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9.1 | |
Examining the Routing Table | |
| 9.1.1 | |
The show ip route command |
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One of the primary functions of a router
is to determine the best path to a given destination. A router learns
paths, also called routes, from an administrator's configuration or
from other routers by way of routing protocols. Routers store this
routing information in routing tables using on-board dynamic random
access memory (DRAM). A routing table contains a list of the best
available routes. Routers use the routing table to make packet
forwarding decisions. The show ip route command displays the contents of the IP routing table. This table contains entries for all known networks and subnetworks, as well as a code that indicates how that information was learned. The following are some additional commands that can be used with the show ip route command:
A routing table maps network prefixes
to an outbound interface.
The example routing table shows four routes for directly connected networks. These routes, labeled with a C, are available to directly connected networks. RTA drops any packet destined for a network that is not listed in the routing table. In order to forward to other destinations the routing table for RTA will have to include more routes. These new routes may be added using one of two methods:
Administratively defined routes are
said to be static because they do not change until a network
administrator manually programs the changes. Routes learned from other
routers are dynamic because they can change automatically as
neighboring routers update each other with new information. Each
method has fundamental advantages and disadvantages.
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When RTA receives a packet destined for
192.168.4.46, it looks for the prefix 192.168.4.0/24 in its table. RTA
then forwards the packet out an interface (Ethernet0) based on the
routing table entry. If RTA receives a packet destined for 10.3.21.5,
it sends that packet out Serial 0/0.
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