Content Overview Routing is nothing
more than directions for getting from one network to another.
These directions, also known as routes, can be dynamically
given to the router by another router, or they can be
statically assigned to the router by an administrator. This
module introduces the concept of dynamic routing protocols,
describes the classes of dynamic routing protocol, and gives
examples of protocols in each class. A network administrator
chooses a dynamic routing protocol based upon many
considerations. The size of the network, the bandwidth of
available links, the processing power of the network routers,
the brands and models of routers on the network, and the
protocols already in use on the network are all factors that
must be considered in choosing a routing protocol. This module
will provide more details about the differences between routing
protocols that help network administrators make a choice.
Students completing this module should be able to:
- Explain the significance of static routing
- Configure static and default routes
- Verify and
troubleshoot static and default routes
- Identify the
classes of routing protocols
- Identify distance vector
routing protocols
- Identify link-state routing
protocols
- Describe the basic characteristics of common
routing protocols
- Identify interior gateway
protocols
- Identify exterior gateway protocols
- Enable Routing Information Protocol (RIP) on a router
Content 6.1 Introduction to Static
Routing 6.1.1 Introducing routing Routing
is the process that a router uses to forward packets toward the
destination network. A router makes decisions based upon the
destination IP address of a packet. All devices along the way
use the destination IP address to point the packet in the
correct direction so that the packet eventually arrives at its
destination. In order to make the correct decisions, routers
must learn the direction to remote networks. When routers use
dynamic routing, this information is learned from other
routers. When static routing is used, a network administrator
configures information about remote networks manually.Because
static routes must be configured manually, any network topology
changes require the network administrator to add and delete
static routes to account for the changes. In a large network
this manual maintenance of routing tables could require a
tremendous amount of administrative time. On small networks
with few possible changes, static routes require very little
maintenance. Because of the extra administrative requirements,
static routing does not have the scalability of dynamic
routing. Even in large networks, static routes that are
intended to accomplish a specific purpose are often configured
in conjunction with a dynamic routing protocol. Web
Links Introduction to Routing
http://www.cisco.com/networkers/ nw99_pres/301.pdf
Content 6.1 Introduction to Static
Routing 6.1.2 Static route operation Static
route operations can be divided into these three parts:
- Network administrator configures the route
- Router
installs the route in the routing table
- Packets are
routed using the static route
Since a static route
is manually configured, the administrator must configure the
static route on the router using the ip route command.
The correct syntax for the ip route command is shown in
Figure . In Figures and , the network administrator of the
Hoboken router needs to configure a static route pointing to
the 172.16.1.0/24 and 172.16.5.0/24 networks on the other
routers. The administrator could enter either of two commands
to accomplish this objective. The method in Figure specifies
the outgoing interface. The method in Figure specifies the
next-hop IP address of the adjacent router. Either of the
commands will install a static route in the routing table of
Hoboken. The only difference between the two is in the
administrative distance assigned to the route by the router as
it is placed in the routing table. The administrative distance
is an optional parameter that gives a measure of the
reliability of the route. A lower value for the administrative
distance indicates the more reliable route. Thus, a route with
a lower administrative distance will be installed before an
identical route with a higher administrative distance. The
default administrative distance when using next-hop address is
1, while the default administrative distance when using the
outgoing interface is 0. If an administrative distance other
than the default is desired, a value between 0 and 255 is
entered after the next-hop or outgoing interface as follows:
waycross(config)#ip route 172.16.3.0 255.255.255.0
172.16.4.1 130 If the router cannot reach the outgoing
interface that is being used in the route, the route will not
be installed in the routing table. This means if that interface
is down, the route will not be placed in the routing table.
Sometimes static routes are used for backup purposes. A static
route can be configured on a router that will only be used when
the dynamically learned route has failed. To use a static route
in this manner, simply set the administrative distance higher
than that of the dynamic routing protocol being used. Lab
Activity e-Lab Activity: Static Route Operation In this
lab, the student will learn how to create a static route.
Lab Activity e-Lab Activity: Static Routes In this lab, the
students will practice using static routes by troubleshooting a
network connected in the fashion outlined in the topology map.
Content 6.1 Introduction to Static
Routing 6.1.3 Configuring static routes
This section lists the steps for configuring static routes and
gives an example of a simple network for which static routes
might be configured.Use the following steps to configure static
routes: - Determine all desired destination networks,
their subnet masks, and their gateways. A gateway can be either
a local interface or a next hop address that leads to the
desired destination.
- Enter global configuration
mode.
- Type the ip route command with a
destination address and subnet mask followed by their
corresponding gateway from Step one. Including an
administrative distance is optional.
- Repeat Step three
for as many destination networks as were defined in Step
one.
- Exit global configuration mode.
- Save the
active configuration to NVRAM by using the copy
running-config startup-config command.
The
example network is a simple three-router configuration. Hoboken
must be configured so that it can reach the 172.16.1.0 network
and the 172.16.5.0 network. Both of these networks have a
subnet mask of 255.255.255.0. Packets that have a destination
network of 172.16.1.0 need to be routed to Sterling and packets
that have a destination address of 172.16.5.0 need to be routed
to Waycross. Static routes can be configured to accomplish this
task. Both static routes will first be configured to use a
local interface as the gateway to the destination networks.
Since the administrative distance was not specified, it will
default to 0 when the route is installed in the routing table.
Note that an administrative distance of 0 is the same as a
directly connected network. The same two static routes can also
be configured using a next-hop address as their gateway. The
first route to the 172.16.1.0 network has a gateway of
172.16.2.1. The second route to the 172.16.5.0 network has a
gateway of 172.16.4.2. Since the administrative distance was
not specified, it defaults to 1. Lab Activity e-Lab
Activity: Configuring Static Routes In this lab, the student
will learn to configure some static routes. Web Links
ip route Command http://www.cisco.com/en/US/products/sw/
iosswrel/ps1835/products_command_reference_
chapter09186a00800ca75a.html#1018067
Content
6.1 Introduction to Static Routing