Chapter 1 Ip Routing Protocol Overview; Ip Route And Routing Table Overview; Ip Route And Route Segment - 3Com 3C13636 Configuration Manual

3Com Router 3000 Ethernet Family
Configuration Guide

Chapter 1 IP Routing Protocol Overview

1.1 IP Route and Routing Table Overview

1.1.1 IP Route and Route Segment

Routers are adopted for route selection on the Internet. According to the destination
address of a received packet, a router selects an appropriate route (via a network) and
forwards the packet to the next router. The last router is responsible for forwarding the
packet to the destination host.
For example, in Figure 1-1, from host A to host C, there are totally three networks and
two routers. If a node is connected to another node through a network, there will be a
route segment between these two nodes. They are thus considered adjacent in the
Internet. Similarly, two routers connected to the same network are adjacent routers.
The number of route segments between a router and certain host in the same network
is taken as zero. In the following figure, the bold arrows represent these route segments.
A router does not concern about which physical links constitute this route segment.
Host A
Host C
Figure 1-1 Route segment
As network size may vary greatly, the actual "lengths" of router segments may be
different from each other. Therefore, for different networks, the number of route
segments multiplies a weighted coefficient can measure the actual length of the path.
If a router in a network is regarded as a node in the network and a route segment in the
Internet is regarded as a link in the Internet, routing in the Internet is similar to routing in
a simple network. Routing through the shortest route is not always the most ideal way.
For example, routing through 3 high-speed LAN route segments may be much faster
than that through 2 low-speed WAN route segments.
Route Segment
3Com Corporation
1-1
Chapter 1 IP Routing Protocol Overview
Host B