3Com Switch 7750 Configuration Guide Guide

262 CHAPTER 30: IP ROUTING PROTOCOL OVERVIEW
greatly, the actual length of router segments may be different from each other.
Therefore, you can put different weights to different route segments (so that, for
example, a route segment can be considered as two segments if the weight is
two), In this way, the length of the path can be measure by the number of
weighted route segments.
If routers in networks are regarded as nodes in networks and route segments in
the Internet are regarded as links in the Internet, routing in the Internet is similar
to that in a conventional network.
Routing through the shortest route is not always the most ideal way. For example,
routing across three high-speed LAN route segments may be much faster than
routing across two low-speed WAN route segments.
Route Selection Through
the Routing Table
The key for a router to forward packets is the routing table. Each router maintains
a routing table. Each entry in this table contains an IP address that represents a
host/subnet and specifies which physical port on the router should be used to
forward the packets destined for the host/subnet. And the router forwards those
packets through this port to the next router or directly to the destination host if
the host is on a network directly connected to the router.
Each entry in a routing table contains:
■ Destination address: It identifies the address of the destination host or network
of an IP packet.
■ Network mask: Along with the destination address, it identifies the address of
the network segment where the destination host or router resides. By
performing "logical AND" between destination address and network mask,
you can get the address of the network segment where the destination host or
router resides. For example, if the destination address is 129.102.8.10 and the
mask is 255.255.0.0, the address of the network segment where the
destination host or router resides is 129.102.0.0.A mask consists of some
consecutive 1s, represented either in dotted decimal notation or by the number
of the consecutive 1s in the mask.
■ Output interface: It indicates through which interface IP packets should be
forwarded to reach the destination.
■ Next hop address: It indicates the next router that IP packets will pass through
to reach the destination.
■ Preference of the route added to the IP routing table: There may be multiple
routes with different next hops to the same destination. These routes may be
discovered by different routing protocols, or be manually configured static
routes. The one with the highest preference (the smallest numerical value) will
be selected as the current optimal route.
According to different destinations, routes fall into the following categories:
■ Subnet route: The destination is a subnet.
■ Host route: The destination is a host.
In addition, according to whether the network where the destination resides is
directly connected to the router, routes falls into the following categories: