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Chapter 4
OSPF Areas
Area border router An area border router (ABR) is a router that has
multiple area assignments. An interface may belong to only one area. If a
router has multiple interfaces and if any of these interfaces belong to dif-
ferent areas, the router is considered an ABR.
Autonomous system boundary router An autonomous system bound-
ary router (ASBR)
is a router with an interface connected to an external
network or a different AS. An external network or autonomous system
refers to an interface belonging to a different routing protocol, such as
EIGRP. An ASBR is responsible for injecting route information learned by
other Interior Gateway Protocols (IGPs) into OSPF.
Non-broadcast multi-access Non-broadcast multi-access (NMBA) net-
works are networks such as Frame Relay, X.25, and ATM. This type of
network allows for multi-access but has no broadcast ability like Ether-
net. NBMA networks require special OSPF configuration to function
Broadcast (multi-access) Networks such as Ethernet allow multiple
access as well as provide broadcast ability. A DR and BDR must be elected
for multi-access broadcast networks.
Point-to-point This type of network connection consists of a unique
NMBA configuration. The network can be configured using Frame Relay
and ATM to allow point-to-point connectivity. This configuration elimi-
nates the need for DRs or BDRs.
Router ID The Router ID is an IP address that is used to identify the
router. Cisco chooses the Router ID by using the highest IP address of all
configured loopback interfaces. If no loopback addresses are configured,
OSPF will choose the highest IP address of the functional physical
All of these terms play an important part in understanding the operation
of OSPF. You must come to know and understand each of these terms. As
you read through the chapter, you will be able to place the terms in their
proper context.
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