Configuring Static and Default Routing

8/14/2019 Configuring Static and Default Routing

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Configuring static and default routing

Takeaway: You may know how a packet traverses the internetwork via IP routing, but

how do routers manage to help in this process? Routing tables aid routers in knowing exactly

how to forward packets throughout your network. Todd Lammle is here to explain thisprocess to you.

My last Daily Drill Down, IP routing in 40 short steps,described how a packet is sent from

an originating host on one network over to another host on a different network through an

internetwork, as well as exactly what happens to those packets during that process. This Daily

Drill Down is going to take things a step further and focus on both IP routing and how routers

use a routing table to accomplish the task of forwarding those packets properly throughout an

internetwork.

There are three different ways a routing table is built:

Statically

By default

Dynamically

I'm going to talk about the first twostatic and default routing tables that are built by

network administrators.

A little review

But first, a little review...Remember that an internetwork is defined as two or more networks

connected with a router or routers. Also recall that routers don't keep track of, or care even

the slightest bit about, hosts, but they are extremely concerned about networks and the best

path to access each one.

Logical addressing (IP, for example) is what's used to identify each host on the internetwork.

Routers read the network portion of an IP address to figure out where in the Net world a host

is and then use a routing table to determine the best path to the network that the destination

host is located on.

Once that network is located, the packet is sent to the destination network by forwarding thepacket, hop-to-hop, until it reaches the specific router that's directly connected to the

destination host's network. From there, the destination host's unique hardware address is used

to get the packet to the host that's supposed to receive it.

It's all about maps

Routers have maps, or at least a form of them. They must have a map of the entire

internetwork to explain to them where each logical network is located, as well as to guide

their decision in choosing the quickest, most efficient way to get there. This map is called a

routing table, and each routed protocol you use has to have its own map. For instance, if

you're running IP, IPX, and AppleTalk on your network, each of your routers will have three

mapsone for each routing protocolall describing the same physical networks in adifferent way.
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It's kind of a language barrier thing; the reason each protocol has separate routing tables is

because each protocol really is like a different language. Say you've built a gated community,

and in it, you have a street you named Cat Street. Everyone on that block speaks English, and

the street sign is in English. Then a Spanish family moves in that doesn't speak any English,

so you add a sign that reads Avenida Gato. Next, a French family moves inthey don't speakEnglish or Spanishso you add Le Chat to the sign. You now have three separate signs

describing Cat Street in three different ways.

Take a look at Figure A, which has two 2500 routers connected with a serial link.

Figure A

Each router must have all three networks in the routing table in order to send packets through

the internetwork.

By default, each router will have the directly connected networks in its routing table. Before

we take a look at the routing tables, let's view the configuration used on each router.

Here's the basic configuration for the 2500A router.

Here's the basic configuration for the 2500B router.

The 2500B router had the DCE end of the serial link, so the clock rate command needed to be

added. We should now have two networks in each routing table. Let's view each table with

the show ip route command (or the short form: sh ip route). Here's the 2500A router.

And here's the 2500B router.

The 2500A router is directly connected to subnets 32 and 64. The 2500A must have a route

entered for the 96 subnet. The 2500B router is directly connected to the 64 and 96 subnets.

The 2500B router must have an entry for the 32 subnet.
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Configuring static routes

Static routes are routes configured and entered into the routing table by the administrator.

Typically, in larger networks, creating nothing but static routes would be a gruesome task (if

not impossible), so dynamic routing is usually used (examples are RIP and OSPF). However,

in smaller networks, static routing can work well; it saves overhead on the router CPU and

bandwidth on the serial links that dynamic routing protocols greedily consume as fast as mygolden retriever will steal and swallow an unattended turkey sandwich!

To configure a static route, use the global configuration command ip route. Since the 2500A

router must understand how to get to the 96 subnet, let'sconfigure a static route that describes

to the router what to do when it receives a packet with a destination IP network of

192.168.10.96.

The ip route command is broken down as follows:

ip route: The command issued to add a route to a routing table

192.168.10.96: The destination network 255.255.255.224: The subnet mask used on the network

192.168.10.66: Where to send a packet with a destination IP network of

192.168.10.96

Notice that the IP routing table now has an entry for the 192.168.10.96 subnet via

192.168.10.66, which is the next hop gateway from the 2500A router.

This is working great; we're halfway to finishing our routing tables. The reason we're only

half done is that the 2500B router still doesn't know how to send packets to the 32 subnet. If a

packet is sent from HostA on the 32 subnet over to HostB on the 96 subnet, it'll definitely get

to HostB, and HostB will respond by sending a new packet back to the configured default

gateway. The problem is that 2500B will discard the packet since it doesn't know how to get

to the 32 subnet. So, let's configure 2500B with a route to network 192.168.10.32.

The ip route command is broken down as follows:

ip route: The command used to add a static route

192.168.10.32: The destination route we want router 2500B to know about

255.255.255.224: The mask used in the network

192.168.10.65: The next hop router used to get to subnet 32

The routing table for the 2500B router now knows how to get to subnet 32 and packets can be

sent from HostA to HostB and back again.

Configuring default routing

Since we have an Internet connection off the 2500B serial 0 interface, we need to add a

default route to the routing table of the 2500B router. This is just like adding a static route,

except wildcards of all zeros (0s) are used instead of a network and mask.

Here is an exampleof configuring the serial 1 interface on the 2500B router and then settingup default route. The ISP provided an IP address of 200.43.89.65/30 for the interface. Since /
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30 is a block size of 4, the valid hosts are 65 and 66. We can set our next hop to 66 since we

were given 65 for our routers interface.

The default route command is broken down as follows:

ip route: The command used to add a static or default route 0.0.0.0: The wildcard used to say any network not already in the routing table

0.0.0.0: Wildcard mask to say any network mask

200.43.89.66: Next hop gateway

If you don't know the next hop gateway for some reason, you can always create the command

like this:

ip route 0.0.0.0 0.0.0.0 s1

This will tell the router to send packets that aren't in the routing table out serial 1. Also,

notice that I used the ip classless command. This tells the route not to drop packets that aredestined for a network that's not in the routing table, but to use the default route instead. If

you don't use the ip classless command, packets like that would be dropped before being sent

to the default route. (ip classless is on by default in IOS 12.x.)

Okay, so the routing table on the 2500B router now looks like this.

The S* is a static default route. Notice also that the gateway of last resort is now set as well.

Since a router cannot set a default gateway and since it actually is the default gateway for a

network, routers use a gateway of last resort instead, which is really a default route.

A couple of tips

Routers need current, up-to-date maps. If a routing table doesn't have a route

to each network that it's going to be required to send packets to, then packets will be

dropped.

Troubleshooting a routing table problem is tough. But by using the ping

program and Traceroute command, you can find exactly where a packet is failing in

an internetwork. Each of these commands will be discussed in future articles.

ConclusionBy now it should be pretty clear both why it's important to understand how a routing table is

used in an internetwork and how important it is to be able to read a routing table and

understand the output. There are many tools that can help you understand this concept, but

the best tool is experience. Use the tools Ive shown you, and eventually, youll be whipping

up routing tables faster than you can say cat in three different languages.
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Configuring Static and Default Routing