3com 3031 Instruccion De Instalación

C

HAPTER

 40: OSPF C

ONFIGURATION

Differences between NBMA and p2mp:

■

In OSPF, NBMA refers to a network that is fully matched, non-broadcast and 
multi-accessible. While a p2mp network is unnecessarily fully matched.

■

On a NBMA network, it is necessary to elect DR and BDR; while a p2mp 
network has no DR and BDR.

■

NBMA is the default network type. For example, if ATM is adopted as the link 
layer protocol, OSPF defaults the network type on the interface to NBMA, 
regardless of whether the network is fully connected. In contrast, p2mp is not a 
default network type. No link layer protocol is regarded to be p2mp by default. 
A p2mp network must be compulsorily changed from other networks types. 
The common practice is to change a non fully-matched NBMA into a p2mp 
network.

■

NBMA forwards packets by unicast and neighbors shall be configured 
manually. While p2mp forwards packets by multicast.

Perform the following configuration in interface view.

After the interface has been configured with a new network type, the original 
network type of the interface is removed automatically.

For a NBMA network, OSPF router cannot discover the adjacent router through 
broadcasting the Hello packets, you must manually specify an IP address for the 
adjacent router, and whether the adjacent router is eligible for election.

Perform the following in the OSPF View.

By default, the preference for the neighbor of NBMA interface is 1.

The preference levels set with the 

 command and the 

 

command are used in different purpose:

■

The preference levels set with the 

 command are for real DR 

election.

The preference levels set with the 

 command indicate if the neighbors are 

authorized to elect. The preference level 0 means the neighbor is not authorized 
and no hello messages are sent to the neighbor. This is an effective way to reduce 
hollo packet number in electing DR and BDR. But if the local router is DR or BDR, it 

Table 565   Configuring network type of an OSPF interface

Configure network type 
of an OSPF interface

Table 566   Configuring the adjacent point

Configure a peer for the NBMA interface

peer ip-address [ dr-priority 
dr-priority-number ]

Remove the configured peer for the NBMA 
interface

undo peer ip-address