IS-IS summarization

IS-IS summarization
You can see from Figure 11.9 that IS-IS is summarizing into the Level 2, or backbone, area
from the Level 1 area. You might notice that in OSPF a router can be in two areas, but in IS-IS
a router can be in only one area. The transition from one area to another happens on the link
between routers and not within the router itself.
Areas in IS-IS are what we call in OSPF totally stubby, meaning that the only routes within
an area are routes contained in the area and a default route. Because IS-IS has natural totally
stubby areas, the amount of system resources needed for a Level 1 router is smaller and can
mean that you can have many routers within a single Level 1 area. This is a good thing, but it
could lead to suboptimal routing because each Level 1 router will send traffic to the nearest
Level 2 router to exit the area. This router may not be the closest to the traffic’s final destination.
To attempt to combat the suboptimal routing problem, IS-IS has a feature called route leaking.
Route leaking occurs when Level 2 routes are sent, or leaked, into a Level 1 area from the
Level 2 router. This allows the Level 1 router to make a more informed routing decision regarding
the leaked routes. It will send traffic to the Level 2 router that is closest to the destination,
and not the closest Level 2 router.