3Com REMOTE ACCESS SYSTEM 1500, Management Manual

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Overview
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Dynamic, Static, and
Default Routes
You can configure the RAS 1500 to use constantly updated routing tables
(dynamic routes that use protocols such as IP RIPv1 or RIPv2) or to use
only your pre-configured routing tables (static routes).
Dynamic Routes
Network devices running RIPv1 or RIPv2 broadcast the destination
addresses to which they can forward packets. Other routers build routing
tables by listening to the broadcasts of other devices that they are directly
connected to.
If the RAS 1500 does not periodically hear a broadcast for a given route,
the RAS 1500 assumes the route is unavailable and deletes it from the
routing table.
Static Routes
Static routes are user-defined. By adding entries to the Routes Table, you
tell the RAS 1500 how to forward packets bound for specific networks
rather than relying on RIP to dynamically learn the routes. If you have
defined a static route to a given location, the RAS 1500 assumes you
want to use that route and ignores dynamic routing broadcasts pointing
to the same location. Static routes remain in the table until removed by
the administrator.
Default Routes
A default route is used to route to networks not specifically listed in the
routing table. It can be used on routers that have just one connection to
remote sites. It can be used as an alternative to using a dynamic routing
protocol across a WAN link, if that is appropriate for the network
topology.
How the RAS 1500
Routes Packets
When the RAS 1500 receives a packet, it looks up the packet destination
in the routing table. If a static route is found, the RAS 1500 send the
packet to the gateway listed in the routing table. If the RAS 1500 does
not find a static route, it uses a dynamic route that it learned from other
routers. If the routing table contains no dynamic routes to the
destination, it sends the packet to the specified default route gateway. If
no such gateway has been defined, the RAS 1500 discards the packet.