Building Adjacency - D-Link DGS-3324SR User Manual

D-Link DGS-3324SR Layer 3 Stackable Gigabit Ethernet Switch
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.

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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