Web Management Guide-R06

Chapter 19 | Unicast Routing
Configuring the Open Shortest Path First Protocol (Version 2)
– 723 –
throughput and connectivity. OSPF utilizes IP multicast to reduce the amount
of routing traffic required when sending or receiving routing path updates. The
separate routing area scheme used by OSPF further reduces the amount of
routing traffic, and thus inherently provides another level of routing protection.
In addition, all routing protocol exchanges can be authenticated. Finally, the
OSPF algorithms have been tailored for efficient operation in TCP/IP Internets.
◆ OSPFv2 is a compatible upgrade to OSPF. It involves enhancements to protocol
message authentication, and the addition of a point-to-multipoint interface
which allows OSPF to run over non-broadcast networks, as well as support for
overlapping area ranges.
◆ When using OSPF, you must organize your network (i.e., autonomous system)
into normal, stub, or not-so-stubby areas; configure the ranges of subnet
addresses that can be aggregated by link state advertisements; and configure
virtual links for areas that do not have direct physical access to the OSFP
backbone.
■
To implement OSPF for a large network, you must first organize the
network into logical areas to limit the number of OSPF routers that actively
exchange Link State Advertisements (LSAs). You can then define an OSPF
interface by assigning an IP interface configured on this router to one of
these areas. This OSPF interface will send and receive OSPF traffic to
neighboring OSPF routers.
■
You can further optimize the exchange of OSPF traffic by specifying an area
range that covers a large number of subnetwork addresses. This is an
important technique for limiting the amount of traffic exchanged between
Area Border Routers (ABRs).
■
And finally, you must specify a virtual link to any OSPF area that is not
physically attached to the OSPF backbone. Virtual links can also be used to
provide a redundant link between contiguous areas to prevent areas from
being partitioned, or to merge backbone areas. (Note that virtual links are
not supported for stubs or NSSAs.)
Defining
Network Areas
Based on Addresses
OSPF protocol broadcast messages (i.e., 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 Routing Protocol > OSPF > Network Area (Add) page 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 the area identifier 0.0.0.0. By
default, all other areas are created as normal transit areas.