to OSPF Totally NSSA Area, ISIS Level 1 sub domain allows redistribution. Also from the upper level (IS-IS Level2) only default route is allowed. No other information is sent from IS-IS Level 2 sub domain into IS-IS Level 1 sub domain.

Default route is sent from the ISIS L1-L2 router into the IS-IS L1 routers in ATT (Attached) bit.

Of course all the routers in the same sub domain keeps the same database. Which mean, all the router in the same level have the same routing table.

Why IS-IS Level 1 is used ?

If you are designing IS-IS network in greenfield environment, don’t start with the IS-IS level 1. Don’t put them all your routers in Level 1 IS-IS sub domain.

New IS-IS deployment should start with IS-IS level 2.

ISIS level 1 is used in multi level IS-IS design. Similar to OSPF Multi Area design, the reason is scalability.

If you have 1000s of routers, you have to create multiple levels to keep the flooding domain small. Otherwise link or node failures, link flaps or other events create CPU and memory problems on the routers in the same level.

Last but not least, you would start with IS-IS level 2 in greenfield IS-IS design for easier migration from flat IS-IS design to multi level IS-IS design.

 
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