Error reporting and error correction
8.1.2
Saturday, August 13, 2011
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
Summary of Module 7
Summary
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 the metrics and the logical address of the first router on the path to each network contained in the table.
RIP uses many techniques to reduce routing loops and counting to infinity. RIP permits a maximum hop count of 15. A destination greater than 15 hops away is tagged as unreachable.
The split horizon rule specifies that it is not useful to send information about a route back in the direction from which it came. In some network configurations, it may be necessary to disable split horizon.
Route poisoning is used to overcome large routing loops and provide information when a network is down. It also keeps a router from receiving incorrect updates.
Holddown timers help prevent counting to infinity but also increase convergence time. The default holddown for RIP is 180 seconds. Triggered updates are also sent if routing information changes. The router sends triggered routing update on its other interfaces rather than waiting on the routing update timer to expire.
RIP v2 enhancements include the ability to carry additional packet routing information, an authentication mechanism to secure table updates, and support for VLSM. By default, routing updates are broadcast every 30 seconds.
RIP is enabled with the router rip command. The network command is then used to tell the router on which interfaces to run RIP.
A supernet route is a route that covers a greater range of subnets with a single entry. The ip classless global configuration command is used to forward packets to the best supernet route when a router receives packets destined for an unknown subnet of a network.
The two most common commands used to verify that RIP is properly configured are the show ip route and show ip protocols commands. The show ip route command shows the routes that are installed in the routing table and the status of each route. The show ip protocols command is used to verify the state of the active routing protocol as well as the installed routes specific to the protocol.
To display RIP routing updates as they are sent and received, use the debug ip rip command.
The passive-interface command prevents routers from sending routing updates through a router interface. This keeps update messages from being sent through a router interface so that other systems on a network will not learn about routes dynamically.
The show ip route command is used to find equal cost routes for load balancing. RIP uses round robin load balancing. Routers take turns to forward packets over equal cost paths. IGRP is a distance vector routing protocol that measures distances to mathematically compare routes. It sends routing updates at 90 second intervals to advertise networks for an AS. IGRP uses a composite metric. This metric is calculated as a function of bandwidth, delay, load, and reliability. IGRP advertises three types of routes. These include interior, system, and exterior. There are many features such as holddowns and split horizons that provide stability. Use the show ip protocols and the show ip route commands to verify that IGRP is properly configured. In addition, the ping and trace commands are used to troubleshoot errors.
This page summarizes the topics discussed in this module.
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 the metrics and the logical address of the first router on the path to each network contained in the table.
RIP uses many techniques to reduce routing loops and counting to infinity. RIP permits a maximum hop count of 15. A destination greater than 15 hops away is tagged as unreachable.
The split horizon rule specifies that it is not useful to send information about a route back in the direction from which it came. In some network configurations, it may be necessary to disable split horizon.
Route poisoning is used to overcome large routing loops and provide information when a network is down. It also keeps a router from receiving incorrect updates.
Holddown timers help prevent counting to infinity but also increase convergence time. The default holddown for RIP is 180 seconds. Triggered updates are also sent if routing information changes. The router sends triggered routing update on its other interfaces rather than waiting on the routing update timer to expire.
RIP v2 enhancements include the ability to carry additional packet routing information, an authentication mechanism to secure table updates, and support for VLSM. By default, routing updates are broadcast every 30 seconds.
RIP is enabled with the router rip command. The network command is then used to tell the router on which interfaces to run RIP.
A supernet route is a route that covers a greater range of subnets with a single entry. The ip classless global configuration command is used to forward packets to the best supernet route when a router receives packets destined for an unknown subnet of a network.
The two most common commands used to verify that RIP is properly configured are the show ip route and show ip protocols commands. The show ip route command shows the routes that are installed in the routing table and the status of each route. The show ip protocols command is used to verify the state of the active routing protocol as well as the installed routes specific to the protocol.
To display RIP routing updates as they are sent and received, use the debug ip rip command.
The passive-interface command prevents routers from sending routing updates through a router interface. This keeps update messages from being sent through a router interface so that other systems on a network will not learn about routes dynamically.
The show ip route command is used to find equal cost routes for load balancing. RIP uses round robin load balancing. Routers take turns to forward packets over equal cost paths. IGRP is a distance vector routing protocol that measures distances to mathematically compare routes. It sends routing updates at 90 second intervals to advertise networks for an AS. IGRP uses a composite metric. This metric is calculated as a function of bandwidth, delay, load, and reliability. IGRP advertises three types of routes. These include interior, system, and exterior. There are many features such as holddowns and split horizons that provide stability. Use the show ip protocols and the show ip route commands to verify that IGRP is properly configured. In addition, the ping and trace commands are used to troubleshoot errors.
Troubleshooting IGRP
Troubleshooting IGRP
7.3.8 This page will introduce some commands that can be used to troubleshoot IGRP.
Most IGRP configuration errors involve a mistyped network statement, discontiguous subnets, or an incorrect AS Number.
The following commands are used to troubleshoot IGRP:
• show ip route
• debug ip igrp events
• debug ip igrp transactions
• ping
• traceroute
Figure shows output from the debug ip igrp events command.
Figure shows output from the debug ip igrp transactions command.
If the AS number is wrong and then corrected, it results in the output shown in Figure .
The Lab Activity will show students how to use the IGRP debug commands.
This page concludes this lesson. The next page will summarize the main points from this module.
Verifying IGRP configuration
Verifying IGRP configuration
7.3.7 This page will teach students how to verify an IGRP configuration.
To verify that IGRP has been configured properly, enter the show ip route command and look for IGRP routes signified by an "I".
Additional commands for checking IGRP configuration are as follows:
• show interfaceinterface
• show running-config
• show running-config interfaceinterface
• show running-config
begin interfaceinterface
• show running-config
begin igrp
• show ip protocols
To verify that the Ethernet interface is properly configured, enter the show interface fa0/0 command. Figure illustrates the output.
To see if IGRP is enabled on the router, enter the show ip protocols command. Figure illustrates the output.
The commands illustrated in Figures - verify the network statements, IP addressing, and routing tables.
In the Lab Activities, students will verify an IGRP configuration and then use IGRP to set up dynamic routing.
The next page will teach students how to troubleshoot IGRP
Migrating RIP to IGRP
Migrating RIP to IGRP
7.3.6 This page will teach students how to convert a router from RIP to IGRP.
When Cisco created IGRP in the early 1980s, it was the first company to solve the problems associated with the use of RIP to route datagrams between interior routers. IGRP examines the bandwidth and delay of the networks between routers to determine the best path through an internetwork. IGRP converges faster than RIP. This prevents routing loops that are caused by disagreement over the next routing hop. Further, IGRP does not share the hop count limitation of RIP. As a result of this and other improvements over RIP, IGRP enabled many large, complex, topologically diverse internetworks to be deployed.
Use the following steps to convert from RIP to IGRP:
1. Enter show ip route to verify that RIP is the routing protocol on the routers to be converted.
2. Configure IGRP on Router A and Router B.
3. Enter show ip protocols on Router A and Router B. 4. Enter show ip route on Router A and Router B.
The Lab Activities will show students how to configure a default route, use RIP to propagate the information, and then convert the router to IGRP.
The next page will explain how to verify that IGRP has been configured properly
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