1.4.1 IP packet-forwarding methods

(1) Contents of the routing table

A routing table contains multiple entries, each of which consists of the items described below. To check the contents of the routing tables maintained by the Switch, execute the show ip route command.

Destination:

The destination network address and the bit length of the subnet mask for the destination network address. The subnet mask masks the destination IP address in a received IP packet when the routing table is searched. If the destination network is not divided into subnetworks, this field indicates the length of the mask bits corresponding to the network class of the network address (for example, 8 for Class A). This field indicates 32 when the IP packet is forwarded based on the host address.

Next Hop:

The IP address of the next router to which the IP packet is forwarded. When the Switch is using the multipath function, multiple next hops exist.

The name of the interface where Interface:Next Hop is located.

Metric: The metric of the route.

Protocol: Learner protocol.

Age: The number of seconds since the route was checked or changed.

(2) Looking up the routing table

The forwarding function searches the routing table for the entry that matches the destination IP address in the received IP packet. The matching entry contains the destination network address, and is found when the Switch applies the subnet mask in the routing table in an AND operation with the destination IP address in the received IP packet and the result is the same as an entry in the routing table. The following figure shows how a routing table is searched to determine the destination network address.

Figure 1-4: Routing table retrieval