IS-IS Overview

What is Routing Protocol IS-IS ?

ISIS Protocol (Intermediate System to Intermediate System Protocol) is a Link-State routing protocol that is developed by ANSI ISO. It is an open standard and classless Interior Gateway Protocol (IGP). IS-IS Protocol uses Dijkstra SPF (Shortest Path First) algorithm like OSPF, to build the IS-IS Protocol databases and calculate the best path. It uses Cost value at the best path calculation.

 

ISIS Protocol is a stable protocol and it has a very fast convergence. It has also large scalability that IS-IS Protocol is a vey good protocol for Service Providers and large enterprises. IS-IS Protocol is also a very good protocol used with MPLS-TE.

 

Dynamic Link-State protocol IS-IS Protocol is designed to used in CLNS (Connectionless Network Service). What is CLNS?

 

CLNS (Connectionless Network Service) : Is a network layer datagram service that requires no circuit establishment.

 

And there is also CMNS (Connection Mode Network Service).

 

CMNS (Connection Mode Network Service) : Is a network layer datagram service that requires path establishment between transport layers.

 

IS-IS Protocol supports both CLNP (Connectionless Network Protocol) and IP (Internet Protocol).

 

So, we can say that, IS-IS Protocol has three options:

 

• IS-IS (for ISO CLNS)
• ISO-IGRP
• Integrated IS-IS (For both ISO CLSN and IP)

 

IS-IS Protocol is a Hierarchical routing protocol like OSPF. IS-IS network is also consist of small Areas connected to the Backbone. So like OSPF, IS-IS Protocol has also two hierarchical levels :

 

• Level 1 (Areas)
  
• Level 2 (Backbone)

 

Basic IS-IS TopologyBackbone IS-IS is a little different than OSPF. IN OSPF, there is a Backbone Area 0 and all the areas connected to this area. This Backbone Area 0 is a continuous area. But in IS-IS Protocol, the backbone can consist of distinct areas that consist of interconnected L2 and L1/2 routers. But also these backbone routers needs to be directly connected and continuous.

 

Level 1 routing is used within any area, between Level 1 routers and between areas ( between L2 routers) Level 2 routing is used. For the connection of these two levels, border routers named Level 1/2 is used.

 

ISIS Protocol Router Types

 

In the Hierarchicel structure of IS-IS Protocol, different types of routers are used. There are three types routers in IS-IS Protocol. These are:

• Level 1 Router
• Level 2 Router
• Level 1/2 Router