330 CHAPTER 29: IS-IS CONFIGURATION
by configuring the routing hierarchy on the interface. For example, the level-1
interface can only establish Level-1 adjacency, while the level-2 interface can only
establish Level-2 adjacency.
By having this function, you can prevent the Level-1 hello packets from
propagating to the Level-2 backbone through the Lever-1-2 router. This can result
in bandwidth saving.
Route leaking
An IS-IS routing domain is comprised of only one Level-2 area and multiple Level-1
areas. A Level-1 area is connected with the Level-2 area rather than other Level-1
areas.
The routing information of the 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 with the
Level-1 area by default.
Since the Level-1 router simply sends the routing information for destinations
outside the area to the nearest Level-1-2 router, this may cause a problem that the
best path cannot be selected.
To solve this problem, route leaking was introduced. The Level-2 router can
advertise the Level-2 routing information to a specified Level-1 area. By having the
routing information of other areas, the Level-1 router can make a better routing
choice for the packets destined outside the area.
IS-IS Network Type Network type
IS-IS supports two network types:
■ Broadcast network, such as Ethernet, Token-Ring.
■ Point-to-point network, such as PPP, HDLC.
n
For the Non-Broadcast Multi-Access (NBMA) network, such as ATM, you need to
configure point-to-point or broadcast network on its configured subinterfaces.
IS-IS does not run on Point to Multipoint (P2MP) links.
DIS and pseudo nodes
On an IS-IS broadcast network, a router has to be selected as the Designated
Intermediate System (DIS).
The Level-1 and Level-2 DISs are selected respectively. You can assign different
priorities for different level DIS selections. 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 (which is the MAC address on a broadcast network) will be
selected. A router can be the DIS for different levels.
As shown in Figure 102, the same level routers on the same network segment can
establish adjacencies. This is different from OSPF.
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