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This chapter 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
and therefore will try to elect a DR and a BDR. Common NBMA
implementations include Frame Relay, X.25, and SMDS. NBMA networks
follow rules at Layer 2 that prevent the delivery of broadcasts and
multicasts. Figure
summarizes the OSPF network types.
NBMA networks can create problems with
OSPF operation, specifically with the exchange of multicast hello
packets. In the example shown in Figure , RTA, RTB, and RTC belong
to the same IP subnetwork and will attempt to elect a DR and a BDR.
However, these routers can not hold a valid election if they can not receive
multicast hellos from every other router on the network. Without
administrative intervention, a strange election takes place. As far as
RTA is concerned, RTC is not participating. Likewise, RTC goes through
the election process oblivious to RTA. This botched election can lead
to problems if the central router, RTB, is not elected the DR.
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 to you depend on
your NBMA network topology.
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