Designated Router Election - D-Link DES-3326SR Manual

Neighbors
Routers that are connected to the same area or segment become neighbors in that area. Neighbors are elected via
the Hello protocol. IP multicast is used to send out Hello packets to other routers on the segment. Routers
become neighbors when they see themselves listed in a Hello packet sent by another router on the same segment.
In this way, two-way communication is guaranteed to be possible between any two neighbor routers.
Any two routers must meet the following conditions before the become neighbors:
• Area ID − two routers having a common segment − their interfaces have to belong to the same area on
that segment. Of course, the interfaces should belong to the same subnet and have the same subnet
mask.
• Authentication − OSPF allows for the configuration of a password for a specific area. Two routers on
the same segment and belonging to the same area must also have the same OSPF password before they
can become neighbors.
• Hello and Dead Intervals − The Hello interval specifies the length of time, in seconds, between the
hello packets that a router sends on an OSPF interface. The dead interval is the number of seconds that
a router's Hello packets have not been seen before its neighbors declare the OSPF router down. OSPF
routers exchange Hello packets on each segment in order to acknowledge each other's existence on a
segment and to elect a Designated Router on multi-access segments. OSPF requires these intervals to
be exactly the same between any two neighbors. If any of these intervals are different, these routers will
not become neighbors on a particular segment.
• Stub Area Flag − any two routers also have to have the same stub area flag in their Hello packets in
order to become neighbors.
Adjacencies
Adjacent routers go beyond the simple Hello exchange and participate in the link-state database exchange
process. OSPF elects one router as the Designated Router (DR) and a second router as the Backup Designated
Router (BDR) on each multi-access segment (the BDR is a backup in case of a DR failure). All other routers on
the segment will then contact the DR for link-state database updates and exchanges. This limits the bandwidth
required for link-state database updates.

Designated Router Election

The election of the DR and BDR is accomplished using the Hello protocol. The router with the highest OSPF
priority on a given multi-access segment will be com the DR for that segment. In case of a tie, the router with
the highest Router ID wins. The default OSPF priority is 1. A priority of zero indicates a router that can not be
elected as the DR.
D-Link DES-3326SR Layer 3 Switch
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