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To pass the BSCI exam and earn your CCNP, you have got to know ISIS inside and out.  There are several similarities amongst ISIS and OSPF, but one particular major difference is that ISIS has 3 different types of routers - Level 1 (L1), Level two (L2), and L1/L2.

L1 routers are contained in a single area, and are linked to other locations by an L1/L2 router.  The L1 uses the L1/L2 router as a default gateway to reach destinations contained in other places, much like an OSPF stub router uses the ABR as a default gateway.

L1 routers have no precise routing table entries relating to any destination outdoors their own region they will use an L1/L2 router as a default gateway to reach any external networks.  ISIS L1 routers in the identical location need to synchronize their databases with every other.

Just as we have L1 routers, we also have L2 routers.  Anytime we're routing in between locations (inter-area routing), an L2 or L1/L2 router need to be involved.  All L2 routers will have synchronized databases as effectively.

Both L1 and L2 routers send out their personal hellos.  As with OSPF, hello packets allow ISIS routers to form adjacencies.  The essential 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 exact same link, they will not type 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 locations.  The L1/L2 router will have two separate databases, even though - one for L1 routes and an additional for L2 routes.  L1/L2 is the default setting for Cisco routers running ISIS.  The L1/L2 router is the router that tends to make it feasible for an L1 router to send data to an additional location.

In the subsequent portion of my ISIS tutorial, we'll take a far more comprehensive look at those ISIS hellos! [http://www.ionizeroasis.com/dolphin-voc-filter.html advertisers] [http://www.ionizeroasis.com/life-7600-water-ionizer.html TM] [http://www.amazon.com/shops/AYG2PTRHXHGPF/ water ionizers reviews]