WXES2106Network Technology

Semester 1 2004/2005

Chapter 7

TCP/IP Suite Error and Control Messages

CCNA2: Module 8, 9

Contents

Introduction ICMP Message Control Message Examining the Routing Table Network Testing Troubleshooting Router

Introduction

IP is an unreliable method for delivery of network data It is known as a best effort delivery mechanism. Internet Control Message Protocol (ICMP) is an error

reporting protocol for IP. ICMP reports on the status of the delivered packet only

to the source device. ICMP messages are encapsulated into datagrams in the

same way any other data is delivered using IP.

Introduction

If a router receiving a packet that it is unable to deliver to its ultimate destination, the router sends an ICMP host unreachable message to the source.

Introduction

If the destination device receives the ICMP echo request, it formulates an echo reply message to send back to the source of the echo request.

If the sender receives the echo reply, this confirms that the destination device can be reached via the IP protocol.

Introduction

Introduction

Excessively long route The packet that exceed the maximum hop count,

which is also known as reaching its time-to-live (TTL). The hop limit of RIP is 15. When the TTL of the datagram value reaches zero, the packet is discarded.

Two routers continually route a datagram back and forth between them never reaching its destination.

ICMP Message

Each ICMP message start with these fields: Type

Indicates the type of ICMP message being sent Code

Includes further information specific to the message type

Checksum Verify the integrity of the data

ICMP Message

ICMP Message

Destination unreachable message Hardware failures, improper protocol configuration,

down interfaces and incorrect routing information May also be sent when packet fragmentation is

required in order to forward a packet. May also be generated if IP related services such as

FTP or Web services are unavailable.

ICMP Message

Code Values for Destination Unreachable Message

Control Message

Control messages are used to inform hosts of conditions such as network congestion or the existence of a better gateway to a remote network.

A common ICMP control message is the ICMP redirect/change request.

This type of message can only be initiated by a router. When a host connects to a segment that has two or

more directly connected routers. The default gateway of the host may need to use a redirect/change request to inform the host of the best path to a certain network.

Control Message

Redirect/change request Message and Code

Control Message

Default gateways send ICMP redirect/change request messages when The interface on which the packet comes into the

router is the same interface on which the packet gets routed out.

The subnet/network of the source IP address is the same subnet/network of the next-hop IP address of the routed packet.

The datagram is not source-routed. The route for the redirect is not another ICMP redirect

or a default route. The router is configured to send redirects.

Control Message The ICMP timestamp request message allows a host to

ask for the current time according to the remote host. The remote host uses an ICMP timestamp reply

message to respond to the request. Originate timestamp

The time on the requesting host just before the timestamp request is sent

Receive timestamp The time that the destination host receives the ICMP

timestamp request. Transmit timestamp

Filled in before the ICMP timestamp reply is returned.

Control Message Address mask

If a host does not know the subnet mask, it may send an address mask request to the local router or broadcast.

When the router receives the request, it will respond with an address mask reply.

Router Discovery The host sending a router solicitation message to all

routers, using the multicast address 224.0.0.2 as the destination address.

When a router that supports the discovery process receives the router discovery message, a router advertisement is sent in return.

Control Message

A host generates an ICMP router solicitation message in response to a missing default gateway.

Router Discovery Message

Control Message ICMP source-quench messages asks senders to

reduce the rate at which they are transmitting packets during congestion

However, most Cisco routers do not send source-quench messages by default, because the source-quench message may itself add to the network congestion.

It is used effectively in SOHO

Examine Routing Table

Routers store the routing information in routing tables using on-board dynamic random access memory (DRAM).

The show ip route command displays the contents of the IP routing table.

Examine Routing Table Routers keep a default route, or a gateway of last

resort. Default routes are used when the router is unable to match a destination network with a more specific entry in the routing table.

Before routers can dynamically exchange information, an administrator must configure at least one router with a default route. ip default-network or ip route 0.0.0.0 0.0.0.0

Examine Routing Table

Examine Routing Table The administrative distance of the route is the key information that the

router uses in deciding which is the best path to a particular destination. The lower the administrative distance, the more trustworthy the

source.

Examine Routing Table

Routing protocols use metrics to determine the best route to a destination.

The smaller the metric number, the better the path. Static metric: bandwidth and delay Dynamic metric: Load and reliability

Examine Routing Table IGRP Metric

Metric=K1 *Bandwidth + (K2 * Bandwidth)/256-load) + K3 * Delay

The default constant values are K1=K3=1 and K2=K4=K5=0 so:

Metric=Bandwidth + Delay Command to determine the last routing update

Router#show ip route Router#show ip route network Router#show ip protocols Router#show ip rip database

Network Testing Troubleshooting is a process that allows a user to find

problems on a network. Basic testing of a network should proceed in sequence

begin with Layer 1 and work to Layer 7. Layer 1 errors can include:

Broken cables, disconnected cables, cables connected to the wrong ports

Intermittent cable connection, Wrong cables used Transceiver problems DCE cable problems, DTE cable problems Devices turned off

Network Testing

Troubleshooting

Network Testing Layer 2 errors can include:

Improperly configured serial interfaces or Ethernet interfaces

Improper encapsulation set Improper clockrate settings on serial interfaces Network interface card (NIC) problems

Layer 3 errors can include: Routing protocol not enabled Wrong routing protocol enabled Incorrect IP addresses or subnet masks

Network Testing Indicator lights are a useful tool for troubleshooting The ping utility is used to test network connectivity. The telnet utility is a virtual terminal protocol that is part

of the TCP/IP protocol suite. It allows verification of the application layer software between source and destination stations.

Troubleshooting Router The Cisco IOS contains a rich set of commands for

troubleshooting. Router#show interfaces

check the status and statistics of the interfaces Router#show interfaces FastEthernet

view the status of the FastEthernet interfaces Router#show interfaces serial0/0

view the status of Serial 0/0

Troubleshooting Router Layer 1 Problem

Cable Connectors Interfaces

Layer 2 Problem No keepalives No clock rate Mismatch in encapsulation type

Router#show cdp neighbors Displays information about directly connected

neighbors.

Troubleshooting Router Traceroute

It is used to discover the routes that packets take when traveling to their destination

Test the network layer (Layer 3) on a hop-by-hop basis and provide performance benchmarks.

The output of the traceroute command generates a list of hops that were successfully reached.

If an asterisk (*) appears, the packet failed. Sends out a sequence of User Datagram Protocol

(UDP) datagrams from the router to an invalid port address on the remote host.

This router then responds with an ICMP Time Exceeded Message (TEM)

Troubleshooting Router Router#show ip protocols

display information about routing protocols and the routing table

Router#show ip route displays the contents of the IP routing table.

Router#show controllers serial determine the type of cable connected

The debug command is used to display dynamic data and events.

By default, the router sends the debug output and system messages to the console.

Troubleshooting Router Router#terminal monitor

redirected debug output to the remote terminal Router#service timestamps debug uptime

provides the time when the debug event occurred and the duration of time between events.

Router#no debug all turn off all diagnostic output.

Router#undebug all turn off all diagnostic output.