CCNP Certification BSCI Exam Tutorial: ISIS Router Types

By Chris Bryant, CCIE #12933

OSPF and ISIS are both link state protocols, but the two protocols have as many differences as they do similarities! ISIS has router types that can somewhat mirror what OSPF does with stub and total stub areas, and in this tutorial we'll examine each of these router types.

ISIS has three types of routers; Level 1 (L1), Level 2(L2), and L1/L2. L1 routers are contained in a single area, and are connected to other areas by an L1/L2 router.  The L1 uses the L1/L2 router as a default gateway to reach destinations contained in other areas, much like an OSPF total stub router uses the ABR as a default gateway.

Here, all routers except R2 are L1 routers.  L1 routers are intra-area routers and act much like an OSPF total stub router.  They don't have any specific routing table entries regarding any destination outside their own area; they will use R2 as a default gateway to reach any external networks.  ISIS L1 routers must have synchronized databases.

Just as we have L1 routers, we also have L2 routers.  Anytime we're routing between areas (inter-area routing), an L2 or L1/L2 router must be involved.  All L2 routers will have synchronized databases as well. 

Both L1 and L2 routers send out their own hellos.  As with OSPF, hello packets allow ISIS routers to form adjacencies.  The key difference here is that L1 routers send out L1 hellos, and L2 routers send out L2 hellos.  If you have an L1 router and an L2 router on the same link, they will not form an adjacency.

An ISIS router can act as an L1 and an L2 router at the same time; these routers are L1/L2 routers.  An L1/L2 router can have neighbors in separate ISIS areas, as R2 does in the example above.  The L1/L2 router will have two separate databases, though - one for L1 routes and another for L2 routes.  L1/L2 is the default setting for Cisco routers running ISIS.  The L1/L2 router is the router that makes it possible for an L1 router to send data to another area.

As I mentioned, we not only have different ISIS router types, we have different ISIS Hello packets as well! We'll take a look at those in a future CCNP certification tutorial. See you then!

To your success,

Chris Bryant

CCIE #12933

chris@thebryantadvantage.com