CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 73 pdf

Figure 15-13 Dynamic Route Static routing allows routers to properly route a packet from network to network based on manually configured information.. When the path between Routers A and

Verifying Static Route Configuration

It is important to verify that the static routes are present in the routing table and that

routing is working as expected after the static routes are configured 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

Use the following steps to verify static route configuration:

Step 1 In privileged mode, enter the command show running-config to view the

active configuration

Step 2 Verify that the static route has been entered correctly If the route is not

correct, it will be necessary to go back into global configuration mode to remove the incorrect static route and enter the correct one

Step 3 Enter the command show ip route.

Step 4 Verify that the route that was configured is in the routing table

Troubleshooting Static Route Configuration

Having knowledge of troubleshooting tools and procedures is just as important in

static routing as in any other aspect of networking You can use the show interfaces

command to check the state and configuration of the interface that is to be used for the

route gateway Using the ping command helps you to determine whether end-to-end

connectivity exists If an echo reply is not received after a ping, you can use the

tracer-oute command to determine which rtracer-outer in the rtracer-oute path is dropping the packets

The routing process must happen on each router the packet travels through, or the

packet will be dropped In many cases, packets actually reach their destination, but the

remote network router has no knowledge of a route to reply to the sender

Use the following steps to troubleshoot a static route configuration:

Step 1 Make sure that the link that is to be used as the gateway by the route is

available

Step 2 Enter the command show interfaces, and verify that the interface is up

and that the line protocol is up

Example 15-5 Default Route for Waycross

Waycross(config)# ip route 0.0.0.0 0.0.0.0 s1

Example 15-6 Default Route for Sterling

Sterling(config)# ip route 0.0.0.0 0.0.0.0 s0

Step 3 Verify that the IP address being used on the interface is correct.

Step 4 ping the IP address on the remote router interface that is connected

directly to the route gateway If the ping is not successful, the problem is

not related to routing The interfaces of one or both of the directly con-nected routers might be configured incorrectly, or a physical problem might exist with the link Return to Step 1 to troubleshoot

Step 5 If the ping of the far-end router fails, use the traceroute command to

determine which router in the route path is dropping the packet

Step 6 Log into the router with the failed traceroute Return to Step 1 and start

again

Step 7 If the ping is successful, attempt to ping the far-end router If this ping is

successful, complete end-to-end connectivity has been achieved The test

of the static route is complete

Dynamic Routing Overview

Dynamic routingis necessary to allow networks to update and adapt quickly to changes The network shown in Figure 15-13 adapts differently to topology changes depending

on whether it uses statically or dynamically configured routing information

Figure 15-13 Dynamic Route

Static routing allows routers to properly route a packet from network to network based

on manually configured information In the example, Router A always sends traffic destined for Router C to Router D The router refers to its routing table and follows

A

X

D

B

C

the static knowledge residing there to relay the packet to Router D Router D does the

same and relays the packet to Router C Router C delivers the packet to the

destina-tion host

If the path between Router A and Router D fails, Router A is not capable of relaying

the packet to Router D using that static route Until Router A manually is reconfigured

to relay packets by way of Router B, communication with the destination network

is impossible Dynamic routing offers more flexibility According to the routing table

generated by Router A, a packet can reach its destination over the preferred route

through Router D

However, a second path to the destination is available by way of Router B When

Router A recognizes that the link to Router D is down, it adjusts its routing table,

making the path through Router B the preferred path to the destination The routers

continue sending packets over this link

When the path between Routers A and D is restored to service, Router A again can

change its routing table to indicate a preference for the counterclockwise path through

Routers D and C to the destination network Dynamic routing protocols also can direct

traffic from the same session over different paths in a network for better performance

This is known as load sharing

Routing Protocol Examples

This section provides a brief overview of some of the most common routing protocols

and their key characteristics

RIP originally was specified in RFC 1058 Its key characteristics include the following:

■ It is a distance vector routing protocol

■ It uses hop count as the metric for path selection If the hop count is greater than

15, the packet is discarded

■ By default, routing updates are broadcast every 30 seconds

IGRP is a distance vector routing protocol developed by Cisco IGRP sends routing

updates at 90-second intervals, advertising networks for a particular autonomous system

IGRP offers the following design characteristics and features:

■ Versatility for automatically handling indefinite or complex topologies

■ Flexibility for handling segments with different bandwidth and delay characteristics

■ Scalability for functioning in large networks

By default, IGRP uses two metrics, bandwidth and delay IGRP can be configured to use a combination of variables to determine a composite metric Possible configurations include the following variables:

■ Bandwidth

■ Delay

■ Load

■ Reliability OSPF is a link-state routing protocol used for IP Link-state protocols keep a detailed topology, which allows the protocol to use calculations that prevent loops With OSPF, the subnet mask also is transmitted, enabling features such as variable-length subnet masking (VLSM) and route summarization

EIGRP is a balanced hybrid routing protocol developed by Cisco EIGRP has charac-teristics in common with both distance vector protocols and link-state protocols EIGRP calculates the best route to each network or subnet and provides alternative routes that can be used if the current route fails EIGRP also transmits the subnet mask for each routing entry Therefore, features such as VLSM and route summarization easily are supported

BGP is an exterior routing protocol BGP is designed to operate between autonomous systems BGPs can be used between two ISPs or between a company and an ISP

Purpose of a Routing Protocol and Autonomous Systems

The goal of a routing protocol is to build and maintain the routing table This table contains the learned networks and associated ports for those networks Routers use routing protocols to manage information received from other routers and information generated from the configuration of its own interfaces

The routing protocol identifies all available routes, places the best routes into the rout-ing table, and removes routes when they are no longer valid The router uses the infor-mation in the routing table to forward routed protocol packets

The routing algorithm is fundamental to dynamic routing Whenever the topology of

a network changes because of growth, reconfiguration, or failure, the network knowl-edge base also must change The network knowlknowl-edge needs to reflect an accurate and consistent view of the new topology

When all routers in an internetwork are operating with the same knowledge, the inter-network is said to have converged Fast convergence is desirable because it reduces the time period of incorrect routing decisions

Autonomous systems divide the global internetwork into smaller and more

manage-able networks Each AS has its own set of rules and policies and an AS number that

distinguishes it from all other autonomous systems in the world

Dynamic Routing Operations

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

(see Figure 15-14)

Figure 15-14 Routing Protocols Maintain Routing Information

Dynamic routing relies on a routing protocol to share knowledge among routers A

routing protocol defines the set of rules used by a router when it communicates with

neighboring routers For example, a routing protocol describes the following:

■ How to send updates

■ What knowledge is contained in these updates

■ When to send this knowledge

■ How to locate recipients of the updates

How Distances on Network Paths Are Determined by

Various Metrics

When a routing algorithm updates a routing table, its primary objective is to determine

the best information to include in the table Each routing algorithm interprets what is

best in its own way The algorithm generates a number called the metricvalue for each

path through the network Typically, the smaller the metric number is, the better the

path is, as shown in Figure 15-15

Routing Protocol

Routing Table

Routing Protocol

Routing Table

Figure 15-15 Metrics Used to Define Best Path

You can calculate simple metrics based on a single characteristic such as path, or you can calculate more complex metrics by combining several characteristics The metric characteristics that most commonly are used by routers are as follows:

■ Bandwidth—The data capacity of a link (Normally, a 10-Mbps Ethernet link is

preferable to a 64-kbps leased line.)

■ Delay—The length of time required to move a packet along each link from

source to destination

■ Load—The amount of activity on a network resource such as a router or a link.

■ Reliability—Usually a reference to the error rate of each network link.

■ Hop count—The number of routers that a packet must travel through before

reaching its destination

■ Ticks—The delay on a data link using IBM PC clock ticks (approximately

55 milliseconds, or 1/18 second)

■ Cost—An arbitrary value, usually based on bandwidth, monetary expense, or

other measurement, that is assigned by a network administrator

Identifying the Classes of Routing Protocols

Most routing algorithms can be classified under one of the following three categories:

■ Distance vector

■ Link-state

■ Balanced hybrid

T1

56 Bandwidth

Delay

Load Reliability

Hop count Ticks

Cost

A

B

Thedistance vector routing protocolapproach determines the direction, or the vector,

and the distance to any link in the internetwork The link-state routing protocol

approach, also called shortest path first (SPF), recreates the exact topology of the entire

internetwork The balanced hybrid routing protocol approach combines aspects of the

link-state and distance vector algorithms

Distance Vector Routing Protocol Features

Distance vector routing algorithms pass periodic copies of a routing table from router

to router These regular updates between routers communicate topology changes

Dis-tance vector–based routing algorithms also are known as Bellman-Ford algorithms

In Figure 15-16, each router receives a routing table from its neighboring routers

Router B receives information from Router A Router B adds a distance vector number,

such as a number of hops, which increases the distance vector Then Router B passes

this new routing table to its other neighbor, Router C This step-by-step process occurs

in all directions between all neighbor routers

Figure 15-16 Distance Vector Concepts

The algorithm accumulates network distances so that it can maintain a database of

network topology information However, distance vector algorithms do not provide

routers with the exact topology of an internetwork because each router is aware of

only its neighbor routers

A

B

C

D

Routing Table RoutingTable RoutingTable RoutingTable Pass periodic copies of a routing table to neighbor routers and accumulate distance vectors.

Each router that uses distance vector routing begins by identifying its neighbors Fig-ure 15-17 shows distance vector discovery The interface that leads to each directly connected network is shown as having a distance of 0 As the distance vector network discovery process proceeds, routers discover the best path to destination networks based

on the information they receive from each neighbor For example, Router A learns about other networks based on the information that it receives from Router B Each

of the other network entries in the routing table has an accumulated distance vector to show how far away that network is in a given direction

Figure 15-17 Distance Vector Network Discovery

Routing table updates occur when the topology changes As with the network discov-ery process, topology change updates proceed step by step from router to router, as shown in Figure 15-18 Distance vector algorithms call for each router to send its entire routing table to each of its adjacent neighbors The routing tables include information about the total path cost as defined by its metric and the logical address of the first router on the path to each network contained in the table The metric is made up of several components, as shown in Figure 15-19

Figure 15-18 Distance Vector Topology Changes

A

0 0 1 2

W X Y Z

Routing Table

0 0 1 1

X Y Z W

Routing Table

0 0 1 2

Y Z X W Routing Table

B

Process to Update This Routing Table

Router A Sends Out This Updated Routing Table

A

Process to Update This Routing Table

Topology Change Causes Routing Table Update

Figure 15-19 Distance Vector Routing Metric Components

Routing Updates Explained

Each router receives a routing table from its directly connected neighboring routers

For example, in Figure 15-18, Router B receives information from Router A Router B

adds a distance vector number (such as a number of hops), which increases the distance

vector and passes this new routing table to its another neighbor router This same

step-by-step process occurs in all directions between direct-neighbor routers

A distance vector is comparable to the signs along a highway Highway signs direct

drivers toward a destination and indicate the distance to that destination Farther down

the highway, additional signs point toward the same destination, but now the distance

is shorter As long as the distance continues to become shorter, the traffic is on the right

path

Link-State Routing Basics

The second basic algorithm used for routing is the link-state algorithm Link-state

algo-rithms are also known as Dijkstras algorithm or as shortest path first (SPF) algoalgo-rithms

They maintain a complex database of topology information Whereas the distance

vector algorithm has nonspecific information about distant networks and no

knowl-edge of distant routers, a link-state routing algorithm maintains full knowlknowl-edge of

dis-tant routers and how they interconnect Link-state routing uses the following:

■ Link-state advertisements (LSAs)—Small packets of routing information that

are sent between routers

■ Topological database—A collection of information gathered from LSAs

■ Shortest path first (SPF) algorithm—A calculation performed on the database

resulting in the SPF tree

■ Routing table—A list of the known paths and interfaces

Routing Metric Internetwork

Engineers have implemented this link-state concept in Open Shortest Path First (OSPF) routing RFC 1583 contains a description of OSPF link-state concepts and operations Figure 15-20 illustrates these link-state concepts

Figure 15-20 Link-State Concepts

Network Discovery Processes for Link-State Routing

LSAs are exchanged between routers, starting with directly connected networks Each router, in parallel with others, constructs a topological database consisting of all the exchanged LSAs

The SPF algorithm computes network accessibility The router constructs this logical topology as a tree, with itself as the root, consisting of all possible paths to each net-work in the link-state protocol internetnet-work It then sorts these paths using SPF The router lists the best paths and the interfaces to these destination networks in the rout-ing table The router also maintains other databases of topology elements and status details

How Link-State Protocols Exchange Routing Information

Link-state network-discovery mechanisms are used to create a common picture of the entire network All link-state routers share this view of the network This is similar to having several identical maps of a town In Figure 15-21, four networks (W, X, Y, and

Link-State Advertisement Packets

Topological Database

SPF Algorithm

SPF Tree

Routing Table