Qtech QSW-3900-48-SFP-DC [102/245] Dr and bdr

QTECH Software Configuration Manual

8-101

8.1.4 DR and BDR

8.1.4.1 DR/BDR introduction

On broadcast or NBMA networks, any two routers exchange routing information with each other. If n

routers are present on a network, n(n-1)/2 adjacencies are required. Any change on a router in the network generates

traffic for routing information synchronization, consuming network resources. The Designated Router is defined to

solve the problem. All other routers on the network send routing information to the DR, which is responsible for

advertising link state information.

If the DR fails to work, routers on the network have to elect another DR and synchronize information with

the new DR. It is time-consuming and prone to routing calculation errors. The Backup Designated Router (BDR) is

introduced to reduce the synchronization period.

The BDR is elected along with the DR and establishes adjacencies for routing information exchange with

all other routers. When the DR fails, the BDR will become the new DR in a very short period by avoiding adjacency

establishment and DR reelection. Meanwhile, other routers elect another BDR, which requires a relatively long

period but has no influence on routing calculation.

Other routers, also known as DRothers, establish no adjacency and exchange no routing information with

each other, thus reducing the number of adjacencies on broadcast and NBMA networks.

In the following figure, real lines are Ethernet physical links, and dashed lines represent adjacencies. With

the DR and BDR in the network, only seven adjacencies are enough.

DR and BDR in a network

8.1.4.2 DR/BDR election

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 that :

· The DR election is available on broadcast, NBMA interfaces rather than P2P, or P2MP

interfaces.

· A DR is an interface of a router and belongs to a single network segment. The router’s