Routing Tables

Routing Tables
At this point you may be wondering, “What is a routing table?” The first thing you need to understand
is what a route is. The easiest way to explain a route is to think of using an online map. You
are required to enter your current location, or source location, and your destination. After you
enter this information, the online map will do its nice little calculation and print the best route to
take you from your source location to the destination. A route in the world of internetworking is
essentially the same, with each router keeping track of the next hop in the route between itself and
the next downstream router toward the destination. Once a router has learned a route, it places
it in a repository for future use, assuming it has not already learned a route that it considers to be
better. This repository is known as a
routing table
.
In order to view the IP routing table on your router, you need to use the command
show ip
route
. Let’s take a look at an actual routing table:
2501A#
sh ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B – BGP D -
EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area N1 - OSPF NSSA
external type 1, N2 - OSPF NSSA external type 2 E1 - OSPF external type 1, E2 -
OSPF external type 2, E – EGP i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2,
* - candidate default U - per-user static route, o - ODR, P - periodic
downloaded static route T - traffic engineered route
Gateway of last resort is not set
172.16.0.0/16 is subnetted, 1 subnets
C 172.16.50.0 is directly connected, FastEthernet0/0
C 192.168.24.0 is directly connected, FastEthernet0/0
10.0.0.0/8 is subnetted, 1 subnets
C 10.10.10.0 is directly connected, Serial0/0
R 175.21.0.0/16 [120/1] via 10.10.10.1, 00:00:18, Serial0/0
2501A#
Now you may be wondering what all of this means. So, let’s break it down.
6
Chapter 1 
Routing Principles
The
Codes
section at the very top tells you how the route was learned. As you may have
noticed, there are many different ways a route can be learned. It’s not important for you to
memorize all of the possible codes. What is important is for you to know how to use the
Codes
section to find out how a route was learned.
Next, note the line
Gateway of last resort is not set
. The gateway of last resort, also
known as a default route, is where your router will send IP packets if there isn’t a match in the
routing table. In this example, the gateway of last resort has not been set. This means if the
router receives a packet destined for an unknown network, it will drop the packet and send an
ICMP destination network unreachable message to the originator of the packet.
The next items in the routing table are the routes the router knows about. Let’s go ahead and
break down a route into its components. We will use the following example:
R 175.21.0.0/16 [120/1] via 10.10.10.1, 00:00:18, Serial0
R
The means by which the route entry was learned on this router. In this case, the
R
stands for
RIP. From this, you can deduce that the entry you are looking at was learned by the RIP routing
protocol.
175.21.0.0/16
The network address and prefix length (number of bits set to 1 in the subnet
mask) of the destination network.
[120
The administrative distance of the route. (We will explain administrative distance a little
later in this chapter.)
/1]
The metric of the route specific to the routing protocol used to determine the route. RIP
uses hops as its metric. A
hop
is how many routers away—excluding this router—the destination
network is. In this example, there is one router between this router and the destination.
via 10.10.10.1
The next-hop address for the route. This is the address that the packet will
need to be sent to in order for the packet to reach its destination.
00:00:18
The length of time since the route has been updated in the routing table. In this
example, the route was updated 18 seconds ago.
Serial0
The interface the route was learned through. This is also the interface the packet will
be switched to in order for the packet to be forwarded toward its destination. If you see another
IP address here, at least one additional lookup will have to occur within the same routing table,
which is defined as a recursiv