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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)
– 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 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

Level 1 Routers exchange topology information for its own area. It is like the OSPF Internal Routers. Level 1 Router has an LSDB (Label Switch Databases) for its own area.

Level 2 Routers are responsible for exchanging topology databases between the areas. It is like the OSPF Backbone Routers. Level 2 Router has LSDB for inter-area topology. L2 Routers interconnects the different areas consist of many Level 1 routers.

Level 1/2 Routers exchange information between L1 and L2 domain. They are for routing both for intra-area and inter-area routing. It is like OSPF ABRs (Area Border Router). It both communicate L1 routers and L2 routers. L1/2 routers connect L1 routers to outside of the area, it is the exit point of the area.

By default Cisco, Alcatel-Lucent and Juniper routers are all works as Level 1/2 router for IS-IS.

All routers update their databases with Link-State PDUs and so all the IS-IS routers has the same topology view.

L1 routers uses LSPs (Link State Packet) to build topology for local area. L2 routers uses LSPs to build the topology between different areas. L1/2 routers work as a border router between L1 and L2 domains like ABR in OSPF. So, it has both L1 and L2 topologies.

Routing In IS-IS Protocol



In IS-IS Protocol there are ES (End System) and IS (Intermediate System) devices. This is not different than the normal hosts and the routers. Hosts called ES, routers called IS in the IS-IS Protocol world.

IS-IS Protocol Devices, End System (ES), Intermediate System (IS)

IS-IS Protocol Devices, End System (ES), Intermediate System (IS)

There are four OSI routing levels are used in IS-IS Protocol. These routing levels are:

– Level 0 Routing : Routing between ESs and ISs on the same subnet.
– Level 1 Routing : Routing between ISs within the same area.
– Level 2 Routing : Routing between different areas withing the same domain.
– Level 3 Routing : Routing between the seperate ISIS domains.

Four OSI Routing Levels in IS-IS

Four OSI Routing Levels in IS-IS

There are two sub routing protocols in IS-IS Protocol. These are ES-IS and IS-IS

ES-IS is like ARP (Address Resolution Protocols) in IP networks. It is also called Level 0 Routing. ESs listens the ISs hello packets and ISs listens to ES Hello packets. By doing this, they discover each other.

IS-IS Protocol is used in the other three routing levels. For Level 1 Routing it is used between the nodes in different subnets in the same area. For Level 2 Routing, it is used between areas. And lastly, for Level 3 Routing, it is used between different domains.

Summarization is very important for a routing protocol. In IS-IS Protocol, summarization is also support to reduce the Level 2 routes. The default route is also used in IS-IS Protocol with “default information originate” command.

By the way, the exit point of an area is mentioned with ATT bit. If this bit is set, it means that it is the exit point Level 1/2 router of that area.

Redistribution is also supported in IS-IS Protocol. If you want to redistribute from one routing protocol to IS-IS Protocol, you can use both “internal” and “external” metric types. “Internal” means that, the redistributed route will compete with the other internal routes to be selected. “External” means an additional metric value will be given to the redistributed route ( redistribution metric +128).

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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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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