1© 2004, Cisco Systems, Inc. All rights reserved.

CCNA 2 v3.1 Module 6 Routing and Routing Protocols

Objectives

Route Types

Introducing Routing

• In order to forward packets correctly, routers must learn the path to remote networks.

• There are methods by which a router can learn these routes:

1. Dynamic routing - information is learned from other routers, and routing protocols adjust routes automatically.

2. Static routing - network administrator configures information about remote networks manually. They are used to reduce overhead and for security.

– Because of the extra administrative requirements, static routing does not have the scalability of dynamic routing.

– In most networks static routes are often used in conjunction with a dynamic routing protocol.

Static Routes

• Static routes are manually configured using the ip route command

• The IP route command can set the next hop by specifying either:

1. The outgoing interface, or

2. The next hop IP address of the adjacent router

See examples in next slides

• Command Format

ip route network_address network_mask next_hop | interface_admin_dist

1. Specifying the Outgoing Interface

2. Specifying the Next-hop IP Address

Administrative Distance

• The Administrative Distance is the trustworthiness of the source of the route.

– The router by default assigns a Administrative Distance of 1 to static routes.

– It is assumed that if the administrator takes the time to figure out what route the packet should take then this routing information must be very reliable

– Only directly connected routes have a default Administrative Distance that is trusted more (directly connected default Administrative Distance is 0).

• Administrative Distance should not be confused with the Metric of the route. The metric of the route is how good an individual route is

– When a router selects a route to a particular destination to put in the routing table, it first examines the Administrative Distance of all the routes available to that destination

– If it has alternate routes to the same destination it will use the route with the lowest Administrative Distance

Configuring Static Routes

• We can configure static routes to all destinations

Non-directly Connected Networks

• Or we can set a default route to be used for any destination that does not have a routing table entry

Verifying Static Route Configuration

• The command show running-config is used to view the active configuration in RAM to verify that the static route was entered correctly.

• The show ip route command is used to make sure that the static route is present in the routing table

This command sets a default route on a router: ip route 0.0.0.0 0.0.0.0 [next-hop-address | outgoing interface]

Router(config)#

Static RoutesStatic routes between networks are manually configured by an administrator.

Static routes are added with the following command:

Router(config)# ip route 192.168.2.0 255.255.255.0 E0

ip route 0.0.0.0 0.0.0.0 192.168.1.1

Network Address Subnet Mask Interface

Static routes out interfaces have an administrative distance of 0.

Static routes to next hop addresses have administrative distance of 1.

You can specify a non-default administrative distance for a static route:

Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1 130

Default routes are used to route packets with destinations that do not match any of the other routes in the routing table

Troubleshooting Static Route Configuration

• The show interfaces command

• The ping command

• The traceroute command

Verifying / Troubleshooting the Static Route

Verifying static route configuration

• After static routes are configured it is important to verify that they are present in the routing table and that routing is working as expected

• The command show running-config is used to view the active configuration in NVRAM to verify that the static route was entered correctly

• The show ip route command is used to make sure that the static route is present in the routing table

Troubleshooting static route configuration

• The show interfaces command can be used to check the state and configuration of the interface that is to be used for the route gateway

• The ping command is used to determine if end-to end connectivity exists

• If an echo reply is not received after a ping, traceroute will be used to determine which router in the route path is dropping the packets

The show ip route Command Output

The ping and traceroute Commands

Routed Versus Routing Protocol

Autonomous Systems

AS 10

AS 20

Autonomous Systems

• Autonomous Systems divide the global internetwork into smaller, more manageable networks.

• An Autonomous System is a collection of networks under a common administration (a single organisation, ISP or systems administrator) and sharing a common routing strategy.

• Typically the world wide organisation ICANN, the ISP, or the administrator assigns a unique AS number to the Autonomous System.

• The Autonomous System number uniquely distinguish it from other Autonomous Systems around the world.

• Each Autonomous Systems has its own set of rules and policies.

Dynamic Routing Operations

Classes of Routing Protocols

Routing Protocols

The success of dynamic routing depends on two basic router functions:

• Maintenance of a routing table

• Timely distribution of knowledge, in the form of routing updates, to other routers.

Types of routing protocols

• Distant Vector – determines the distance (hop count) and direction (vector – next hop) to a

destination network

• Link-State – maintains the complete network topology and determines the shortest path

to each destination

• Hybrid protocols (like EIGRP) – contain some elements of both.

• Different routing protocols use different metrics to determine the best route to a network.

• Administrative Distances are used to rate the trustworthiness of the various route entries.

Distance Vector Concepts

Distance Vector Routing Protocols

• The distance-vector routing algorithm passes complete routing tables to neighbor routers.

• RIP is a distance vector routing protocol:

Uses hop count as its metric

Router(config)#

Configuration Example:

router ripRouter(config-router)# network 172.16.0.0

Routing Metric Components

Link-State Concepts

Link-State Network Discovery

Link-State Topology Changes

Link-State Concerns

Routing Protocols – Path Determination

• Routing protocols create and maintain routing tables

• From these tables routing of packets can be performed

• Routing consists of two basic mechanisms:

– Path Determination:

• Router uses the routing table to determine the best path to the destination

– Switching (forwarding):

• Accept a packet on the incomming interface

• and switches it to the outgoing interface

– (based on result of path determination)

Default Route

The Default Route entry in the right hand router sends packets for any unknown destination network out S1.

IP Routing Configuration Tasks

Using the router and network Commands

Routing Protocols

Interior/Exterior Routing Protocols

1. ________________________ are designed for use in networks that are under the control of a single organization.

2. Exterior Gateway Protocols are designed for use between different networks that are under the control of ___________________.

3. ________________________ are typically used between ISPs

or between a company and an ISP.

EGP and IGP Overview

IGP IGPEGP

Interior Gateway Protocols

different organizations

Exterior Gateway Protocols

Routing Protocols Overview

Protocol Features

Distance vector, hop count metric, maximum 15 hops, broadcasts updates every 30 secs.

Cisco proprietary distance vector, bandwidth / load / delay / reliability composite metric, broadcast updates every 90 secs.

Cisco proprietary, enhanced distance vector (hybrid), load balancing, uses DUAL to calculate shortest path.

Routing updates are triggered by topology changes.

Link-state, open standard, Uses SPF algorithm. Routing updates are sent as topology changes occur.

Distance vector exterior routing protocol, used between ISPs, used to route traffic between ASs.

RIP

IGRP

EIGRP

OSPF

BGP

Summary