To pass the BSCI exam and earn your CCNP, you've got to know ISIS
inside and out. There are many similarities between ISIS and OSPF, but
one major difference is that ISIS has three different 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 stub router uses the ABR as a default gateway.
L1 routers have no specific routing table entries regarding any
destination outside their own area; they will use an L1/L2 router as a
default gateway to reach any external networks. ISIS L1 routers in the
same area must synchronize their databases with each other.
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. 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.
In the next part of my ISIS tutorial, we'll take a more detailed look at those ISIS hellos!