CCNA :) Be a Good Network Administrator

Echo messages 8.1.7 This page will provide information about ICMP messages. As with any type of packet, ICMP messages have special formats. Each ICMP message type shown in Figure has its own unique characteristics. All ICMP message formats start with the same three fields: • Type • Code • Checksum The type field indicates the type of ICMP message being sent. The code field includes further information specific to the message type. The checksum field, as in other types of packets, is used to verify the integrity of the data. Figure shows the message format for the ICMP echo request and echo reply messages. The relevant type and code numbers are shown for each message type. The identifier and sequence number fields are unique to the echo request and echo reply messages. The identifier and sequence fields are used to match the echo replies to the corresponding echo request. The data field contains additional information that may be a part of the echo reply or echo request messag...

Detecting excessively long routes 8.1.6 This page will explain how excessively long routes are created. Situations can occur in network communication where a datagram travels in a circle, never reaching its destination. This might occur if two routers continually route a datagram back and forth between them, thinking the other should be the next hop to the destination. When there are several routers involved, a routing cycle is created. In a routing cycle, a router sends the datagram to the next hop router and thinks the next hop router will route the datagram to the correct destination. The next hop router then routes the datagram to the next router in the cycle. This can be caused by incorrect routing information. The limitations of the routing protocol can result in unreachable destinations. The hop limit of RIP is 15, which means that networks that are greater than 15 hops will not be learned through RIP. In either of these cases, an excessively long route exists. Whether th...

Detecting excessively long routes 8.1.6 This page will explain how excessively long routes are created. Situations can occur in network communication where a datagram travels in a circle, never reaching its destination. This might occur if two routers continually route a datagram back and forth between them, thinking the other should be the next hop to the destination. When there are several routers involved, a routing cycle is created. In a routing cycle, a router sends the datagram to the next hop router and thinks the next hop router will route the datagram to the correct destination. The next hop router then routes the datagram to the next router in the cycle. This can be caused by incorrect routing information. The limitations of the routing protocol can result in unreachable destinations. The hop limit of RIP is 15, which means that networks that are greater than 15 hops will not be learned through RIP. In either of these cases, an excessively long route exists. Whether th...

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 de...

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. ...

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 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...

Error reporting and error correction 8.1.2

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.

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 under...

Summary 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 co...

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 protocols • 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 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 tea...

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...

Migrating RIP to IGRP 7.3.6

Migrating RIP to IGRP 7.3.6

Configuring IGRP 7.3.5 This page will introduce the commands that are used to configure IGRP. To configure the IGRP routing process, use the router igrp configuration command. To shut down an IGRP routing process, use the no form of this command. The command syntax is as follows: RouterA(config)#router igrpas-number RouterA(config)#no router igrpas-number The AS number identifies the IGRP process. To specify a list of networks for IGRP routing processes, use the network router configuration command. To remove an entry, use the no form of the command. Figure shows an example of how to configure IGRP for AS 101. The Lab Activities will help students configure IGRP. The next page will explain how to convert a router from RIP to IGRP.

IGRP stability features 7.3.4 This page will describe three features that are designed to enhance the stability of IGRP: • Holddowns • Split horizons • Poison reverse updates Holddowns Holddowns are used to prevent regular update messages from reinstating a route that may not be up. When a router goes down, neighbor routers detect this from the lack of regularly scheduled update messages. Split horizons Split horizons are derived from the premise that it is not useful to send information about a route back in the direction from which it came. The split horizon rule helps prevent routing loops between adjacent routers. Poison reverse updates Poison reverse updates are used to prevent larger routing loops. Increases in routing metrics usually indicate routing loops. Poison reverse updates then are sent to remove the route and place it in holddown. With IGRP, poison reverse updates are sent only if a route metric has increased by a factor of 1.1 or greater. IGRP also maintains ...

IGRP routes 7.3.3 This page will introduce the three types of routes that IGRP advertises: • Interior • System • Exterior Interior Interior routes are routes between subnets of a network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes. System System routes are routes to networks within an autonomous system. The Cisco IOS software derives system routes from directly connected network interfaces and system route information provided by other IGRP routers or access servers. System routes do not include subnet information. Exterior Exterior routes are routes to networks outside the autonomous system that are considered when a gateway of last resort is identified. The Cisco IOS software chooses a gateway of last resort from the list of exterior routes that IGRP provides. The software uses the gateway of last resort if a better route is not found and the destination is not a connected network. If the auto...

IGRP metrics 7.3.2 This page will describe the metrics that IGRP uses. The show ip protocols command displays parameters, filters, and network information about the routing protocols in use on the router. The algorithm used to calculate the routing metric for IGRP is shown in the graphic. It defines the value of the K1 to K5 metrics and provides information about the maximum hop count. The metric K1 represents bandwidth and the metric K3 represents delay. By default the values of the metrics K1 and K3 are set to 1, and K2, K4, and K5 are set to 0. This composite metric is more accurate than the hop count metric that RIP uses to choose a path to a destination. The path that has the smallest metric value is the best route. IGRP uses the following metrics: • Bandwidth – The lowest bandwidth value in the path • Delay – The cumulative interface delay along the path • Reliability – The reliability on the link toward the destination as determined by the exchange of keepalives • Loa...

IGRP IGRP features 7.3.1 This page will explain the main features and functions of IGRP. IGRP is a distance vector IGP. Distance vector routing protocols measure distances to mathematically compare routes. This measurement is known as the distance vector. Routers that use distance vector protocols must send all or a portion of their routing table in a routing update message at regular intervals to each neighbor router. As routing information spreads throughout the network, routers perform the following functions: • Identify new destinations • Learn of failures IGRP is a distance vector routing protocol developed by Cisco. IGRP sends routing updates at 90 second intervals. These updates advertise all the networks for a particular AS. Key design characteristics of IGRP are a follows: • The versatility to automatically handle indefinite, complex topologies • The flexibility needed to segment with different bandwidth and delay characteristics • Scalability for functioning in ve...

Integrating static routes with RIP 7.2.10 This page will explain how static routes can be configured on a router that uses RIP. Static routes are user-defined routes that force packets to take a set path from a source to a destination. Static routes become very important if the Cisco IOS software does not learn a route to a particular destination. They are also used to specify a gateway of last resort, which is commonly referred to as a default route. If a packet is destined for a subnet that is not explicitly listed in the routing table, the packet is forwarded to the default route. A router that runs RIP can receive a default route through an update from another router that runs RIP. Another option is for the router to generate the default route itself. Use the no ip route global configuration command to remove static routes. The administrator can override a static route with dynamic routing information by adjusting the administrative distance values. Each dynamic routing prot...

Load balancing across multiple paths 7.2.9 This page will further explain how routers use load balancing to transmit packets to a destination IP address over multiple paths. The paths are derived either statically or with dynamic protocols, such as RIP, EIGRP, OSPF, and IGRP. When a router learns multiple routes to a specific network, the route with the lowest administrative distance is installed in the routing table. Sometimes the router must select a route from among many, learned through the same routing process with the same administrative distance. In this case, the router chooses the path with the lowest cost or metric to the destination. Each routing process calculates its cost differently and the costs may need to be manually configured in order to achieve load balancing. If the router receives and installs multiple paths with the same administrative distance and cost to a destination, load-balancing can occur. Cisco IOS imposes a limit of up to six equal cost routes in a ...

Load balancing with RIP 7.2.8 This page will describe load balancing and explain how RIP uses this feature. Load balancing is a concept that allows a router to take advantage of multiple best paths to a given destination. These paths are either statically defined by a network administrator or calculated by a dynamic routing protocol such as RIP. RIP is capable of load balancing over as many as six equal-cost paths. The default is four paths. RIP performs what is referred to as “round robin” load balancing. This means that RIP takes turns forwarding packets over the parallel paths. Figure shows an example of RIP routes with four equal cost paths. The router will start with an interface pointer to the interface connected to Router 1. Then the interface pointer cycles through the interfaces and routes in a deterministic fashion such as 1-2-3-4-1-2-3-4-1 and so on. Since the metric for RIP is hop count, the speed of the links is not considered. Therefore, the 56-Kbps path will be g...

Load balancing with RIP

Preventing routing updates through an interface 7.2.7 This page will teach students how to prevent routing updates. Route filtering regulates the routes that are entered into or advertised out of a route table. These have different effects on link-state routing protocols than they do on distance vector protocols. A router that runs a distance vector protocol advertises routes based on what is in its route table. As a result, a route filter influences which routes the router advertises to its neighbors. Routers that run link-state protocols determine routes based on information in the link-state database, rather than the route entries advertised by neighbor routers. Route filters have no effect on link-state advertisements or the link-state database. For this reason, the information on this page only applies to distance vector IP routing protocols such as RIP and IGRP. The passive-interface command prevents the transmission of routing updates through a router interface. When upda...

Troubleshooting RIP update issues 7.2.6 This page will teach students how to troubleshoot RIP update issues. Most of the RIP configuration errors involve an incorrect network statement, discontiguous subnets, or split horizons. An effective command that is used to find RIP update issues is the debug ip rip command. The debug ip rip command displays RIP routing updates as they are sent and received. The example in Figure shows the output from the debug ip rip command after a router receives a RIP update. After the router receives and processes the update, it sends the updated information out its two RIP interfaces. The output shows the router uses RIP v1 and broadcasts the update with the broadcast address 255.255.255.255. The number in parenthesis represents the source address encapsulated into the IP header of the RIP update. There are several key indicators to look for in the output of the debug ip rip command. Problems such as discontiguous subnetworks or duplicate networks c...

Verifying RIP configuration 7.2.5 This page will describe several commands that can be used to verify that RIP is properly configured. Two of the most common are the show ip route command and the show ip protocols command. The show ip protocols command shows which routing protocols carry IP traffic on the router. This output can be used to verify most if not all of the RIP configuration. Some of the most common configuration items to verify are as follows: • RIP routing is configured. • The correct interfaces send and receive RIP updates. • The router advertises the correct networks. The show ip route command can be used to verify that routes received by RIP neighbors are installed in the routing table. Examine the output of the command and look for RIP routes signified by "R". Remember that the network will take some time to converge so the routes may not appear immediately. Additional commands to check RIP configuration are as follows: • show interfaceinterfa...