Routing Versus Forwarding - Bay Networks 5390 Administering

Internet Protocol (IP) Routing
router is a hop in the path to the address; the next router in the path is called the next hop; and the
total number of routers that must be crossed is called the hop count (also known as the metric). The
next hop in a routing table is always on a network directly attached to the router containing the
table. The RIP metric is an integer from 1 through 15, with 1 representing the best (shortest) route.
Together, the IP address of a destination host or network, the next hop, and the metric constitute a
route.
If one of the routers or networks in a particular RIP route fails, a time-out mechanism prevents other
routers from keeping the route in their routing tables and advertising it as viable. If Router A's table
contains a route learned from Router B, and three minutes elapse between Router A hearing from
Router B, Router A marks the route in question as invalid by setting its metric to 16; this value is
referred to as infinity. Router A advertises the route, with a metric of 16, for two more minutes, so
that other routers can learn the route is unusable. At the end of that time, Router A deletes the route
from its routing table.

Routing versus Forwarding

Do not confuse IP routing with IP forwarding. IP forwarding is the process of sending an IP datagram
to the next hop on the way to its destination. IP routing is the algorithm that determines the next
hop to use. The Model 5390 kernel performs IP forwarding; Model 5390 RIP performs routing.
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