Cisco IS-IS Configuration Primer

With the expansion of today’s networks over the last 30 years, routing protocols have become a vital central part of these networks to ensure efficient routing and reachability.

Along with the well known routing protocols like OSPF and EIGRP there is also some other routing protocols which get less attention. These include BGP and IS-IS.

BGP (Border Gateway Protocol) is used at the core of the modern Internet and focuses on linking large scale networks, IS-IS (Intermediate System to Intermediate System) is a routing protocol which closely resembles OSPF and actually has been around just as long. IS-IS is a protocol which is used as a competitor to OSPF as it is standards based and is supported by multiple vendors.

The commonly used version of IS-IS is called integrated IS-IS which is simply IS-IS being used to route IP traffic; it is important to note that this was not the original design intent with IS-IS.

Initially the IS-IS protocol was designed to be used to route OSI traffic and used the Connectionless Network Protocol (CLNP) and was used by the Connectionless Network Service (CLNS). The only detail from the original IS-IS implementation which must be understood on today’s networks is CLNS addressing as it is used for IS-IS administration.

IS-IS defines four routing levels from level 0 through 3. On modern implementations only levels 1 and 2 are used. There is a rather simple difference between the two: Level 1 is used for routes within an area and Level 2 is used for routes between areas. Using level 3 would be for routing between Autonomous Systems.

Now IS-IS areas are very similar to those areas used by OSPF with some differences which must be understood. IS-IS does not have a central area requirement as OSPF does, however areas can only be connected via Level 2 configured routers.

Now if you think about this, that means that each router between areas is typically (but not required) a Level 1 and a Level 2 router; this allows the same router to see level 1 internal routes and pass them to another area via the Level 2 component.

The other major difference is that with OSPF, routers are allowed to be in more than one area at the same time to pass routes between those different areas; with IS-IS,  routers are wholly in only one area, the links between the areas become the boundary between the areas.

One of the most complicated things for most people to grasp with IS-IS is the addressing structure. As most are not familiar with CLNS, some basics must be understood to properly select an addressing structure.

IS-IS uses an ISO style of address which includes an Area, ID (system ID) and a SEL field.

The Area is split into three different parts including the AFI (1 byte), IDI and High Order DSP (HODSP – internal AS Area). When implementing IS-IS the IDI is typically omitted as it is not required with Level 1 and Level 2 routing. The AFI is short for Authority and Format Identifier and are assigned, the most common being 39 (Country code), 47 (International code) and 49 (Private).

The HODSP is used to assign the area used by the routers and is interpreted by Cisco equipment as anything between the AFI and the System ID, so the length is variable.  The ID is a 6 byte (on Cisco equipment) field which is used to specify the system ID of the device being addressed (IS-IS addresses are assigned to the system not the interface).

The SEL field is typically not changed from a value of 00 as this signifies the device; an address with a SEL of 00 is also referred to as a Network Entity Title (NET).

For example, if a router was given the address:

IS-IS Example49.0001.1234.5678.9012.00

than 49 would be the AFI,
0001 would be the area,
1234.5678.9012 would be the system ID and
00 would be the SEL,
this address could also be used as a NET address as the SEL is 00.

Basic IS-IS configuration

On Cisco equipment there are some basic configuration parameters which are required to get IS-IS up and running. The commands to get IS-IS up and running on a router include:

  1. router isis – This command is used to enable the start of the IS-IS process on the router
  2. net address – This command is used to configure the NET address which will be used to identify this router
  3. ip router isis – This command is configured on each interface which will participate with IS-IS

By default, a Cisco router will operate in Level-1/Level-2 mode which supports both levels of routing. This makes IS-IS easier to configure but requires additional memory and processor on each device.

If only Level-1 or Level-2 routing is required on a specific device you can enable only the level required on the device by using the is-type [level-1 | level-2] command while in IS-IS router configuration mode.

It is also possible to disable a level of routing on a specific interface by using the isis circuit-type [level-1 | level-2 only] command while in interface configuration mode.

 in Cisco

