Default Route - 3Com CoreBuilder 2500 User Manual

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HAPTER OUTING WITH
IP T
ECHNOLOGY
Routing table data is updated statically or dynamically:
Statically — You manually enter static routes in the routing table.
Static routes are useful in environments where no routing protocol is
used or where you want to override some of the routes that are
generated with a routing protocol. Because static routes do not
automatically change in response to network topology changes,
manually configure only a small number of reasonably stable routes.
Static routes do not time out.
Dynamically — Routers use a protocol such as the Routing
Information Protocol (RIP) to automatically exchange routing data.
Routes are recalculated at regular intervals. This process helps you to
keep up with network changes and allows the system to reconfigure
routes quickly and reliably. Interior Gateway Protocols (IGPs), which
operate within networks, provide this automated method. The
CoreBuilder 2500 system uses RIP and Open Shortest Path First (OSPF)
Protocol to configure its routing tables dynamically.
RIP operates using both active and passive devices. Active devices, usually
routers, broadcast RIP messages to all devices in a network or subnetwork
and update their internal routing tables when they receive a RIP message.
Passive devices, usually hosts, listen for RIP messages and update their
internal routing tables, but do not send RIP messages.
An active router sends an RIP message every 30 seconds. This message
contains the IP address and a metric (distance) from the router to each
destination in the router's internal table. In RIP, each router through which
a packet must travel to reach a destination counts as one network hop.
OSPF routes packets within and between predefined autonomous
systems and areas based on the cost of network links. The OSPF protocol
handles network topology changes with a minimum of administrator
involvement and routing traffic.

Default Route

In addition to the routes to specific destinations, a routing table can
contain a default route. The router uses the default route to forward
packets that do not match any other routing table entry. A default route
is often used in place of routes to numerous destinations that all have the
same gateway IP address and interface number. The default route can be
configured statically, or it can be learned dynamically.