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CCNA2

Module 9

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One of the primary functions of a router is to determine the best path to a given destination.

A router learns paths, also called routes, from an administrator's configuration or from other routers by way of routing protocols.

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

A routing table contains a list of the best available routes

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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

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Default routing begins with the administrator Before routers can dynamically exchange

information, an administrator must configure at least one router with a default route

ip default-networkor ip route 0.0.0.0 0.0.0.0

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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

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at each interface, as the packet moves across the network, the routing table is examined and the router determines the next hop.

The packet is then forwarded using the MAC address of that next hop.

The IP source and destination headers do not change, at any time.

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The Layer 3 address is used to route the packet from the source network to the destination network.

The source and destination IP addresses remain the same. The MAC address changes at each hop or router

A data-link layer address is necessary because delivery within the network is determined by the address in the Layer 2 frame header, not the Layer 3 packet header.

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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 administrative distance is a number that measures the trustworthiness of the source of the route information. The lower the administrative distance, the more trustworthy the source

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Different routing protocols have different default administrative distances

A route is not installed in the routing table if the administrative distance from another source is lower

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Routing protocols use metrics to determine the best route to a destination. The metric is a value that measures the desirability of a route.

Some routing protocols use only one factor to calculate a metric

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IGRP calculates the metric by adding the weighted values of different characteristics of the link to the network in question

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In the following example the values bandwidth, bandwidth divided by load, and delay, are weighted with the constants K1, K2, and K3.

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

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Some routing protocols support multiple paths to the same destination. Unlike single path algorithms, these multi-path algorithms permit traffic over multiple lines, provide better throughput, and are more reliable

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The commands telnet and ping are two important commands that are used for testing a network.

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Layer 1 errors can include:

Broken cables Disconnected cables Cables connected to the wrong ports Intermittent cable connection Wrong cables used for the task at hand (must use rollovers,

crossover cables, and straight-through cables correctly) Transceiver problems DCE cable problems DTE cable problems Devices turned off

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Layer 2 errors can include:

Improperly configured serial interfaces Improperly configured Ethernet interfaces Improper encapsulation set (HDLC is default

for serial interfaces) Improper clockrate settings on serial

interfaces Network interface card (NIC) problems

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Layer 3 errors can include:

Routing protocol not enabled Wrong routing protocol enabled Incorrect IP addresses Incorrect subnet masks

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The ping command is used at Layer 3 to test connectivity.

At Layer 7 the telnet command may be used to verify the application layer software between source and destination stations.

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Ping

The ping command can be invoked from both user EXEC mode and privileged EXEC mode.

The ping command can be used to confirm basic network connectivity

To use extended ping, type ping at the command line, then press the Enter key without entering an IP address

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Telnet

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.

This is the most complete test mechanism available. The telnet utility is normally used to connect remote devices, to gather information, and to run programs.

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Troubleshooting Router

The Cisco IOS contains a rich set of commands for troubleshooting. Among the more widely used are the show commands

show interfaces show interfaces FastEthernet show interfaces serial0/0. show version

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This is determined by whether keepalives are successfully received.

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If the show interfaces output shows the last clearing of the counters as never, use the show version command to find out how long the router has been functional.

Use the clear counters command to reset the counters to zero

These counters should always be cleared after an interface problem has been corrected

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The show interfaces command is perhaps the single most important tool to discover Layer 1 and Layer 2 problems with the router.

The first parameter (line) refers to the physical layer

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The second parameter (protocol) indicates whether the IOS processes that control the line protocol consider the interface usable

This is determined by whether keepalives are successfully received.

Keepalives are defined as messages sent by one network device to inform another network device that the virtual circuit between the two is still active

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If the interface is up and the line protocol is down, a Layer 2 problem exists. Among the possible causes are:

No keepalives No clock rate Mismatch in encapsulation type

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Troubleshooting using show cdp

The output from the show cdp neighbors command displays information about directly connected neighbors

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Troubleshooting using traceroute

The traceroute command is used to discover the routes that packets take when traveling to their destination.

Traceroute can also be used to help test the network layer (Layer 3) on a hop-by-hop basis and provide performance benchmarks.

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Troubleshooting using show ip route

The show ip route command is perhaps the single most important command for troubleshooting routing issues

This command displays the contents of the IP routing table.

The output from the show ip route command shows the entries for all known networks and subnetworks, and how that information was learned

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Troubleshooting using show ip protocols

The show ip protocols command displays values about IP routing protocol information on the entire router.

This command can be used to confirm which protocols are configured, which networks are being advertised, which interfaces are sending updates, and the sources of routing updates

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Troubleshooting using show controllers serial

By examining the show controllers serial command output, the type of cable that the controller detects can be determined. This is useful for finding a serial interface with no cable, the wrong type of cable, or a defective cable.

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Debug

The debug commands assist in the isolation of protocol and configuration problems

The debug command is used to display dynamic data and events

The dynamic output of the debug command comes at a performance cost, producing high processor overhead that may disrupt normal router operation

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Verifying RIP ConfigurationDublin# show ip protocols

RIP routing is configured

Interfaces sending and receiving RIP updates

Router is advertising the correct networks

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Check power. Check the NIC or interface’s ‘Link Light’. Make sure interface cards are inserted correctly. Ensure appropriate, known-good cable is correctly attached (both ends). Verify that switch ports are in the correct collision domain (VLAN), and

have appropriate options set (spanning tree). Check transceiver is attached and functional.

Layer 1 Troubleshooting

BHM# show interface s0

Serial0 is up, line protocol is up< output omitted > Last clearing of "show interface" counters never< output omitted >

Received 73 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 135 packets output, 7361 bytes, 0 underruns 0 output errors, 0 collisions, 37 interface resets 0 output buffer failures, 0 output buffers swapped out 54 carrier transitions

Physical layer (line status)Up: has Layer 1 connectivity Down: L1 problemAdministratively down: disabled

Faulty hardware, cable or noise

Bad line, bad hardware

To reset: BHM# clear counters

Line interruptions, faulty hardware

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BHM#

Layer 2 TroubleshootingIf the interface is up and the line protocol is down, a Layer 2 problem exists.Indicates whether the IOS processes consider the interface usable.If the interface misses three consecutive keepalives, line protocol is down.     When the line is down, the protocol is always down.show interface serial 0Serial0 is up, line protocol is upHardware is HD64570Internet address is 172.17.0.2/16MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255Encapsulation HDLC, loopback not set, keepalive set (10 sec)Last input 00:00:01, output 00:00:00, output hang neverLast clearing of "show interface" counters never < output omitted >

Router# show controllers serial 0

HD unit 0, idb = 0x1086D4, driver structure at 0x10E568buffer size 1524 HD unit 0, V.35 DCE cable, clockrate 56000

Data-Link layer (up or down)Are keepalives being received?

Encapsulation must match at both ends of connection

In lab situations where the router is the DCE the clockrate must be set

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Layer 3 TroubleshootingBoth Ping and Traceroute are used to troubleshoot Layer 3 problems.The ping utility is used to test basic network connectivity.The ping command can be used with IP and most other Layer 3 protocols.Extended ping offers a more extensive range of test options.

vista# ping SanJose1Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds:..!!!Success rate is 60 percent (3/5),round-trip min/avg/max = 32/33/36 ms

‘!’ indicate a successful echo‘.’ indicate the application timed out waiting for an echo

YORK# trace ROMEType escape sequence to abort.Tracing the route to ROME (172.16.33.5) 1 LONDON 192.168.1.6 16 msec 16 msec 16 msec 2 PARIS 192.168.1.37 16 msec * 16 msec 3 ROME 172.16.33.5 16 msec 16 msec 4 msec

Traceroute is used to discover routes packets take to their destination. Also used to test on hop-by-hop basis and provide performance benchmarks.The last good hop can be used to indicate a problem area (failed link).

* packet failed

RTT used to approximate delay on the link

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