Designated Router Election - D-Link xStack DES-3800 Series User Manual
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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 become 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.
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Attempt − On non-broadcast multi-access networks (such as Frame Relay or X.25), this state
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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
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communication has not yet been established.
Two-way − Bi-directional communication with a neighbor has been established. The router
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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
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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
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description packets.
Loading − The routers are finalizing the information exchange. Routers have link-state
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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
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routers will have the same link-state database.
Adjacencies on Point-to-Point Interfaces
OSPF Routers that are linked using point-to-point interfaces (such as serial links) will always form
adjacencies. The concepts of DR and BDR are unnecessary.
OSPF Packet Formats
All OSPF packet types begin with a standard 24-byte header and there are five packet types. The
header is described first, and each packet type is described in a subsequent section.
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