Cisco Troubleshoot: VLAN mismatch

Hardware: Cisco Catalyst 2960 S Series Switch
Use "show version" to display the hardware configuration, Cisco IOS version, names and sources of configuration files, boot images (and boot sequences).

Symptoms:
The "show logging" displays a large volume of CDP messages potentially bogging down the network with warnings about vlan mismatch.

What is a VLAN?
Diagnosis:
Vlan mismatch could indicate that the switch does not have the VLAN 2 created on it, and is being asked to route traffic from a VLAN that is not advertised on the switch. Usually, only the default native VLAN 1 is configured by factory default, but the attached router is advertising VLAN 2.

Fix: Add VLAN 2 to the swtich
Access: Console Access or Cisco SDM (web GUI)

• If Console login is not configured by default, user must telnet to swtich to configure the "line con 0"
• If Telnet is disabled/ not configured by default and must be enabled first through the Cisco SDM
• web GUI.
• Cisco SDM is a web interface to configure the switch as an alternative to the Command Line Interface

HOW TO Use the Cisco SDM
1) Launch the webpage http://ipaddressoftheswitch
2) Login as cisco, cisco; You will be asked to change after the first login
3) Enable Telnet access to the switch from the Advanced Settings page
4) Use the web menus and configuration tabs, Add VLAN 2 and assign all the switchports to VLAN2

If you cannot open the Cisco SDM webpage because the username login is invalid and cannot be recovered, you can resort to restore the Switch to factory default to use the default login.


Alternatively, you may login as EXEC Privileged, global configuration mode and use the Cisco commands
switch (config)# line con 0
login
password mypassword

This indicates the maximum number of 5 sessions for telnet.
Additional lines may be specified with line vty 5 15
switch (config)# line vty 0 4
password
password myTelnetpassword


Verify Telnet and Assign ports to VLAN 2
Before closing the Cisco SDM or the CLI, verify that you can telnet into the switch from another machine on the network.
switch (config)# int range fa 0/1 - 24
switchport mode acces vlan 2
no schutdown

Do a "show running" on the switch and all the switchports should be ssigned to VLAN2.
This command configures ssh (and removes Telnet access. Beware!)
line vty 0 15
login local
transport input ssh

(if you leave that blank after ssh, then there is no longer any telnet acces)