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Chapter 4
OSPF Areas
F I G U R E 4 . 4
NBMA broadcast implementation
This configuration guarantees that all routers have connectivity and that
all will be able to participate in the DR/BDR election process. Once the DR
and BDR have been chosen, the meshed networks act as a broadcast net-
work. All LSA updates are sent to the DR and BDR, and the DR floods the
updates out every interface.
One of the major weaknesses with this configuration is that if one of the
PVCs fails (especially if it is a PVC between a DROther and the DR), then
communication is also halted between the two adjacent peers.
This environment requires that all OSPF neighbors be manually config-
ured. This is the default setting for the router. By manually configuring each
neighbor, OSPF knows exactly which neighbors need to participate and
which neighbor is identified as the DR. Also, communication between neigh-
bors is done via unicast instead of multicast. This configuration also requires
a full mesh and has the same weakness as the broadcast environment.
This environment uses subinterfaces on the physical interface to create point-
to-point connections with other OSPF neighbors. No DR or BDR is elected
since the link is treated as a point-to-point circuit. This allows for faster
Copyright ©2001 SYBEX , Inc., Alameda, CA