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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
• 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 elected 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 cannot 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:
• Down − No information has been received from any router on the segment.
• Attempt − On non-broadcast multi-access networks (such as Frame Relay or X.25), this
state indicates that no recent information has been received from the neighbor. An effort
should be made to contact the neighbor by sending Hello packets at the reduced rate set
by the Poll Interval.
• Init − The interface has detected a Hello packet coming from a neighbor but bi-
directional communication has not yet been established.
• Two-way − Bi-directional communication with a neighbor has been established. The
router has seen its address in the Hello packets coming from a neighbor. At the end of
this stage the DR and BDR election would have been done. At the end of the Two-way
stage, routers will decide whether to proceed in building an adjacency or not. The
decision is based on whether one of the routers is a DR or a BDR or the link is a point-
to-point or virtual link.
• Exstart − (Exchange Start) Routers establish the initial sequence number that is going to
be used in the information exchange packets. The sequence number insures that routers
always get the most recent information. One router will become the primary and the
other will become secondary. The primary router will poll the secondary for information.
• Exchange − Routers will describe their entire link-state database by sending database
description packets.
• Loading − The routers are finalizing the information exchange. Routers have link-state
request list and a link-state retransmission list. Any information that looks incomplete or
outdated will be put on the request list. Any update that is sent will be put on the
retransmission list until it gets acknowledged.
• Full − The adjacency is now complete. The neighboring routers are fully adjacent.
Adjacent routers will have the same link-state database.
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