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Partitioning the Backbone
OSPF also allows virtual links to be configured to connect the parts of the backbone that are discontinuous. This is the
equivalent to linking different area 0s together using a logical path between each area 0. Virtual links can also be
added for redundancy to protect against a router failure. A virtual link is configured between two border routers that
both have a connection to their respective area 0s.
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 they become neighbors:
1. Area ID - Two routers having a common segment
that segment. Of course, the interfaces should belong to the same subnet and have the same subnet
mask.
2. 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.
3. 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.
4. 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
DES-3810 Series Layer 3 Managed Ethernet Switch Web UI Reference Guide
their interfaces have to belong to the same area on
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