The Shortest Path First Tree; Internal Versus External Routing - IBM RackSwitch G8000 Application Manual

The Shortest Path First Tree

Internal Versus External Routing

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RackSwitch G8000: Application Guide
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.
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 autonomous 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.