H3C Technologies H3C S7500E Series Switches - Manual (Page 151)

5-6
Route leaking
An IS-IS routing domain is comprised of only one Level-2 area and multiple Level-1 areas. A Level-1
area consists of a group of Level-1 routers and is connected with a Level-2 area rather than other
Level-1 areas.
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 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 type
IS-IS supports two network types:
z Broadcast network, such as Ethernet, Token-Ring.
z Point-to-point network, such as PPP, HDLC.
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 likelihood the router becomes the DIS. If
there are multiple routers with the same highest DIS priority, 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:
z A router with priority 0 can also participate in the DIS election.
z When a router is added to the network and becomes the new DIS, an LDP flooding process is
triggered.
As shown in Figure 5-4 , the same level routers on a network including non-DIS routers establish
adjacencies with each other.