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Configuring rip to advertise a summary route, Protocols and standards – H3C Technologies H3C S12500-X Series Switches User Manual

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65

BDR—Elected along with the DR to establish adjacencies with all other routers. If the DR fails, the

BDR immediately becomes the new DR, and other routers elect a new BDR.

Routers other than the DR and BDR are called "DROthers." They do not establish adjacencies with one

another, so the number of adjacencies is reduced.
The role of a router is subnet (or interface) specific. It might be a DR on one interface and a BDR or

DROther on another interface.
In

941H

Figure 20

, solid lines are Ethernet physical links, and dashed lines represent OSPF adjacencies. With

the DR and BDR, only seven adjacencies are established.

Figure 20 DR and BDR in a network

NOTE:

In OSPF, "neighbor" and "adjacency" are different concepts. After startup, OSPF sends a hello packet on
each OSPF interface. A receiving router checks parameters in the packet. If the parameters match its own,

the receiving router considers the sending router an OSPF neighbor. Two OSPF neighbors establish an

adjacency relationship after they synchronize their LSDBs through exchange of DD packets and LSAs.

489B

DR and BDR election

DR election is performed on broadcast or NBMA networks but not on P2P and P2MP networks.
Routers in a broadcast or NBMA network elect the DR and BDR by router priority and ID. Routers with a

router priority value higher than 0 are candidates for DR and BDR election.
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 router

priority wins. If router priorities are the same, the router with the higher router ID wins.
If a router with a higher router priority is added to the network after DR and BDR election, the router
cannot become the DR or BDR immediately as no DR election is performed for it. Therefore, the DR of a

network might not be the router with the highest priority, and the BDR might not be the router with the

second highest priority.

185B

Protocols and standards

RFC 1765, OSPF Database Overflow

RFC 2328, OSPF Version 2

RFC 3101, OSPF Not-So-Stubby Area (NSSA) Option

DR

BDR

DR other

DR other

DR other

Physical links

Adjacencies

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