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Chapter 14 OSPF Configuration
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However, in NBMA networks and virtual links, Hello packets are
unicast to neighbor routers.
OSPF uses three types of timers related to Hello packets:
1. Call interval
Call interval is an attribute of an interface, which defines at
which interval a router sends a Hello packet out each
interface. The default call interval depends on network type.
In broadcast and point-to-point networks, the default call
interval is 10s. In NBMA and point-to-multipoint networks,
the default call interval is 30s. Two adjacent routers must
agree with call interval to become neighbors.
2. Dead interval of router
A router dead interval refers to a time interval between
receiving of last Hello packet from its neighbor and detection
of offline status of neighbor. Default dead interval is four
times the call interval (the same is true for all types of
networks).
3. Polling interval
Polling interval is only used in NBMA networks.
OSPF neighbors are a group of routers on same network. These
routers have agreed with some configuration parameters. To
form adjacency, routers must be neighbors.
To form adjacency, routers must analyze Hello packets of each
other to confirm whether they have agreed on necessary
parameters. The parameters are as follows: area ID, area tag,
authentication information, call interval and dead interval of
router.
When two routers form an adjacent relation, they can exchange
routing information. Whether two routers can form an adjacent
relation depends upon network type connected to routers.
Point-to-point networks and virtual links only have two routers,
so routers form adjacency automatically. A point-to-multipoint
network can be regarded as a set of point-to-point networks, so
adjacency is formed between each pair of routers.
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In a broadcast or an NBMA network, adjacency may not be
formed between two neighbors. If adjacency has been set up
among all routers (the number of routers is "n" for example)
on a network, each router will have "(n-1)" adjacent
connections and the network will have "n (n-1)/2" adjacent
connections. In a large multi-access network, if each router
traces so many neighbor routers, the router will have too
heavy traffic, and furthermore, the routing information
between each neighboring routers will waste many network
bandwidth.
Therefore, OSPF defines a Designated Router (DR) and a Backup
Designated Router (BDR). The DR and BDR must form an
adjacent relation with each OSPF router on network and each
OSPF
Neighbors
Adjacency and
Router
Designation
NBMA Network
Designated
Router (DR)