D-Link xStack Reference Manual page 211

®
xStack DGS-3620 Series Layer 3 Managed Stackable Gigabit Switch Web UI Reference Guide
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 must 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 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.
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.
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.
All OSPF packets (except for Hello packets) forward link-state advertisements. Link-State Update packets, for
example, flood advertisements throughout the OSPF routing domain.
OSPF packet header
Hello packet
210