3com 5500-ei pwr Instrução De Instalação

 

4-11 

solve this problem, DR is defined in OSPF so that all routers send information to the DR only and the 

DR broadcasts the network link states in the network. 

If the DR fails, a new DR must be elected and synchronized with the other routers on the network. The 

process takes quite a long time; in the process, route calculation is incorrect. To shorten the process, 

BDR is introduced in OSPF.  

A BDR provides backup for a DR. DR and BDR are elected at the same time. Adjacencies are also 

established between the BDR and all the other routers on the segment, and routing information is also 

exchanged between them. Once the DR becomes invalid, the BDR becomes a DR. Since no re-election 

is needed and the adjacencies already exist, the switchover process is very short. Now, a new BDR 

should be elected. Although this election process will also take quite a long time, route calculation will 

not be affected. 

On an OSPF network, a router which is neither DR nor BDR is called DR Other. It establishes 

adjacencies with the DR and BDR, but not with other DR Others. This reduces not only the number of 

adjacencies among routers, but also the network traffic and occupied bandwidth resources on the 

broadcast or NBMA network. 

, the solid lines represent physical Ethernet connections and the dotted lines represent adjacencies 

established. The figure shows that, with the DR/BDR mechanism adopted, seven adjacencies suffice 

among the five routers. 

Figure 4-7 DR/BDR 

DR

BDR

DRother

DRother

DRother

 

 

The DR and BDR in a network are elected by all routers rather than configured manually. The DR 

priority of an interface determines its qualification for DR/BDR election. Interfaces attached to the 

network and having priorities higher than ‘0” are election candidates.  

The election votes are hello packets. Each router sends the DR elected by itself in a hello packet to all 

the other routers. If two routers on the network declare themselves as the DR, the router with the higher 

DR priority wins. If DR priorities are the same, the router with the higher Router ID wins. In addition, a 

router with the priority 0 cannot become the DR/BDR. 

Note the following points: 

z 

DR election is required for broadcast or NBMA interfaces but is not required for P2P or P2MP 

interfaces.