H3C S7500 Series Ethernet Switches Operation Manual

Operation Manual – Routing Protocol
H3C S7500 Series Ethernet Switches Chapter 5 IS-IS Configuration
5-3
A router that functions as a Level-1 and a Level-2 router is called a Level-1-2 router. It
can form the Level-1 neighbor relationship with the Level-1 and Level-1-2 routers in
the same area, or form Level-2 neighbor relationship with the Level-2 and Level-1-2
routers in different areas. A Level-1 router must be connected to other areas through a
Level-1-2 router. The Level-1-2 router maintains two LSDBs, where the Level-1 LSDB
is for routing within the area, and the Level-2 LSDB is for routing between areas.
 Note:
z The Level-1 routers in different areas can not form the neighbor relationship.
z Level-2 routers can reside in different areas.
Figure 5-1 shows a network topology running the IS-IS protocol. It is similar to the
multiple-area OSPF topology. Area 1 is a set of the Level-2 routers, called backbone
network. The other 4 areas are non-backbone networks connected to the backbone
through Level-1-2 routers.
Area 1
Area 3
Area 5
Area 4
Area 2
L1
L1/L2
L2
L2
L2
L1
L1/L2
L1/L2
L2
L1 L1
L1/L2
L1 L1
Figure 5-1 An example of the IS-IS topology I
Figure 5-2 shows another network topology running the IS-IS protocol. The Level-1-2
routers connect the Level-1 and Level-2 routers, and also forms the IS-IS backbone
together with the Level-2 routers. There is no area defined as the backbone in this
topology. The backbone is composed of all contiguous Level-2 routers which can
reside in different areas.