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6.1 | ![]() |
OSPF Overview | |
6.1.4 | ![]() |
OSPF network types |
Adjacency is required to allow for OSPF
routers to share routing information, a router will try to become
adjacent to at least one other router on each IP network to which it
is connected. Some routers may try to become adjacent to all their
neighbor routers, and others may try with only one or two. OSPF
routers determine which routers to become adjacent to based on what
type of network connects them.
OSPF interfaces automatically recognize
three types of networks: broadcast multiaccess, nonbroadcast
multiaccess (NBMA), and point-to-point networks.
The type of network dictates how OSPF routers should relate to each other. An administrator may have to override the detected network type in order for OSPF to operate properly. Some networks are defined as multiaccess because it cannot be predicted just how many routers are connected to them. There may be one, two, or more routers. A campus that uses a switched Ethernet core may have half a dozen routers connected to the same backbone network. A school district might have 10, 12, or 25 remote-site routers connected by way of Frame Relay PVCs to the same IP subnet. A significant number of routers can exist on a multiaccess network. The designers of OSPF developed a system to avoid the overhead that would be created if every router established full adjacency with every other router. This system restricts who can become adjacent to whom by employing the services of one of the following:
On point-to-point networks, only two nodes exist. Therefore, a focal point for routing information is not needed. No DR or BDR is elected. Both routers become fully adjacent to one and other.
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