KTI KGS-3120, Руководство по эксплуатации, Страница: 113 / 221

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border routers that have a common area, with one border router connected to the backbone.
Partitioning the Backbone
OSPF also allows virtual links to be configured to connect the parts of the backbone that are discon-
tinuous. 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 be-
tween 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 seg-
ments. 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 ex-
change 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.