Given the basics covered in part I, IS-IS configuration isn’t that hard. It already clearly shows some differences with OSPF, but it’s when using multiple areas that there is a clear distinction in logic.

OSPF-Areas

First a small recap of OSPF areas: you have a backbone area, area 0, to which all other areas must connect. A router can be in multiple areas, an interface can be in only one area for a given OSPF process. Routes between areas are known by default, but setting an area to stub can change this to just a default route.

ISIS-Areas

IS-IS is different: as you may have guessed by the ‘net’ command of part I, a router can only be part of one area. Area borders are between routers. An area is made up of routers with level 1 neighborships. A router with a level 2 neighborship towards another router is considered a backbone router. Since level 2 neighborships can be between routers in different areas (the second part of ‘net’ command can differ), these routers connect areas.

The moment a router has a level 2 neighborship and becomes a backbone router, it will automatically propagate a default route towards its level 1 neighbors. This gets flooded throughout the area. To reach another area, packets will be sent automatically towards the nearest backbone router. The Backbone router has a second topology table for level 2 that lists information of all subnets in all areas (which requires more memory). The packet will then be transported over the backbone to the appropriate area. For this reason, the backbone must be continuous: otherwise there would be multiple islands of routers propagating default routes.

From that point of view, the level 2 backbone becomes an overlay on top of the areas that connects everything: an extra ‘level’, likely the reason for the terminology. While this design works and is very scalable it may introduce suboptimal routing. Inter-area traffic will go to the nearest backbone router, but there may be other backbone routers in the area that can route the packets to the destination in a better way. For example, in the above image, the bottom router in the purple middle area may decide to follow the default route to the left backbone router for a packet destined for the right blue area.

Configuration is still straightforward:

Router(config)#interface GigabitEthernet0/1
Router(config-int)#ip address 10.0.2.5 255.255.255.252
Router(config-int)#ip router isis
Router(config-int)#isis circuit-type level-1
Router(config-int)#exit
Router(config)#interface GigabitEthernet0/2
Router(config-int)#ip address 10.0.3.1 255.255.255.252
Router(config-int)#ip router isis
Router(config-int)#isis circuit-type level-2-only
Router(config-int)#exit
Router(config)#interface GigabitEthernet0/3
Router(config-int)#ip address 10.0.2.9 255.255.255.252
Router(config-int)#ip router isis
Router(config-int)#
Router(config-int)#exit
Router(config)#router isis
Router(config-router)#log-adjacency-changes
Router(config-router)#net 49.0001.0000.0000.0008.00

This example configures a router for a level 1 neighborship on Gi0/1 (inside the area), a level 2 neighborship on Gi0/2 (between areas) and a level 1 & 2 neighborship on Gi0/3 (inside the area, but still backbone). Note the missing ‘is-type’ command in the routing process, which makes the router default to both a level 1 and level 2 router. A router in another area has a different area number in the net command:

Router(config)#interface GigabitEthernet0/2
Router(config-int)#ip address 10.0.3.2 255.255.255.252
Router(config-int)#ip router isis
Router(config-int)#isis circuit-type level-2-only
Router(config-int)#exit
Router(config)#router isis
Router(config-router)#log-adjacency-changes
Router(config-router)#net 49.0002.0000.0000.0009.00

Note that an IS-IS router is not required to have a level 1 neighborship. It is possible to have a ‘pure’ backbone router with only level 2 neighborships, which makes the router only use one topology table again, just like a level 1-only router.

The topology tables for both levels can be checked with show isis topology l1 and show isis topology l2. Same for the database, just replace the word ‘topology’ with ‘database’. The show clns is-neighbors and show isis neighbors commands both show all IS-IS neighbors and the level of the neighborship.

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