H3C Technologies H3C SR8800 - Manual (Page 156)

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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.
Since a Level-1 router simply sends packets destined for other areas to the nearest Level-1-2 router, this
may cause that the best paths cannot be selected. To solve 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 types
IS-IS supports the following network types:
• Broadcast network, such as Ethernet and Token-Ring.
• Point-to-point network, such as PPP and HDLC.
NOTE:
For a Non-Broadcast Multi-Access (NBMA) interface, such as an ATM interface, you need to configure
subinterfaces for it and configure the interface type for the subinterfaces as point-to-point or broadcast.
IS-IS cannot run on point to multipoint (P2MP) links.
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 respectively. You can assign different priorities for 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 that:
• 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 level routers on a network including non-DIS routers establish
adjacencies with each other.