Dr And Bdr - 3Com S7906E Configuration Manual

An NBMA network is fully meshed, which means any two routers in the NBMA network have a direct
virtual link for communication. If direct connections are not available between some routers, the type of
interfaces associated should be configured as P2MP, or as P2P for interfaces with only one neighbor.
Differences between NBMA and P2MP networks:
NBMA networks are fully meshed, non-broadcast and multi access. P2MP networks are not
required to be fully meshed.
It is required to elect the DR and BDR on NBMA networks, while DR and BDR are not available on
P2MP networks.
NBMA is the default network type, while P2MP is a conversion from other network types, such as
NBMA in general.
On NBMA networks, packets are unicast, and neighbors are configured manually on routers. On
P2MP networks, packets are multicast.

DR and BDR

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.
Figure 1-7 DR and BDR in a network
DR
DRother
BDR
DRother DRother
1-9