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To pass the BSCI exam and earn your CCNP, you've got to know ISIS inside and out. There are many similarities amongst ISIS and OSPF, but a single major distinction is that ISIS has 3 different sorts of routers - Level 1 (L1), Level two (L2), and L1/L2. L1 routers are contained in a single place, and are linked to other locations by an L1/L2 router. The L1 makes use of the L1/L2 router as a default gateway to reach destinations contained in other places, much like an OSPF stub router makes use of the ABR as a default gateway. L1 routers have no particular routing table entries regarding any destination outside their personal area they will use an L1/L2 router as a default gateway to reach any external networks. ISIS L1 routers in the exact same region ought to synchronize their databases with each other. Just as we have L1 routers, we also have L2 routers. Anytime we're routing amongst areas (inter-place routing), an L2 or L1/L2 router should 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 permit ISIS routers to type adjacencies. The important difference right 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 identical link, they will not form an adjacency. An ISIS router can act as an L1 and an L2 router at the very same time these routers are L1/L2 routers. An L1/L2 router can have neighbors in separate ISIS regions. The L1/L2 router will have two separate databases, though - a single for L1 routes and yet another 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 attainable for an L1 router to send information to an additional location. In the subsequent portion of my ISIS tutorial, we'll take a more comprehensive look at these ISIS hellos! alkaline water