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6.4 | ![]() |
Configuring OSPF Over NBMA | |
6.4.1 | ![]() |
NBMA overview |
This module has focused on two types of
OSPF networks in detail, broadcast multiaccess and point-to-point
networks. Even if there is only one router, broadcast multiaccess
networks elect a DR and a BDR to serve as focal points for routing
information. In contrast, point-to-point OSPF networks do not elect a
DR because they can never include more than two nodes.
Another type of OSPF network,
Nonbroadcast Multiaccess (NBMA), can include more than two nodes as
shown in Figure
NBMA networks can create problems with
OSPF operation, specifically with the exchange of multicast Hello
packets. RTA, RTB, and RTC belong to the same IP subnetwork and will
attempt to elect a DR and a BDR.
The Cisco IOS offers several options for configuring OSPF to overcome NBMA limitations, including the OSPF neighbor command, point-to-point subinterfaces, and point-to-multipoint configuration. The solutions that are available depend on the current NBMA network topology.
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