CCNA 2 v3.1 Module 9 Basic Router Troubleshooting

Objectives 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 This module will describe methods for examining and interpreting 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 This module will introduce several of the most important of these tools and provide practice in their use.

The show ip route Command The show ip route command displays the contents of the IP routing table.

Examining the Routing Table A routing table contains a list of the best available routes. Routers use routing tables to make packet forwarding decisions. Dublin# show ip route Codes, how the routes were learnt Networks being advertised Administrative Distance Metric Next Hop Time since last update Out interface

Static Routing

Dynamic Routing

Default Route Default routes are used when there isn’t a more specific entry in routing table. The router uses the default route to reach the gateway of last resort. 

Configuring a Default Route Alternatively use the default network command Router(config)# ip default-network 172.16.1.2

Determining Route Source and Destination

Routing – Role of Layer 2 and Layer 3 Addressing Hop Source MAC Destination MAC Source IP Destination IP 1 2 3 00:ac:2d:34:56:11 00:01:46:26:23:a4 192.168.1.5 192.168.3.10 00:01:46:26:23:a4 00:04:24:98:55:44 192.168.1.5 192.168.3.10 00:04:24:98:55:44 00:2a:3b:3c:a4:33

Role of Packets and Frames in Path Traversal

Determining L2 and L3 Addresses

Determining Administrative Distance AD measures the trustworthiness of the source of the route information. The lower the AD, the more trustworthy the source. Different routing protocols have different default ADs. When there are two paths available to a destination the path with the lowest AD is used in the routing table.

Determining the Route Metric Routing algorithms generate a number, called the metric value, for each path through the network . The metric value is used to determine the best route to a destination. The various protocols base their metrics on different factors: 

Routing Metrics Hop Count number of hops Bandwidth speed of the link Delay the time taken to traverse the path Reliability the error rate on a link, from 1(bad) to 255 (good). Load the amount of traffic on the link from 1(good) to 255(bad) MTU Largest packet allowed on link Cost Arbitrary meaning – assigned by administrator – lower is better

Routing Metric Calculations RIP Hop Count IGRP Metric = [K1 * Bandwidth + (K2 * Bandwidth)/(256-load) + K3*Delay] * [K5/(reliability + K4)] Defaults: K1 = K3 = 1 and K2 = K4 = K5 = 0. If K5 = 0, then [K5/(reliability + K4)] =0. Metric = Bandwidth + Delay.

Viewing the Routing Table Use the following commands to find the last routing update: show ip route show ip route network show ip route connected show ip route static show ip protocols show ip rip database

Show ip route Command

Observing Multiple Paths to a Destination 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.

Introduction to Network Testing

Structured Approach to Troubleshooting

Typical Layer 1 Errors Broken cables Disconnected cables Cables connected to the wrong ports Intermittent cable connection Wrong cables used for the task at hand Transceiver problems DCE cable problems DTE cable problems Devices turned off

Troubleshooting Using show interfaces Useful for troubleshooting Layer1, Layer 2 in addition to Layer 3 addressing

Layer 1 Troubleshooting – using show interfaces BHM# show interface s0 Serial0 is up, line protocol is up < output omitted > Last clearing of "show interface" counters never 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 problem Administratively down: disabled Faulty hardware, cable or noise Bad line, bad hardware To reset: BHM# clear counters Line interruptions, faulty hardware

Layer 1 Troubleshooting – using show interfaces The show interfaces serial command

Layer 1 Troubleshooting – show controllers show controllers serial 0 HD unit 0, idb = 0x1086D4, driver structure at 0x10E568 buffer size 1524 HD unit 0, V.35 DCE cable, clockrate 56000 Router#

Troubleshooting Using show controllers serial The show controllers serial Command

Typical Layer 2 Errors Improperly configured serial interfaces Improperly configured Ethernet interfaces Improper encapsulation set Improper clock rate settings on serial interfaces Network interface card (NIC) problems

Troubleshooting Using show cdp neighbors

Troubleshooting Using show cdp neighbors detail

Layer 2 Troubleshooting – show interfaces Data-Link layer (up or down) Are keepalives being received? BHM# show interface serial 0 Serial0 is up, line protocol is up Hardware is HD64570 Internet address is 172.17.0.2/16 MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255 Encapsulation HDLC, loopback not set, keepalive set (10 sec) Last input 00:00:01, output 00:00:00, output hang never Last clearing of "show interface" counters never < output omitted > Encapsulation must match at both ends of connection 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

Layer 2 Troubleshooting If the interface is up and the line protocol is down, a Layer 2 problem exists. Indicates whether the IOS processes consider the interface usable. 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 If the interface misses three consecutive keepalives, line protocol is down When the line is down, the protocol is always down    

show interfaces Command

Typical Layer 3 Errors Routing protocol not enabled Wrong routing protocol enabled Incorrect IP addresses Incorrect subnet masks

Layer 3 Troubleshooting Using Ping

Ping Extended ping provides extra facilities for ping ping SanJose1 Type 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 vista# Extended ping provides extra facilities for ping

Troubleshooting Using traceroute Command Test on a hop-by-hop basis – Each time is the RTT

Troubleshooting Routing Issues The show ip route Command

Troubleshooting Routing Issues The show ip protocols Command

Introduction to debug Debug syntax

Debug Used to display dynamic data and events as they occur. Displays current events of the router, example: traffic on an interface error messages generated by nodes on the network protocol-specific diagnostic packets Debug commands effect router performance due to high processor overhead. Debug should be used sparingly to avoid disrupting normal router operation.   

Debug output and system messages are sent to the console. Debug all Router# debug all But don’t use as it can crash the system Debug output and system messages are sent to the console. To redirected to a Telnet client: Router# terminal monitor To add timestamps to debug messages: Router(config)# service timestamps debug uptime To Turn off debugging Router# no debug all Router# undebug all

Layer 7 Troubleshooting Using Telnet

Summary