[Chapter 7] 7.2 The Minimal Routing Table

[Chapter 7] 7.2 The Minimal Routing Table



% ping -s almond

PING almond.nuts.com: 56 data bytes

64 bytes from almond.nuts.com (172.16.12.1): icmp_seq=0. time=11. ms

64 bytes from almond.nuts.com (172.16.12.1): icmp_seq=1. time=10. ms

^C

----almond.nuts.com PING Statistics---2 packets transmitted, 2 packets received, 0% packet loss

round-trip (ms) min/avg/max = 10/10/11

ping displays a line of output for each ICMP ECHO_RESPONSE received. [3] When ping is interrupted, it

displays some summary statistics. All of this indicates successful communication with almond. But if we

check a host that is not on nuts-net, say a host at O'Reilly, the results are different.

[3] Sun's ping would only display the message "almond is alive" if the -s option was not used.

Some other ping implementations do not require the -s option.

% ping 207.25.98.2

sendto: Network is unreachable

Here the message "sendto: Network is unreachable" indicates that peanut does not know how to send data to

the network that host 207.25.98.2 is on. There are only two routes in the peanut routing table and neither is a

route to 207.25.98.0.

Even other subnets on nuts-net cannot be reached using this routing table. To demonstrate this, ping a host on

another subnet. For example:

% ping 172.16.1.2

sendto: Network is unreachable

These ping tests show that the routing table created by ifconfig allows communication only with other hosts

on the local network. If your network does not require access to any other TCP/IP networks, this may be all

you need. However, if it does require access to other networks, you must add more routes to the routing table.



Previous: 7.1 Common

Routing Configurations

7.1 Common Routing

Configurations



TCP/IP Network

Administration

Book Index



Next: 7.3 Building a Static

Routing Table

7.3 Building a Static Routing

Table



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[Chapter 7] Configuring Routing



Previous: 6.5 Summary



Chapter 7



Next: 7.2 The Minimal

Routing Table



7. Configuring Routing

Contents:

Common Routing Configurations

The Minimal Routing Table

Building a Static Routing Table

Interior Routing Protocols

Exterior Routing Protocols

Gateway Routing Daemon

Configuring gated

Summary

Routing is the glue that binds the Internet together. Without it, TCP/IP traffic is limited to a single

physical network. Routing allows traffic from your local network to reach its destination somewhere

else in the world - perhaps after passing through many intermediate networks.

The important role of routing and the complex interconnection of Internet networks make the design

of routing protocols a major challenge to network software developers. Consequently, most

discussions of routing concern protocol design. Very little is written about the important task of

properly configuring routing protocols. However, more day-to-day problems are caused by

improperly configured routers than are caused by improperly designed routing algorithms. As system

administrators, we need to ensure that the routing on our systems is properly configured. This is the

task we tackle in this chapter.



7.1 Common Routing Configurations

First, we must make a distinction between routing and routing protocols. All systems route data, but

not all systems run routing protocols. Routing is the act of forwarding datagrams based on the

information contained in the routing table. Routing protocols are programs that exchange the

information used to build routing tables.

A network's routing configuration does not always require a routing protocol. In situations where the

routing information does not change - for example, when there is only one possible route, the system

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[Chapter 7] Configuring Routing



administrator usually builds the routing table manually. Some networks have no access to any other

TCP/IP networks, and therefore do not require that the system administrator build the routing table at

all. The three most common routing configurations are: [1]

[1] Chapter 4, Getting Started , presents guidelines for choosing the correct routing

configuration for your network.

Minimal routing

A network completely isolated from all other TCP/IP networks requires only minimal routing.

A minimal routing table usually is built by ifconfig when the network interface is configured.

[2] If your network doesn't have direct access to other TCP/IP networks, and if you are not

using subnetting, this may be the only routing table you'll require.

[2] Linux is an exception. ifconfig does not create routing table entries on a Linux

system.

Static routing

A network with a limited number of gateways to other TCP/IP networks can be configured

with static routing. When a network has only one gateway, a static route is the best choice. A

static routing table is constructed manually by the system administrator using the route

command. Static routing tables do not adjust to network changes, so they work best where

routes do not change.

Dynamic routing

A network with more than one possible route to the same destination should use dynamic

routing. A dynamic routing table is built from the information exchanged by routing protocols.

The protocols are designed to distribute information that dynamically adjusts routes to reflect

changing network conditions. Routing protocols handle complex routing situations more

quickly and accurately than the system administrator can. Routing protocols are designed not

only to switch to a backup route when the primary route becomes inoperable; they are also

designed to decide which is the "best" route to a destination. On any network where there are

multiple paths to the same destination, a routing protocol should be used.

Routes are built automatically by ifconfig, manually by the system administrator, or dynamically by

routing protocols. But no matter how routes are entered, they all end up in the routing table.



Previous: 6.5 Summary

6.5 Summary



TCP/IP Network

Administration

Book Index



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Routing Table

7.2 The Minimal Routing

Table



[Chapter 7] Configuring Routing



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[Chapter 6] 6.5 Summary



Previous: 6.4 Installing

SLIP



Chapter 6

Configuring the Interface



Next: 7. Configuring

Routing



6.5 Summary

TCP/IP works with a wide variety of networks. TCP/IP cannot make assumptions about the network it

runs on - the network interface and its characteristics must be identified to TCP/IP. In this chapter we

have looked at several examples of how to configure the physical network interface over which

TCP/IP runs.

ifconfig is the most commonly used interface configuration command. It assigns the interface its IP

address, sets the subnet mask, sets the broadcast address, and performs several other functions.

TCP/IP can also run over telephone lines using dial-up connections. Two protocols are available to do

this: Serial Line IP (SLIP) and Point-to-Point Protocol (PPP). PPP is the preferred choice. It is an

Internet standard and offers better reliability, performance, and security.

There are several steps to setting up a PPP or a SLIP connection: selecting and configuring the serial

protocol, configuring the port and modem, making the dial-up connection, and completing the remote

login. Some programs, such as dip, combine all of these steps into one program. Other programs, such

as pppd and chat, separate the functions.

Configuring the network interface allows us to talk to the local network, while configuring routing

allows us to talk to the world. We touched on routing in Chapter 2 and again in this chapter in our

discussion of routing metrics for ifconfig and default routes for PPP and SLIP. In the next chapter we

look at routing in much greater detail.



Previous: 6.4 Installing

SLIP

6.4 Installing SLIP



TCP/IP Network

Administration

Book Index



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Routing

7. Configuring Routing



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[Chapter 6] 6.4 Installing SLIP



Previous: 6.3 Installing PPP



Chapter 6

Configuring the Interface



Next: 6.5 Summary



6.4 Installing SLIP

Installing Serial Line IP (SLIP) is very similar to installing PPP. As with PPP, support for SLIP is usually

installed in the kernel - but that is only part of the configuration. The SLIP network interface also must be

configured.

PPP and SLIP configuration is complicated by the fact that these serial line protocols support both dedicated and

dial-up connections. For our Linux sample system, this means that two different commands are used to configure

a SLIP interface depending on whether it is a dedicated or a dial-up connection. In this section we discuss both,

beginning with the configuration command for dedicated connections.



6.4.1 slattach

The slattach command "attaches" the SLIP protocol to a specific serial interface. For example:

# slattach /dev/tty03 &

This command tells the SLIP protocol to use /dev/tty03 as its serial interface. The slattach command can

optionally set some configuration parameters for the serial interface. The syntax of slattach on a Slackware 96

Linux system is:

slattach [-h | -c | -6] ttyname [baudrate]

The three options, -h, -c, and -6, select the type of SLIP protocol used. -h selects uncompressed SLIP with full

headers. CSLIP with Van Jacobsen header compression is selected with -c. Use -6 to select six-bit SLIP. If none

of these options is selected, the slattach command defaults to CSLIP.

The baudrate argument sets the interface's transmission speed. Set the speed by entering a number that

corresponds to the bit rate that is used to transmit and receive data on this line, e.g., 56000. Both ends of the line

must set exactly the same bit rate. This may be determined by the characteristics of the leased line, or by the

hardware interfaces for a direct cable connection. Regardless, the transmission speed is a physical characteristic

limited by the equipment on the line. A default transmission speed of 9600 bits per second is used if no

baudrate value is entered on the command line.

The ttyname is the name of the serial interface attached to the leased line or direct cable connection. The serial

interfaces are identified by the system during the boot. dmesg and grep displays the interface names on a

Slackware 96 Linux system:

> dmesg | grep tty



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