3Com 10014299 manual Specify the Router Priority, Ospf Dr-priority value, Undo Ospf dr-priority

Configuring OSPF 429

Specifying the Router It is necessary to establish the peer relationship manually between interfaces for Priority multi-point access network, (NBMA and broadcast type networks). But

establishing peer relationshipoccupies large amounts of system resources when there are hundreds of routers in the network. To resolve this issue, OSPF specifies a “designated router” (DR). All routers within the same network segment send the relationship information to the DR, which broadcasts the link status of each network segment. In this way, the number of the peer relationships between different routers on the multi-access network is significantly reduced.

The priority of a router interface determines the qualifications of the interface in voting for the DR. The interface with a higher priority is considered first when the voting rights conflict.

The DR is not designated manually, but voted by all routers in the local network segment. The routers of Priority>0 in the local network segment can be used as the “candidates”. The router with the greatest priority value is selected among all routers that claim to be DR. If two routers have the same priority, the one with greater router ID is selected. Routers vote by Hello packet. Each router writes the DR into the Hello packet and sends it to all other routers on the network segment. When two routers in the same network segment claim to be the DR, the one with the higher priority is chosen. If the priorities are equivalent, the one with higher router ID is chosen. If the priority of a router is 0, it is not selected as the DR or “backup designated router” (BDR).

If a DR fails due to a specific fault, a new DR must be elected, with synchronization. This can take a long time, during which, the route calculation is not correct. To shorten the process, OSPF puts forward the concept of the “backup designated router” (BDR). The BDR is actually a standby for the DR and is voted together with DR. The BDR also creates relations with all neighboring routers in the network segment and exchanges routing information with them. When the DR fails, the BDR becomes the DR immediately without the need for re-election. Because the neighboring relationship is already created, this takeover process is instantaneous. Of course a new BDR needs to be elected again but during the election, the route calculation is not affected.

It should be noted that:

■The DR in the network segment is not necessarily the router with the highest priority. Similarly, the BDR is not necessarily the router with the second highest priority.

■The DR is a role in a single network segment, based on the router interface. A router can be a DR on one interface and a BDR or DROther on another interface.

■The DR is elected on a broadcast interface or NBMA interface. It is not necessary on a point-to-point interface or point-to-multipoint interface.

Perform the following configurations in interface view.

Table 493 Specify the Router Priority

Operation	Command
Set the priority of the interface when	ospf	dr-priority value
selecting a designated router
Return to the default router priority	undo	ospf dr-priority