IS-IS Overview


Routing With IS-IS Protocol

IS-IS (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.

IS-IS 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? Let’s check…

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.

IS-IS 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

Lesson tags: routing, is-is
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