Is-is network types, Network types, Dis and pseudonodes – H3C Technologies H3C S5560 Series Switches User Manual

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sends packets destined for other areas to the nearest Level-1-2 router. The path passing through the

Level-1-2 router might not be the best. To solve this problem, IS-IS provides the route leaking feature.
Route leaking enables a Level-1-2 router to advertise the routes of other Level-1 areas and the Level-2 area

to the connected Level-1 area so that the Level-1 routers can select the optimal routes for packets.

IS-IS network types

Network types

IS-IS supports broadcast networks (for example, Ethernet and Token Ring) and point-to-point networks

(for example, PPP and HDLC).
For an NBMA interface, such as an ATM interface, you must configure point-to-point or broadcast
subinterfaces. IS-IS cannot run on P2MP links.

DIS and pseudonodes

IS-IS routers on a broadcast network must elect a DIS.
The Level-1 and Level-2 DISs are elected separately. You can assign different priorities to a router for

different level DIS elections. The higher the router priority, the more likely the router becomes the DIS. If
multiple routers with the same highest DIS priority exist, the one with the highest Subnetwork Point of

Attachment (SNPA) address will be elected. On a broadcast network, the SNPA address is the MAC

address. 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 with a higher priority is added to the network, an LSP flooding process is performed
to elect the router as the new DIS.

As shown in

Figure 36

, the same level routers on a network, including non-DIS routers, establish

adjacency with each other.

Figure 36 DIS in the IS-IS broadcast network

The DIS creates and updates pseudonodes, and generates LSPs for the pseudonodes, 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 1-byte Circuit ID (a non-zero value).
Using pseudonodes simplifies network topology and can reduce the amount of resources consumed by

SPF.

L1

L2

L1/L2

L1/L2

DIS

DIS

L1 adjacencies

L2 adjacencies

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