Command

Modules: | 1 | 2 | 3 | 4 | 5 | 6 | 7 | All |

Module 4: Layer 2 Troubleshooting

4.1.2

No component above the data link layer is functional

> Router#show cdp neighbor
> Router#ping
> Router#show interfaces

4.1.6

Layer 2 to Layer 3 address mapping errors

> Router#show arp
> Router(config-if)#no frame-relay inverse-arp
> Router#clear arp-cache
> Router#show cdp neighbor detail
> Router#show frame-relay map

4.2.2

Microsoft Windows end-system commands

> winipcfg
> ipconfig /all
> tracert
> netstat -r
> netstat -n
> arp -a

4.2.3

UNIX/Mac operating systems end-system commands

> ifconfig -a
> traceroute

4.2.4

General Cisco IOS commands

> Router#ping {host | ip-address}
> Router#trace [destination]
> Router#clear counter
> Router#debug ?
> Router#debug {option}
> Router#undebug all

4.2.6

The show cdp neighbor command

> Router(config)#show cdp neighbors
> Router(config)#cdp run
> Router(config-if)#cdp enable

4.3.2

Troubleshooting STP loops

> Switch(config)#spanning-tree vlan vlan-ID
> Switch#show spanning-tree [vlan vlan-ID]

4.3.3

Preventing STP loops

> Switch#show udld [interface-ID]
> Switch(config)#udld enable
> Switch(config-if)#udld enable

4.3.6

Troubleshooting Ethernet switch flooding

> Switch(config-if)#switchport port-security
> Switch(config-if)#switchport port-security maximum maximum
> Switch#show spanning-tree [vlan vlan-ID]
> Switch#show mac-address-table

4.3.7

Troubleshooting VTP misconfiguration

> Switch#show vtp status

4.3.8

Troubleshooting EtherChannel

> Switch(config)#port-channel load-balance [dst-mac | src-mac]

4.4.1

T1 Framing errors

> Router#show controllers t1 [slot/port]
> Router(config-controller)#clock source {line {primary | secondary} | internal}
> Router(config-controller)#framing {sf | esf }
> Router(config-controller)#cablelength long [dbgain-value dbloss-value]
> Router(config-controller)#cablelength short {133 | 266 | 399 | 533 | 655}
> Router(config-controller)#linecode {ami | b8zs | hdb3}

4.4.2

Troubleshooting ISDN primary rate switch types

> Router(config)#isdn switch-type switch-type
> Router(config-if)#isdn switch-type switch-type
> Router(config-controller)#pri-group timeslots range

4.4.3

Troubleshooting ISDN BRI to telco communications

> Router#show isdn status
> Router(config-if)#isdn twait-disable
> Router#debug isdn q921

4.4.4

Troubleshooting ISDN BRI to dialer problems

> Router#debug dialer events

4.4.5

Debugging ISDN call setup failures

> Router#debug isdn q931

4.5.1

Steps for troubleshooting Frame Relay

> Router#show interfaces [type number] [first] [last] [accounting]
> Router#show frame-relay lmi [type number]
> Router#show frame-relay pvc
> Router#show frame-relay map
> Router#debug frame-relay lmi [interface name]
> Router#debug frame-relay events
> Router#debug frame-relay packet [interface name [dlci value]]

4.6.1

Link Control Protocol

> Router#debug ppp negotiation

4.6.2

Troubleshooting PPP authentication PAP

> Router(config)#username name {nopassword | password password | password encryption-type encrypted-password}
> Router(config-if)#ppp pap sent-username username password password
> Router#debug ppp authentication
> Router(config-if)#ppp authentication {protocol1 [protocol2...]} [if-needed] [list-name | default] [callin] [one-time]
> Router(config-if)#ppp chap refuse [callin]

4.6.5

OSPF in an NBMA environment

> Router(config-if)#ip ospf network {broadcast | non-broadcast | {point-to-multipoint [non-broadcast] | point-to-point}}

4.6.6

The keyword broadcast

> Router(config-if)#frame-relay map protocol protocol-address dlci [broadcast] [ietf | cisco]
> Router(config-router)#neighbor ip-address [priority number] [poll-interval seconds] [cost number] [database-filter [all]]
> Router(config-if)#ip ospf network {broadcast | non-broadcast | {point-to-multipoint [non-broadcast] | point-to-point}}

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