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 ?