IS-IS areas

FIGURE 7 . 2
IS-IS areas
The first difference you should have noticed between the OSPF network and the IS-IS
network is that the area boundaries for IS-IS are on the connections, not the routers. You
should have also noticed that the routers are all completely within an area; the routers do
not have some interfaces in one area and other interfaces in another area. Another important
item to note about IS-IS is that the backbone can have any area ID. It is not limited to
Area 0 like OSPF.
ABR ABR
ABR
Area 1 Area 0 Area 2
Area 3
L1
L1
L1 L1
L1
L1
L2
L1/L2 L1/L2
L1/L2
Backbone
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Chapter 7 
Integrated IS-IS
Now you may be wondering what all the L1, L2, and L1/L2 routers are. This would be a
good time to explain these different types of routers:
L1 routers
A
Level 1 (L1) router
is a router in a non-backbone area. L1 routers know only
about intra-area routes. All they know about inter-area routes is a default route to the L1/L2
router for the area. All routers within a Level 1 area contain the same link-state database.
These routers receive link-state PDUs (LSPs) only from within the area. They will not receive
LSPs from other areas. L1 routers will not receive any information from an L2 router. The L1
router would be the equivalent of an internal router for OSPF.
L2 routers
Level 2 (L2) routers
are the
backbone routers
. They handle all of the inter-area
traffic. An L2 router can belong only to the backbone area. L2 routers will send LSPs to all other
L2 routers and to all L1/L2 routers, regardless of the area the L1/L2 router belongs to. The L2
router can be compared to a backbone router for OSPF.
L1/L2 routers
Level 1/Level 2 (L1/L2) routers
are similar in function to an OSPF ABR. L1/L2
routers will send LSPs to both L1 and L2 routers. The LSPs that the L1/L2 router sends to L1 routers
help it to maintain its Level 1 link-state database. The LSPs that the L1/L2 router sends to L2
routers help it to maintain its Level 2 link-state database. The L1/L2 router contains two linkstate
databases, and information stored in the Level 2 link-state database is not shared with any
L1 routers.
An IS-IS Level 1 area is very similar to an OSPF totally stubby area. This is because all the
L1 routers within the area know only about each other. If they need to reach a network not contained
within the area, they must communicate through the L1/L2 router.
Now that we’ve covered the routers, it’s a good time to talk about the three different levels
of routing for IS-IS:
Level 1 routing
Level 1 routing
is routing between intermediate systems within the same area.
Basically, Level 1 routing is intra-area routing, and it occurs between all routers contained
within the same area.
Level 2 routing
Level 2 routing
occurs between intermediate systems in different areas. All Level 2
routing will cross the backbone at some point. You can think of Level 2 routing as inter-area routing,
and it occurs between routers in different IS-IS areas.
Level 3 routing
Level 3 routing
is routing between different routing domains. This type of
routing will occur when traffic needs to leave the IS-IS routing domain to reach another routing
domain. Level 3 routing is also known as internetwork routing.
Because IS-IS routers are totally contained in one area, the area ID is associated with the
entire router, per routing process, instead of with an interface as is the case with OSPF. IS-IS
allows for up to three area IDs to be associated with one routing process. The main use of multiple
areas being configured on a device is for migrating from one area to another.
You now need to learn how to create an area and how to uniquely identify a router in that
area. This is accomplished through what is known as the network entity title (NET).