Basic Router Troubleshooting: Hetal Presswala

Basic Router Troubleshooting Hetal Presswala August 09, 2008

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.

2 - Confidential

The show ip route Command

The show ip route command displays the contents of the IP routing table. 3 - Confidential

Examining the Routing Table Dublin# show ip route

Networks being advertised Out interface

Codes, how the routes were learnt

Administ rative Distance

Met ric

Next Hop

Time since last update

4 - Confidential

Static Vs Dynamic Routes

5 - Confidential

Static Vs Dynamic

Cont…

6 - Confidential

Need of Default route Default routes are used when there isn’t a more specific entry in routing table

7 - Confidential

How to Configure Default Route

Alternatively use the default network command Router(config)# ip default-network 172.16.1.2

8 - Confidential

Determining Route Source and Destination

9 - Confidential

Routing – Role of Layer 2 and Layer 3 Addressing

Hop

Source MAC

Destination MAC

Source IP

Destination IP

1

00:ac:2d:34:56:11

00:01:46:26:23:a4

192.168.1.5

192.168.3.10

2

00:01:46:26:23:a4

00:04:24:98:55:44

192.168.1.5

192.168.3.10

3

00:04:24:98:55:44

00:2a:3b:3c:a4:33

192.168.1.5

192.168.3.10 10 - Confidential

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.

11 - Confidential

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: 

12 - Confidential

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

13 - Confidential

Show ip route Command

14 - Confidential

Introduction to Network Testing

15 - Confidential

Structured Approach to Troubleshooting

16 - Confidential

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

17 - Confidential

Layer 1 Troubleshooting – using show interfaces Physical layer (line status) Up: has Layer 1 BHM#show interface s0 connectivity Serial0 is up, line protocol is up < output omitted > Down: L1 problem Last clearing of "show interface" counters never Administratively < output omitted > To reset: BHM# clear down: disabled counters 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

Faulty hardware, cable or noise

Line interruption s, faulty hardware

Bad line, bad hardware 18 - Confidential

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

19 - Confidential

Layer 1 Troubleshooting – show controllers

Router# 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

20 - Confidential

Troubleshooting Using show controllers serial

The show controllers serial Command

21 - Confidential

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

22 - Confidential

Troubleshooting Using show cdp neighbors

23 - Confidential

Troubleshooting Using show cdp neighborstail

24 - Confidential

Layer 2 Troubleshooting – show interfaces

Data-Link layer (up or BHM# show interface serial 0 Serial0 is up, line protocol is up down) Hardware is HD64570 Are keepalives being Internet address is 172.17.0.2/16 received? 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

25 - Confidential

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

26 - Confidential

Show interfaces Command

27 - Confidential

Typical Layer 3 Problem



Routing protocol not enabled



Wrong routing protocol enabled



Incorrect IP addresses



Incorrect subnet masks

28 - Confidential

Layer 3 Troubleshooting Using Ping

29 - Confidential

Ping vista# ping ScopeDelhi Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 192.168.1.2, timeout is 2 seconds: ‘!’ indicate a ..!!! successful echo Success rate is 60 percent (3/5), ‘.’ indicate the round-trip min/avg/max = 32/33/36 ms

application timed out waiting for an echo

Extended ping • provides extra facilities for ping

30 - Confidential

Troubleshooting Using traceroute Command

Test on a hop-by-hop basis – Each time is the RTT 31 - Confidential

Troubleshooting Issues The show Routing ip route Command

32 - Confidential

Troubleshooting Routing Issues Command The show ip protocols

33 - Confidential

Layer 7 Troubleshooting Using Telnet

34 - Confidential

Summary

35 - Confidential