Nortel Networks WEB OS 212777 - Manual (Page 73)

Web OS 10.0 Application Guide
Chapter 4: OSPF n 73
212777-A, February 2002
The Shortest Path First Tree
The routing devices use a link-state algorithm (Dijkstra’s algorithm) to calculate the shortest
path to all known destinations, based on the cumulative cost required to reach the destination.
The cost of an individual interface in OSPF is an indication of the overhead required to send
packets across it. The cost is inversely proportional to the bandwidth of the interface. A lower
cost indicates a higher bandwidth.
Internal Versus External Routing
To ensure effective processing of network traffic, every routing device on your network needs
to know how to send a packet (directly or indirectly) to any other location/destination in your
network. This is referred to as internal routing and can be done with static routes or using
active internal routing protocols, such as OSPF, RIP, or RIPv2.
It is also useful to tell routers outside your network (upstream providers or peers) about the
routes you have access to in your network. Sharing of routing information between autono-
mous systems is known as external routing.
Typically, an AS will have one or more border routers (peer routers that exchange routes with
other OSPF networks) as well as an internal routing system enabling every router in that AS to
reach every other router and destination within that AS.
When a routing device advertises routes to boundary routers on other autonomous systems, it
is effectively committing to carry data to the IP space represented in the route being advertised.
For example, if the routing device advertises 192.204.4.0/24, it is declaring that if another
router sends data destined for any address in the 192.204.4.0/24 range, it will carry that data to
its destination.