Configuring the Open Shortest Path First Protocol
20-19
20
Configuring OSPF Areas
OSPF protocol broadcast messages (that is, Link State Advertisements or LSAs) are
restricted by area to limit their impact on network performance. A large network
should be split up into separate OSPF areas to increase network stability, and to
reduce protocol traffic by summarizing routing information into more compact
messages. Each router in an area shares the same view of the network topology,
including area links, route summaries for directly connected areas, and external
links to other areas.
Use the Area Configuration screen to define an OSPF area and the interfaces that
operate within this area. An autonomous system must be configured with a
backbone area, designated by area identifier 0.0.0.0. By default, all other areas are
created as normal transit areas.
Routers in a normal area may import or export routing information about individual
nodes. To reduce the amount of routing traffic flooded onto the network, you can
configure an area to export a single summarized route that covers a broad range of
network addresses within the area (page 20-23). To further reduce the amount of
routes passed between areas, you can configure an area as a stub or a
not-so-stubby area (NSSA).
Normal Area
– A large OSPF domain should be
broken up into several areas to increase network
stability and reduce the amount of routing traffic
required through the use of route summaries that
aggregate a range of addresses into a single
route. The backbone or any normal area can pass
traffic between other areas, and are therefore
known as transit areas. Each router in an area
has identical routing tables. These tables may
include area links, summarized links, or external
links that depict the topology of the autonomous
system.
Stub
– A stub does not accept external routing
information. Instead, an area border router
adjacent to a stub can be configured to send a
default external route into the stub for all
destinations outside the local area or the
autonomous system. This route will also be
advertised as a single entry point for traffic
entering the stub. Using a stub can significantly
reduce the amount of topology data that has to
be exchanged over the network.
By default, a stub can only pass traffic to other areas in the autonomous system via
the default external route. However, you also can configure an area border router to
send Type 3 summary link advertisements into the stub about subnetworks located
elsewhere in the autonomous system.
backbone
area
ABR
area
ABR
backbone
stub
ABR
default
external
route
Summary of Contents for 8926EM
Page 6: ...ii ...
Page 34: ...Getting Started ...
Page 44: ...Introduction 1 10 1 ...
Page 62: ...Initial Configuration 2 18 2 ...
Page 64: ...Switch Management ...
Page 76: ...Configuring the Switch 3 12 3 ...
Page 118: ...Basic Management Tasks 4 42 4 ...
Page 164: ...User Authentication 6 28 6 ...
Page 176: ...Access Control Lists 7 12 7 ...
Page 284: ...Quality of Service 14 8 14 ...
Page 294: ...Multicast Filtering 15 10 15 ...
Page 300: ...Domain Name Service 16 6 16 ...
Page 310: ...Dynamic Host Configuration Protocol 17 10 17 ...
Page 320: ...Configuring Router Redundancy 18 10 18 ...
Page 344: ...IP Routing 19 24 19 ...
Page 356: ...Unicast Routing 20 12 20 Web Click Routing Protocol RIP Statistics Figure 20 5 RIP Statistics ...
Page 386: ...Unicast Routing 20 42 20 ...
Page 388: ...Command Line Interface ...
Page 400: ...Overview of the Command Line Interface 21 12 21 ...
Page 466: ...SNMP Commands 24 16 24 ...
Page 520: ...Access Control List Commands 26 18 26 ...
Page 546: ...Rate Limit Commands 30 2 30 ...
Page 612: ...VLAN Commands 34 24 34 ...
Page 626: ...Class of Service Commands 35 14 35 ...
Page 670: ...DHCP Commands 39 16 39 ...
Page 716: ...IP Interface Commands 41 36 41 ...
Page 768: ...IP Routing Commands 42 52 42 ...
Page 770: ...Appendices ...
Page 791: ......