Ospf Packets - H3C S7500 Series Operation Manual

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Operation Manual – Routing Protocol

H3C S7500 Series Ethernet Switches

Figure 4-2 DR and BDR

IV. DR/BDR election

Instead of being manually configured, DR and BDR are elected by all the routers on

the current network segment. The priority of a router interface determines the

qualification of the interface in DR/BDR election. All the routers with DR priorities

greater than 0 on the current network segment are eligible "candidates".

Hello packets serve as the "votes" in the election. Each router writes the DR it selects

into the Hello packet and sends the packet to each router running OSPF on the

network segment. If two routers on the same network segment declare themselves to

be the DR, the one with the highest DR priority will be preferred. If their priorities are

the same, the one with greater router ID will be preferred. A router whose DR priority

is 0 can neither be elected as the DR nor be elected as the BDR.

Note the following points:

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

P2P or P2MP interfaces.

DR is based on the router interfaces in a certain segment. A router may be a DR

on one interface and a BDR or DR Other on another interface.

If a new router is added after DR and BDR election, the router does not become

the DR immediately even if it has the highest DR priority.

The DR on a network segment is not necessarily the router with the highest

priority. Likewise, the BDR is not necessarily the router with the second-highest

4.1.5 OSPF Packets

OSPF uses five types of packets:

Hello packet

Hello packets are most commonly used OSPF packets, which are periodically sent by

a router to its neighbors. A Hello packet contains the values of some timers, the DR,

the BDR and the known peers.

Chapter 4 OSPF Configuration

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