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The routing information of a Level-1 area is sent to the Level-2 area through the Level-1-2 router; therefore,
the Level-2 router knows the routing information of the entire IS-IS routing domain. But the Level-1-2 router
does not share the information of other Level-1 areas and the Level-2 area with the Level-1 area by default.
Because a Level-1 router simply sends packets destined for other areas to the nearest Level-1-2 router, the
best paths may not be selected. To resolve this problem, route leaking was introduced. A Level-2 router
can advertise Level-2 routing information to a specified Level-1 area. By having the routing information of
other areas, a Level-1 router in the area can make a better routing decision for a packet to another area.
IS-IS network type
Network type
IS-IS supports the following network types:
• Broadcast network, such as Ethernet and Token-Ring
• Point-to-point network, such as PPP and HDLC
DIS and pseudonodes
On an IS-IS broadcast network, a router is elected as the Designated Intermediate System (DIS).
The Level-1 and Level-2 DISs are elected. You can assign different priorities to different level DIS elections.
The higher a router's priority is, the more likely the router becomes the DIS. If multiple routers with the
same highest DIS priority exist, the one with the highest SNPA (Subnetwork Point of Attachment) address
(MAC address on a broadcast network) will be elected. A router can be the DIS for different levels.
IS-IS DIS election differs from OSPF DIS election in the following ways:
• A router with priority 0 can also participate in the DIS election.
• When a router is added to the network and becomes the new DIS, an LDP flooding process is
triggered.
As shown in Figure 52, the same le
vel routers on a network, including non-DIS routers, establish
adjacencies with each other.
Figure 52 DIS in the IS-IS broadcast network
The DIS creates and updates pseudonodes, as well as generates their LSPs, to describe all routers on the
network.
A pseudonode represents a virtual node on the broadcast network. It is not a real router. In IS-IS, it is
identified by the system ID of the DIS and a one-byte Circuit ID (a non zero value).
Using pseudonodes can reduce the resources consumed by SPF and simplify network topology.
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