3com S7906E Manuel De Montage

 

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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. 

 

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 

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

adjacencies with each other.  

Figure 1-4 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.