Selecting Routes Through The Routing Table - 3Com 400 Family Configuration Manual

98 C 6: IP R
HAPTER OUTING
Selecting Routes
Through the Routing
Table
P O
ROTOCOL PERATION
the optimal route. For example, routing through three LAN route segments may
be much faster than routing through two WAN route segments.
Configuring the IP Routing Protocol is described in the following sections:

Selecting Routes Through the Routing Table

■
Routing Management Policy
■
For a router, the routing table is the key to forwarding packets. Each router saves a
routing table in its memory, and each entry in this table specifies the physical port
of the router through which a packet is sent to a subnet or a host. The packet can
reach the next router over a particular path or reach a destination host through a
directly connected network.
A routing table has the following key entries:
A destination address — Identifies the destination IP address or the destination
■
network of the IP packet, which is 32 bits in length.
A network mask — Made up of several consecutive 1s, which can be expressed
■
either in the dotted decimal format, or by the number of the consecutive 1s in
the mask. Combined with the destination address, the network mask identifies
the network address of the destination host or router. With the destination
address and the network mask, you have the address of the network segment
where the destination host or router is located. For example, if the destination
address is 129.102.8.10, the address of the network where the host or the
router with the mask 255.255.0.0 is located is 129.102.0.0.
The output interface — Indicates an interface through which an IP packet
■
should be forwarded.
The next hop address — Indicates the next router that an IP packet will pass
■
through.
The priority added to the IP routing table for a route — Indicates the type of
■
route that is selected. There may be multiple routes with different next hops to
the same destination. These routes can be discovered by different routing
protocols, or they can be the static routes that are configured manually. The
route with the highest priority (the smallest numerical value) is selected as the
current optimal route.
Routes are divided into the following types: subnet routes, in which the
destination is a subnet, or host routes, in which the destination is a host.
In addition, depending on whether the network of the destination host is directly
connected to the router, there are the following types of routes:
Direct route — The router is directly connected to the network where the
■
destination is located.
Indirect route — The router is not directly connected to the network where the
■
destination is located.
To limit the size of the routing table, an option is available to set a default route.
All the packets that fail to find a suitable table entry are forwarded through this
default route.