Cisco IOS Diagnostic Commands


Cisco IOS commands are built in with the software and provide both basic and high-level troubleshooting. Using these commands, you can gather information about the state of the network and devices. This data helps identify potential network performance bottlenecks and decide a corrective action to remove these bottlenecks. However, a major drawback of the Cisco IOS commands is that they consume a significant amount of resources and lead to deterioration in the performance of routers. As a result, these commands need to be properly planned and should be used only on a temporary basis.

There is a wide range of show and debug commands, which can be used for viewing status information and debugging at the event or process level. In addition, the ping and trace commands are available for checking connectivity and routing status over heterogeneous networks.

The Show Command

Cisco provides various IOS commands that display the status of routers, their interface, and utilization of router resources. In addition, the show commands enable isolation of network faults and monitor network behavior during installation and redesign. The functions performed by show commands are:

  • Isolate faults related to applications, interface, nodes, and media

  • Display router behavior during initial installation

  • Monitor the network for congestion

  • Monitor day-to-day operations of the network

  • Provide status updates for servers and clients

Some of the commonly used show commands for troubleshooting are:

Show version: Displays details such as the system hardware configuration, software version, names and sources of configuration files, and boot images.

Show running-config: Displays the current router configuration.

Show startup-config: Shows the router configuration stored in NVRAM.

Show interfaces: Displays statistical data about all the interfaces configured on the router or access server.

Show controllers: Views the statistics of the interface card controller.

Show flash: Provides the layout and content of flash memory.

Show buffers: Shows data for buffer pools on the router.

Show memory summary: Displays the memory pool statistics and summary information about the system memory allocator and provides a block-by-block listing of the memory use.

Show process cpu: Provides information about processes that are active on the router.

Show stack: Shows information about the utilization of processes, and interrupts routines and provides information about the reason for the last system reboot.

Show cdp neighbors: Displays information about the degree of reachability of directly connected Cisco devices. This is a Cisco-proprietary command that assists in determining the operational status of the Physical and Data-link layers.

Show debugging: Displays information about the type of debugging that is enabled for a router.

Like the debug commands, some of the show commands listed are accessible only at the privilege exec mode (enable mode) of the router. Table 4.1 displays the show commands and the services of each command.

Table 4.1 : The Show Commands and Their Services

Command

Service

access-lists

Lists the access lists of the routers

arp

Displays the ARP table

cdp

Displays the CDP information

clock

Displays the system clock

configuration

Displays the contents of the nonvolatile memory

controllers

Displays the interface controller status

flash

Displays information about the flash filesystem

frame-relay

Displays information about the Frame Relay

history

Displays information about the session command history

hosts

Displays the domain name, nameservers, and the host table

interfaces

Displays the interface status and configuration

ip

Displays information about IP protocol

ipx

Displays information about the Novell IPX protocol

isdn

Displays information about ISDN

ntp

Displays information about the Network Time Protocol (NTP)

protocols

Displays information about the currently active network routing protocols

running-config

Displays the current operating configuration

sessions

Displays information about the TELNET connections

startup-config

Displays the startup configuration

terminal

Displays the terminal configuration parameters

users

Displays information about the terminal lines

version

Displays information about the hardware and the software of the system

The Debug Command

The debug command provides detailed information about network traffic on an interface. It also provides error messages generated by nodes along with information about protocol-specific diagnostic packets. Debug commands run in the privilege exec mode of the CLI. However, as stated earlier, this tool needs to be used with caution to minimize the impact on the network. This means that you should use debug commands only for detecting and isolating network problems and not for day-to-day monitoring of network statistics. Debug commands are usually processor intensive, which increases the CPU utilization tremendously and has a negative impact on network performance. This occurs when the network router is preloaded with other tasks.

The adverse impact of debug commands can be avoided by following certain guidelines:

  • Before using the debug command, use the no logging console global configuration to prevent other users from logging on to the console.

  • Executing the debug command at the console port generates character-by-character interrupts, which increases the processor load. To prevent this, it is recommended that you use Telnet to access the privilege exec mode and the terminal monitor command to copy the output of the debug command to the current terminal. This enables remote viewing of the debug command output without connecting to the console port.

  • Spool the output to a file to save the debug command output.

  • Before using the debug command, check if network traffic is at its peak. Execute the command when the network traffic is not at its peak so that the data packet transmission is not hampered.

  • Disable the command after the debugging process is over. To disable a particular debug command, execute the no debug command. You can also disable all types of debugging commands by using the no debug all command.

The output of the debug command differs depending on the type of debug command that is executed. While some debug commands generate a single line of output, others provide multiple lines of output per packet. Using the debug command, you can debug almost all networking features. Table 4.2 displays some the services that use the debug command:

Table 4.2 : The Debug Command and Its Services

Command

Service

nbf

NetBIOS

arp

ARP and HP probe transactions

lapb

LAPB transactions

cls

CLS

apollo

Apollo

domain

Domain Name System (DNS)

broadcast

MAC broadcast packets

aps

Automatic Protection

cbus

CiscoBus events

ipx

IPX

atm

ATM signaling

llc2

LLC2 type II

nhrp

NHRP

callback

Callback activity

sna

SNA

modem

Modem activation

clns

CLNS

serial

Serial interface

compress

COMPRESS traffic

dhcp

DHCP client activity

custom-queue

Custom output queueing

decnet

DECnet

ipc

Interprocess communications

dialer

Dial on Demand

token

Token Ring

dnsix

Dnsix

lnx

qllc/llc2 conversion

cdp

CDP information

dxi

ATM-DXI

eigrp

EIGRP

x25

X.25

ethernet-interface

Ethernet network interface

translate

Protocol translation

filesys

Filesystem

sscop

SSCOP

fras

FRAS Debug

ip

IP

stun

STUN

adjacency

Adjacency

isis

IS-IS

kerberos

KERBEROS

lane

LAN emulation

access-expression

Boolean access expression

lat

LAT

ppp

PPP

sdllc

SDLLC

local-ack

Local acknowledgment

async

Async interface

mop

DECnet MOP server

chat

Chat scripts activity

ntp

NTP

tacacs

TACACS

pad

X.25 PAD

lex

LAN extender

source

Source bridging

probe

Probe proxy requests

radius

RADIUS

rif

RIF cache

sdlc

SDLC

packet

Log unknown packets

confmodem

Modem configuration database

telnet

Telnet connections

smrp

SMRP

channel

Channel interface

xremote

XREMOTE

snmp

SNMP

frame-relay

Frame Relay

spanning

Spanning tree

dspu

DSPU

standby

Hot standby

fastethernet

Fast Ethernet interface

vlan

VLAN

tarp

TARP

tbridge

Transparent bridging

v120

V120

tftp

TFTP

apple

AppleTalk

entry

Incoming queue entries

tunnel

Tunnel information

priority

Priority output queueing

vg-anylan

VG-AnyLAN interface

vines

VINES

snapshot

Snapshot activity

dlsw

Data Link Switching events

xns

XNS

smf

Software MAC filter

Note

Regardless of the precautionary conditions applied while debugging, it is advisable to use third-party tools for troubleshooting, especially when you need to gather information about protocols. A more feasible alternate for gathering information about protocols is a third-party protocol analyzer.

The Ping Commands

The ping command is used to monitor host reachability and connectivity over a network. The command can be executed from the user and privilege exec modes. In the user exec mode, ping allows connection with remote devices, basic level testing, and temporary modification of terminal settings.

The ping command checks basic network connectivity for AppleTalk, IP, Novell, Apollo, ISO Connectionless Network Service (CLNS), VINES, DECnet, or XNS network.

In the case of IP, the ICMP echo messages are sent by the ping command to check the interconnectivity among hosts in a network. If a problem occurs while echoing packets, ICMP reports errors and provides relevant information pertaining to IP packet addressing. For example, Figure 4.4 shows the output of a successful ping command, when ICMP successfully echoes the data packets.

click to expand
Figure 4.4: Output of a successful ping command echoing data packets.

The privilege exec mode allows you to specify the supported IP header options, which enable routers to perform detailed testing.

In the initial stages, some echo messages may timeout due to the need for setting the Address Resolution Protocol (ARP). The ping command works on a round trip path, in which the destination node sends a reply to the echo message, sent from the source node. While troubleshooting, do remember that the ping command

usually fails because the destination host does not find a path to send a reply to the echo message.

The Trace Command

The trace command determines the routes taken by data packets that are transmitted from the source to the destination host. This command receives the error messages generated by routers when the datagram exceeds its TTL and provides the appropriate remedial action.

For example, you can use the trace diagnostic command to identify the route from the host to a specified IP address. Figure 4.5 shows the output of a successful trace command.

click to expand
Figure 4.5: The trace diagnostic command showing its output.

The initial probe datagrams are transmitted with a TTL value of 1. The first router discards the probe datagrams and sends back a Time Exceeded error message. After this, the trace command sends a number of probe datagrams and displays the time of the round trip, increasing the TTL value by 1 after every third probe.

An outgoing datagram can receive two types of error message:

Time exceeded: Is generated when an intermediate router has seen and discarded the probe datagram.

Port unreachable: Is generated when the destination host receives the probe datagram and discards it because it could not be passed to any application.

The trace command is terminated when the destination host responds or the maximum TTL limit is exceeded. In addition, the trace command is terminated when it is interrupted using the escape sequence.




Cisco IP Routing Protocols(c) Trouble Shooting Techniques
Cisco IP Routing Protocols: Trouble Shooting Techniques (Charles River Media Networking/Security)
ISBN: 1584503416
EAN: 2147483647
Year: 2006
Pages: 130

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