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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
138 Switch Management and Operating Concepts
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.
Building Adjacency
Two routers undergo a multi-step process in building the
adjacency relationship. The following is a simplified
description of the steps required: