Use ping to test destination reachability

Use ping to test destination reachability

8.1.5 This page will explain how the ping command can be used to test the reachability of a network.

The ICMP protocol can be used to test the availability of a particular destination. Figure shows ICMP being used to issue an echo request message to the destination device. 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 using the IP protocol.


The echo request message is typically initiated with the ping command as shown in Figure . In this example, the command is used with the IP address of the destination device. The command can also be entered with the IP address of the destination device as shown in Figure . In these examples, the ping command issues four echo requests and receives four echo replies. This confirms IP connectivity between the two devices.


As seen in Figure , the echo reply includes a time-to-live (TTL) value. TTL is a field in the IP packet header used by IP to provide a limitation on packet forwarding. As each router processes the packet, it decreases the TTL value by one. When a router receives a packet with a TTL value of 1, it will decrement the TTL value to 0 and the packet cannot be forwarded. An ICMP message may be generated and sent back to the source machine, and the undeliverable packet is dropped.

The next page will discuss excessively long routes

Unreachable networks

Unreachable networks

8.1.4 This page will explain why some networks are unreachable.

Network communication depends on some basic conditions that must be met. First, the TCP/IP protocol must be properly configured for devices that send and receive data. This includes the installation of the TCP/IP protocol and proper configuration of an IP address and subnet mask. A default gateway must also be configured if datagrams are to travel outside of the local network. Second, intermediary devices must be in place to route the datagram from the source device and its network to the destination network. Routers perform this function. A router also must have the TCP/IP protocol properly configured on its interfaces, and it must use an appropriate routing protocol.

If these conditions are not met, then network communication cannot take place. For instance, the sending device may address the datagram to a non-existent IP address or to a destination device that is disconnected from its network. Routers can also be points of failure if a connecting interface is down or if the router does not have the information necessary to find the destination network. If a destination network is not accessible, it is said to be an unreachable network.

Figures and show a router that receives a packet that cannot be delivered. The packet is undeliverable because there is no known route to the destination. Because of this, the router sends an ICMP host unreachable message to the source.

The next page will teach students how to test network reachability

ICMP message delivery

ICMP message delivery

8.1.3 This page will describe the delivery method that is used by ICMP.

ICMP messages are encapsulated into datagrams in the same way any other data is delivered when IP is used. Figure displays the encapsulation of ICMP data within an IP datagram.

Since ICMP messages are transmitted in the same way as any other data, they are subject to the same delivery failures. This creates a scenario where error reports could generate more error reports and cause increased congestion on a network. For this reason, errors created by ICMP messages do not generate their own ICMP messages. Therefore, it is possible to have a datagram delivery error that is never reported back to the sender of the data.

The next page will discuss unreachable networks.

Error reporting and error correction

Error reporting and error correction

8.1.2 This page will explain how ICMP reports errors for IP. When datagram delivery errors occur, ICMP is used to report these errors back to the source of the datagram. Look at the example in Figure . Workstation 1 tries to send a datagram to Workstation 6, but interface Fa0/0 on Router C goes down. Router C uses ICMP to send a message back to Workstation 1. The message indicates that the datagram could not be delivered. ICMP does not correct any network problems that it encounters, it only reports them.

When Router C receives the datagram from Workstation 1, it knows only the source and destination IP addresses of the datagram. It does not know the exact path that the datagram took. Therefore, Router C can only notify Workstation 1 of the failure and no ICMP messages are sent to Router A and Router B. ICMP reports on the status of the delivered packet only to the source device. It does not send information about network changes to other routers.

The next page will explain how ICMP message delivery occurs.

Error reporting and error correction

Error reporting and error correction

8.1.2

Overview of TCP/IP Error Message / ICMP

Overview of TCP/IP Error Message

ICMP

8.1.1 This page will introduce a protocol that addresses the limitations of IP.

IP is an unreliable method for the delivery of network data. It is known as a best effort delivery mechanism. It has no built-in process to ensure that data is delivered if problems exist with network communication. If an intermediary device such as a router fails, or if a destination device is disconnected from the network, data cannot be delivered. Additionally, nothing in its basic design allows IP to notify the sender that a data transmission has failed. ICMP is the component of the TCP/IP protocol stack that addresses this basic limitation of IP. ICMP does not overcome the unreliability issues in IP. Reliability is provided by upper layer protocols.

The next page will explain how ICMP reports delivery errors.

Module 8: TCP/IP Suite Error and Control Messages / Overview

TCP/IP Suite Error and Control Messages

Overview

IP is limited because it is a best effort delivery system. It has no mechanism to ensure that data is delivered over a network. Data may fail to reach its destination for a variety of reasons such as hardware failure, improper configuration, or incorrect routing information. To help identify these failures, IP uses the Internet Control Message Protocol (ICMP) to notify the sender of the data that there was an error in the delivery process. This module describes the various types of ICMP error messages and some of the ways they are used.

Because IP does not have a built-in mechanism for sending error and control messages, it uses ICMP to send and receive error and control messages to hosts on a network. This module focuses on control messages, which are messages that provide information or configuration parameters to hosts. Knowledge of ICMP control messages is an essential part of network troubleshooting and is important to fully understand IP networks.

This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams. -

Students who complete this module should be able to perform the following tasks:

• Describe ICMP

• Describe ICMP message format

• Identify ICMP error message types
• Identify potential causes of specific ICMP error messages
• Describe ICMP control messages
• Identify a variety of ICMP control messages used in networks
• Determine the causes for ICMP control messages