Determining route source and destination

Determining route source and destination 
9.1.3 This page will explain how a router chooses a path for packet delivery.
For traffic going through a network cloud, path determination occurs at the network layer. The path determination function enables a router to evaluate the available paths to a destination and to establish the preferred handling of a packet. Routing services use network topology information to evaluate network paths. This information can be configured by the network administrator or collected through dynamic processes that are used in the network.
The network layer provides best-effort, end-to-end, packet delivery across interconnected networks. The network layer uses the IP routing table to send packets from the source network to the destination network. After the router determines which path to use, it forwards the packet from one interface to the interface or port that leads to the destination.  
The next page will discuss Layer 2 and Layer 3 addresses.

Determining the gateway of last resort

Determining the gateway of last resort 
9.1.2 This page will teach students how to configure a gateway of last resort and why it is used.
It is not feasible, or even desirable, for a router to maintain routes to every possible destination. Instead, 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. The router uses this default route to reach the gateway of last resort in an effort to forward the packet. 
A key scalability feature is that default routes keep routing tables as lean as possible. They make it possible for routers to forward packets destined to any Internet host without having to maintain a table entry for every Internet network. Default routes can be statically entered by an administrator or dynamically learned using a routing protocol.
Default routing begins with the administrator. Before routers can dynamically exchange information, an administrator must configure at least one router with a default route. Depending on the desired results, an administrator can use either of the following commands to statically configure a default route: 

ip default-network

or

ip route 0.0.0.0 0.0.0.0

The ip default-network command is used to establish a default route in networks that use dynamic routing protocols. The ip default-network command is classful, which means if the router has a route to the subnet indicated by this command, it installs the route to the major net. The ip default-network command must be issued using the major net, in order to flag the candidate default route.
The global command ip default-network 192.168.17.0 defines the Class C network 192.168.17.0 as the destination path for packets that have no routing table entries. Any routes to a network configured with ip default-network will be flagged as a candidate for the default route. 
The ip route 0.0.0.0/0 command can also be used to configure a default route. 

Router(config)#ip route prefix mask {address 1 interface } [distance ]

After configuring a default route or default network, the command show ip route will show the following: 

Gateway of last resort is 172.16.1.2 to network 0.0.0.0

The Lab Activity will show students how to configure a gateway of last resort.
The next page explains how path determination occurs.

Examining the Routing Table / The show ip route command

Examining the Routing Table 
The show ip route command 
9.1.1 This page will explain the functions of the show ip route command.
One of the primary functions of a router is to determine the best path to a given destination. A router learns paths, which are also called routes, from the configurations entered by an administrator or from other routers through routing protocols. Routers store this routing information in routing tables using on-board random access memory (RAM). A routing table contains a list of the best available routes. Routers use the routing table to make packet forwarding decisions.
The show ip route command displays the contents of the IP routing table. This table contains entries for all known networks and subnetworks, as well as a code that indicates how that information was learned. The following are some additional commands that can be used with the show ip route command:

• show ip route connected
• show ip routeaddress
• show ip route rip
• show ip route igrp
• show ip route static

A routing table maps network prefixes to an outbound interface. When RTA receives a packet destined for 192.168.4.46, it looks for the prefix 192.168.4.0/24 in its table. RTA then forwards the packet out interface Ethernet0 based on the routing table entry. If RTA receives a packet destined for 10.3.21.5, it sends that packet out Serial 0/0.
The example routing table shows four routes for directly connected networks. These routes are labeled with a C. RTA drops any packet destined for a network that is not listed in the routing table. The routing table for RTA will have to include more routes before it can forward to other destinations. There are two ways to add new routes:

• Static routing - An administrator manually defines routes to one or more destination networks.
• Dynamic routing - Routers follow rules defined by a routing protocol to exchange routing information and independently select the best path.

Administratively defined routes are said to be static because they do not change until a network administrator manually programs the changes. Routes learned from other routers are dynamic because they change automatically as directly connected routers update each other with new information. Each method has fundamental advantages and disadvantages. 
The Lab Activity will allow students to use the show ip route command to examine routing tables.
The next page will define a gateway of last resort.

Module 9: Basic Router Troubleshooting

Module 9: Basic Router Troubleshooting

Overview A router uses a dynamic routing protocol to learn about routes to destination networks. Most routers use a combination of dynamic routing and manually configured static routes. Regardless of the method used, when a router determines that a route is the best path to a destination, it installs that route in its routing table. This module will describe methods that are used to examine and interpret the contents of the routing table.
Network testing and troubleshooting are perhaps the most time consuming components of every network administrator’s job. Efficient testing and troubleshooting must be done in a logical, orderly, and well-documented fashion. Otherwise, the same problems will reoccur, and the network administrator will never truly understand the network. This module describes a structured approach to network troubleshooting and provides some tools to use in the troubleshooting process.
Routing problems are among the most common and difficult for network administrators to diagnose. There are many tools that make it easier to identify and solve routing problems. This module will introduce several of the most important of these tools and provide practice in their use.
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:

• Use the show ip route command to gather detailed information about the routes installed on the router
• Configure a default route or default network
• Understand how a router uses both Layer 2 and Layer 3 addressing to move data through the network
• Use the ping command to perform basic network connectivity tests
• Use the telnet command to verify the application layer software between source and destination stations
• Troubleshoot by sequential testing of OSI layers
• Use the show interfaces command to confirm Layer 1 and Layer 2 problems
• Use the show ip route and show ip protocol commands to identify routing issues
• Use the show cdp command to verify Layer 2 connectivity
• Use the trace route command to identify the path packets take between networks
• Use the show controllers serial command to ensure the proper cable is attached

Use basic debug commands to monitor router activity 

Module 8 : Summary

Summary

This page summarizes the topics discussed in this module. IP is an unreliable method for delivery of network data. ICMP is an error reporting protocol for IP. When datagram delivery errors occur, ICMP is used to report these errors back to the source of the datagram. ICMP echo request and echo reply messages allow the network administrator to test IP connectivity to aid in the troubleshooting process. Network communication depends on the proper configuration of TCP/IP for both sending and receiving devices. A router also must have the TCP/IP protocol properly configured on its interfaces, and it must use an appropriate routing protocol. To test the availability of a destination use the ICMP ping command. Incorrect routing information can cause a datagram to travel in a circle. The datagram will not reach its destination within the maximum hop count defined by the routing protocol. This is also known as the TTL. The ICMP message format starts with the type, code, and checksum fields. 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. Destination unreachable messages are delivered to the sender when a datagram cannot be forwarded. Codes in the message header provide information about the problem. When a datagram is not forwarded due to an error in the header, an ICMP type 12 parameter problem message is sent to the source of the datagram. Control messages inform hosts of conditions such as network congestion or the existence of a better gateway to a remote network. The ICMP redirect/change request is a common control message. It is initiated by a gateway, which is a term commonly used to describe a router. The following situations will cause default gateways to send ICMP redirect/change request messages: A packet enters a router and leaves from the same interface. The subnet/network of the source IP address is the same as the 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. All ICMP timestamp reply messages contain the originate, receive, and transmit timestamps. The host can subtract the originate time from the transit time to estimate transit time across the network. Transit time will vary based on traffic and congestion on a network.

Router solicitation message / Congestion and flow control messages

Router solicitation message 
8.2.7 This page will explain why router solicitation messages are used.
A host generates an ICMP router solicitation message in response to a missing default gateway.This message is sent using multicast and it is the first step in the router discovery process. A local router will respond with a router advertisement that identifies the default gateway for the local host. Figure identifies the frame format and Figure gives an explanation of each field.
The next page will discuss source quench messages.Congestion and flow control messages
8.2.8 This page will explain how source quench messages are used to solve problems related to network congestion.
If multiple computers try to access the same destination at the same time, the destination computer can be overwhelmed with traffic. Congestion can also occur when traffic from a high speed LAN reaches a slower WAN connection. Dropped packets occur when there is too much congestion on a network. ICMP source quench messages are used to reduce the amount of data lost. The source quench message asks senders to reduce the rate at which they transmit packets. Congestion will usually subside after a short period of time and the source will slowly increase the transmission rate if no other source quench messages are received. Most Cisco routers do not send source quench messages by default, because the source quench message may add to the network congestion.
A small office, home office (SOHO) is a scenario where ICMP source quench messages might be used effectively. A SOHO could consist of four computers that are networked with CAT-5 cable and have a shared Internet connection over a 56K modem. The 10-Mbps bandwidth of the SOHO LAN could quickly overwhelm the 56K bandwidth of the WAN link, which would result in data loss and retransmissions. The gateway host can use an ICMP source quench message to request that the other hosts reduce their transmission rates to prevent continued data loss. A network where congestion on the WAN link could cause communication problems is shown in Figure .
This page concludes this lesson. The next page will summarize the main points from this module.

Router solicitation message

Router solicitation message
8.2.6 This page will explain why router solicitation messages are used.
A host generates an ICMP router solicitation message in response to a missing default gateway. This message is sent using multicast and it is the first step in the router discovery process. A local router will respond with a router advertisement that identifies the default gateway for the local host. Figure identifies the frame format and Figure gives an explanation of each field.
The next page will discuss source quench messages.