Ospf Overview; Features - 3Com corebuilder 3500 Implementation Manual

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OSPF Overview

Features

S P F (OSPF)
HORTEST ATH IRST
The OSPF link-state protocol dynamically responds to changes in network
topology that occur within a group of networks and routers known as an
autonomous system. OSPF tracks the states of links and routers in each
autonomous system, and when a change occurs, calculates new routes
based on the new topology. The OSPF protocol responds to network
topology changes with a minimum of administrator involvement and
routing traffic.
All OSPF routers within an autonomous system build and synchronize
databases of the autonomous system's network topology. Using its
database, each router calculates the shortest path trees to every
destination within the autonomous system. With this dynamic table of
shortest paths, OSPF converges on an optimum route faster than other
routing algorithms, such as the Routing Information Protocol (RIP).
Routers that use a distance-vector protocol like RIP periodically exchange
all or a portion of their tables, but only with their neighbors. Routers
using a link-state protocol like OSPF send small portions of their tables
throughout the network by flooding.
For information about how to perform IP routing, see Chapter 11.
Your system supports OSPF Version 2 as defined in RFC 1583. OSPF
routing on your system includes these features:
Areas — You can subdivide an autonomous system (AS) into more
manageable contiguous networks called areas. Areas increase stability,
conserve router resources, and support route summarization — the
consolidation of network addresses. For more information, see
"Areas" later in this chapter.
Default route metric — You can configure a router to advertise itself
as the default router for the area, and you can specify a cost to be
advertised with the default route. When area routers fail to find a
specific match for a packet's destination, the router then forwards the
packet to the default router, which then forwards the packet to the
most logical destination. For more information, see "Default Route
Metric" later in this chapter.