Identifying Commands and Applications Used to Correct Problems Occurring at the Application Layer

This section presents some commands that you can use to correct problems occurring at the application layer. The selected commands relate to SNMP, NTP, and Dynamic Host Configuration Protocol (DHCP). Of course, you can use other commands to configure and correct application layer problems. The Cisco Internetwork Troubleshooting (CIT) course intends to familiarize you with the troubleshooting process, rather than listing and explaining all Cisco IOS commands. This book also discusses those same commands so that you are well prepared for the CIT exam.

SNMP is an application layer member of the TCP/IP suite. Naturally, operation of this protocol depends on the transport, internet, and network interface layers. As the name implies, SNMP manages (configuration and troubleshooting) network entities. SNMP is based on open standards and can be utilized in multivendor environments. The output shown in Example 12-3 was captured on a Cisco router. Example 12-3 shows the snmp-server command options and a brief explanation for each of them. The snmp-server enable, snmp-server community, and snmp-server host commands are highlighted to emphasize their importance. The snmp-server enable command enables SNMP traps or informs on a Cisco router. The snmp-server community command sets the community string and access privileges (read-only, read, read-write, and view); it essentially configures a community string to act like a password to regulate read-write and read-only access to the agent (router). The snmp-server host command allows you to specify the IP address of the SNMP notification host (recipient).

Example 12-3 Cisco IOS snmp-server Commands and Options

Router(config)#snmp-server enable ? informs Enable SNMP Informs traps Enable SNMP Traps

Router(config)#snmp-server community ?

WORD SNMP community string

Router(config)#snmp-server community somename ?

<1-99> Std IP accesslist allowing access with this community string

<1300-1999> Expanded IP accesslist allowing access with this community string ro Read-only access with this community string rw Read-write access with this community string view Restrict this community to a named MIB view

Router#conf t

Enter configuration commands, one per line. End with CNTL/Z. Router(config)#snmp-server ?

chassis-id community contact enable engineID

group host ifindex inform location manager packetsize queue-length system-shutdown tftp-server-list trap trap-source trap-timeout user view

String to uniquely identify this chassis

Enable SNMP; set community string and access privs

Text for mib object sysContact

Enable SNMP Traps or Informs

Configure a local or remote SNMPv3 engineID

Define a User Security Model group

Specify hosts to receive SNMP notifications

Enable ifindex persistence

Configure SNMP Informs options

Text for mib object sysLocation

Modify SNMP manager parameters

Largest SNMP packet size

Message queue length for each TRAP host

Enable use of the SNMP reload command

Limit TFTP servers used via SNMP

SNMP trap options

Assign an interface for the source address of all traps Set timeout for TRAP message retransmissions Define a user who can access the SNMP engine Define an SNMPv2 MIB view

Example 12-3 Cisco IOS snmp-server Commands and Options (Continued)

Router(config)#snmp-server community somename ro ?

<1-99> Std IP accesslist allowing access with this community string

<1300-1999> Expanded IP accesslist allowing access with this community string <cr>

Router(config)#snmp-server community somename rw ?

<1-99> Std IP accesslist allowing access with this community string

<1300-1999> Expanded IP accesslist allowing access with this community string <cr>

Router(config)#snmp-server host ?

Hostname or A.B.C.D IP address of SNMP notification host

Time synchronization and accurate time and calendar on internetwork devices are of great importance in most environments. NTP serves that purpose; it is a UDP-based (port 123) standard protocol (RFC 1305 describes NTP version 3) that was designed to synchronize the time on a network of devices. Example 12-4 shows ntp commands and configuration options. ntp server, ntp peer, and ntp source commands are highlighted to emphasize their significance. The ntp server and ntp peer commands are straightforward. The first command allows you to specify the address of another device that plays a server role, and the latter allows you to specify another time server device as a peer. The ntp source command specifies which interface's IP address you should use during the NTP communications with other devices.

Example 12-4 Cisco IOS ntp Commands and Options

Router(config)#ntp

?

access-group

Control NTP access

authenticate

Authenticate time sources

authentication-key

Authentication key for trusted time sources

broadcastdelay

Estimated round-trip delay

clock-period

Length of hardware clock tick

master

Act as NTP master clock

max-associations

Set maximum number of associations

peer

Configure NTP peer

server

Configure NTP server

source

Configure interface for source address

trusted-key

Key numbers for trusted time sources

Router(config)#ntp

peer ?

Hostname or A.B.C

.D

IP address of peer

Router(config)#ntp

server ?

Hostname or A.B.C.D IP address of peer

Hostname or A.B.C.D IP address of peer

Example 12-4 Cisco IOS ntp Commands and Options (Continued)

Router(config)#ntp source ?

Async

Async interface

BVI

Bridge-Group Virtual Interface

CTunnel

CTunnel interface

Dialer

Dialer interface

Ethernet

IEEE 802.3

Lex

Lex interface

Loopback

Loopback interface

Multilink

Multilink-group interface

Null

Null interface

Serial

Serial

TokenRing

IEEE 802.5

Tunnel

Tunnel interface

Vif

PGM Multicast Host interface

Virtual-FrameRelay

Virtual Frame Relay interface

Virtual-Template

Virtual Template interface

Virtual-TokenRing

Virtual TokenRing

You can use the service timestamps command to configure how the system log messages and debug messages are time stamped: based on how long the device has been up for, or based on regular date/ time. The information about system events and messages, including the time they occurred, is crucial. Example 12-5 displays the configuration options for the Cisco IOS service timestamps command.

Example 12-5 Cisco IOS service timestamps Commands and Options

Service Timestamps Cisco

Example 12-5 Cisco IOS service timestamps Commands and Options (Continued)

Router(config)#service timestamps debug datetime ?

localtime Use local time zone for timestamps msec Include milliseconds in timestamp show-timezone Add time zone information to timestamp <cr>

DHCP plays a significant role in today's TCP/IP networks. Traditionally, you would configure a special server (or two) as DHCP server(s) and many personal computers as DHCP clients requesting and receiving information such as IP address, subnet mask, default gateway, and so on from the server. You would configure the routers to forward these requests and responses (BOOTP Client and BOOTP Server messages), which are UDP-based broadcasts, by converting them to unicasts. The ip helper-address interface configuration command allows you to configure a router's interface to forward a client's bootp-request (in unicast form) to a DHCP server and insert the network address of the segment where the original requested was generated into the request's DHCP header. Some of today's network devices, such as Cisco routers, allow you to configure the router to be a DHCP server, rather than merely acting as a relay-agent, which has been routers' traditional role. The output captured and displayed in Example 12-6 shows the ip helper-address command and the service dhcp commands and descriptions.

Example 12-6 Cisco IOS ip helper-address and service dhcp Commands and Options

Router(config)#interface ethernet 0/0

Router(config-if)#ip helper-address ?

A.B.C.D IP destination address

Router(config-if)#exit

Router(config)#

Router(config)#service dhcp

Router(config)#ip dhcp

?

conflict

DHCP address conflict parameters

database

Configure DHCP database agents

excluded-address

Prevent DHCP from assigning certain addresses

ping

Specify ping parameters used by DHCP

pool

Configure DHCP address pools

relay

DHCP relay agent parameters

smart-relay

Enable Smart Relay feature

Router(config)#

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