Routing With IS-IS Protocol – Part 1


What is Routing Protocol IS-IS ?

In this lesson, we will focus on one of the Routing protocols. We will learn What is ISIS Protocol? Why We use this protocol. Here , we will focus on the theorical lessons of this protocol. We will learn Cisco IS-IS Configuration in another lesson.


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)
  • 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 Topology, Level 1, Level 2, Level 1/2 Routers

Basic IS-IS Topology
Backbone 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


The other articles on IS-IS Protocol are below:

Routing With IS-IS Protocol – Part 1
Routing With IS-IS Protocol – Part 2 (IS-IS Addresses)
Routing With IS-IS Protocol – Part 3 (IS-IS Packet Types)
Routing With IS-IS Protocol – Part 4 (IS-IS Adjacency)
Routing With IS-IS Protocol – Part 5 (IS-IS versus OSPF)


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