Designated Router Election; Building Adjacency - D-Link xStack DES-3800 Series User Manual

Neighbors
Routers that are connected to the same area or segment become neighbors in that area. Neighbors are elected via the Hello
protocol. IP multicast is used to send out Hello packets to other routers on the segment. Routers become neighbors when they see
themselves listed in a Hello packet sent by another router on the same segment. In this way, two-way communication is
guaranteed to be possible between any two neighbor routers.
Any two routers must meet the following conditions before the become neighbors:
Area ID − Two routers having a common segment − their interfaces have to belong to the same area on that segment. Of
•
course, the interfaces should belong to the same subnet and have the same subnet mask.
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 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.
xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
122