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IS-IS(Intermediate System-to-Intermediate System)




The IS-IS routing protocol is a link-state protocol, as opposed to distance-vector protocols such as Interior Gateway Routing Protocol (IGRP) and Routing Information Protocol (RIP). Link-state offers several advantages over distance-vector protocols. It is faster converging, supports much larger internetworks, and is less susceptible to routing loops. Features of IS-IS includes:

> Hierarchical routing

> Classless behavior

> Fast Convergence

> Very scalable

> Rapid flooding of new information

> Flexible timer tuning


Integrated IS-IS supports three types of networks:


   1)  OSI only (CLNS)

   2)  IP only

   3) Dual (that is, both OSI and IP simultaneously)


A two-level hierarchy is used to support large routing domains. A large domain may be administratively divided into areas. Each system resides in exactly one area.1 Routing within an area is referred to as Level 1 routing. Routing between areas is referred to as Level 2 routing. A Level 2 Intermediate System (IS) keeps track of the paths to destination areas. A Level 1 IS keeps track of the routing within its own area. For a packet destined for another area, a Level 1 IS sends the packet to the nearest Level 2 IS in its own area, regardless of what the destination area is. Then the packet travels via Level 2 routing to the destination area, where it may travel via Level 1 routing to the destination. It should be noted that selecting an exit from an area based on Level 1 routing to the closest Level 2 IS might result in suboptimal routing.2

Basic IS-IS Configuration Steps:


The following steps should be taken when configuring IS-IS on router:


Step 1: Define areas. Prepare an addressing plan for the routers and determine interfaces that will run Integrated IS-IS.


Step 2: Enable IS-IS as an IP routing protocol on the routers.


The "router is-is [tag] global" configuration command enables Integrated IS-IS on the router.


Step 3: Configure the NETs on the routers. This identifies the routers for IS-IS.


After the Integrated IS-IS process is enabled, the router must be identified for IS-IS by assigning a NET to the router with the "net network-entity-title" configuration command. In this command, network-entity-title is the NET that specifies the area address and the system ID for the IS-IS routing process.


Step 4: Enable Integrated IS-IS on the proper interfaces on the routers.


Then finally interfaces that are to use IS-IS to distribute their IP information must be configured using the "ip router isis [tag]" interface configuration command


When routing of CLNS packets is also required, use the "clns router isis [tag]" interface configuration command.


The beauty of ISIS is ,you can configure IPv4 and IPv6,VRF  within same process and enable ISIS under interface,protocol automatically understand.

Configuration example of ISIS for IPv6:

Router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#int e0/0
Router(config-if)#ipv6 enable
Router(config-if)#ipv6 router isis


Verification Command:

sh isis  neighbors

sh clns neighbors

sh isis database detail

Related Information:

Intermediate System-to-Intermediate System Protocol

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