3Com 3C13636 Configuration Manual page 642

3Com Router 3000 Ethernet Family
Configuration Guide
ES
ES
ES
ES
IS
Figure 5-1 IS-IS topology
III. Routing leak
Generally speaking, an IS-IS area can also be called as a Level-1 area. Internal routes
are managed by Level-1 routers. All Level-2 routers form a Level-2 area. Thus, an IS-IS
routing domain may contain multiple Level-1 areas but only one Level-2 area.
A Level-1 area can only connect with a Level-2 area. Different Level-1 areas cannot
connect with each other.
Routing information inside a Level-1 area is advertised to the Level-2 area via
Level-1-2 routers. Therefore, Level-2 routers know routing information in the whole
IS-IS routing domain. However, Level-2 routers do not advertise the routing information
of other Level-1 areas and the Level-2 area they know to Level-1 area by default. Thus,
IS
IS
IS
IS
ES
Area 1
Routing Domain Boundary
IS-IS Area
End system
Intermediate system
Subnetwork Path
Interdomain Routing
Level 1 IS-IS Routing
3Com Corporation
Chapter 5 Integrated IS-IS Configuration
IS
Area 2
Routing Domain 1
Routing Domain 2
IS
ES
5-3
IS ES
IS
Area 3
IS
ES ES
Level 2 IS-IS Routing