Similar and level. 2 (Kaur & Kumar,

to OSPF, IS–IS also supports hierarchical network design allowing a larger
network to be divided into logical divisions called areas. However, there is a
difference between how areas are configured for both routing protocols. Whereas
in OSPF a backbone area is configured and used for connecting other areas,
IS–IS does not include a backbone area. The routers are grouped into a
hierarchy called levels and are used to manage communication between areas. The
levels are simply routers or intermediate systems that are configured to manage
communication within areas and between areas. There are two router levels
defined in the hierarchy. These are level 1 and level. 2 (Kaur & Kumar,

Level 1
(L1) routers are the same as internal routers used in OSPF areas. They all have
their interfaces connected within the same area. All L1 routers exchange
routing information belonging to a specific area. On the other hand, Level 2
routers are used to connect different areas. They are similar to ABRs used in
OSPF. An L2 router is not required to identify the topology within level 1 area
but there is a possibility that an L2 router can be an L1 router in a single
area (Roussinos, 2014). Figure 2.8 shows the IS–IS area structure and all the
router types used to manage communication within the IS–IS routing domain.

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DIS is
the same as DR used in OSPF. However, there is a slight difference between the
two. In OSPF, once the DR and BDR are chosen, all the other routers establish
adjacency relationship with the DR and the BDR so that when the DR fails, the
BDR will become the DR. In IS–IS, all the routers in the broadcast medium form
adjacent relationships with other routers and the DIS. When the DIS fails, any
router can take over as the new DIS. Election of a DIS is based on router
priority. Thus a router with the highest priority is elected as the DIS. If all
the routers have the same priority, MAC address is used as the tiebreaker
(Empson, 2007).




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