TechTip: Configure Cisco Catalyst 2900/3500/3550/XL as a "dumb switch"

There is a number of proprietary Cisco protocols designed to make network administration easier and more streamlined that unfortunately get in a way when you try to setup a "dumb switch" to simply split the connection to multiple ports:

Reset switch configuration to factory defaults

If you have access to the "enable" console on the switch simply enter the following commands:

enable
[enter the switch enable password]
write erase
reload
[do not save configuration if prompted]

Note: if you do not know the switch enable password, you can follow this article to reset it to factory defaults: How-to: Reset Cisco Catalyst 2900/3500/3550/XL to factory defaults

Disable VTP and CDP

VLAN Trunking Protocol, or VTP is Cisco's proprietary Layer 2 protocol designed to propagate VLAN changes done on the root switch to all the child switches in the network. While convenient for streamlining network changes, it is unnecessary on a "dumb" switch that has a single VLAN, simply splitting a network connection to multiple ports. To disable your switch's participation in VTP, set it to "transparent mode" with the following command:

vtp mode transparent

Cisco Discovery Protocol is another proprietary Layer 2 protocol developed by Cisco systems for information sharing between adjacent switches. When enabled, OS version, IP address, on-demand routing and VLAN information is broadcasted from one switch to another (every 60 seconds by default). If left enabled on a "dumb" switch, it will detect VLAN mismatches between the switches, which will most likely be announced with a similar message in the console: CDP-4-NATIVE_VLAN_MISMATCH. To disable CDP on the switch, use the following command:

no cdp run

Note: you can optionally disable CDP on the uplink port only using the "no cdp enable" command in its interface configuration

Configuring a single-VLAN uplink port

Last thing to do is to ensure that the uplink port (port that connects to the other, parent switch) is set to be an "access", not "trunk" mode (unless you plan providing more than one VLAN on the switch, in which case it's not really "dumb" which defeats the whole purpose of this article):

interface [uplink port number: Fa0/# or Gi0/#]
switchport mode access

Saving configuration

When you're done, make sure that you save your configuration, so that you don't lose your changes next time the switch reboots:

copy run start
[enter to confirm]

Summary

So, to summarize, these are the commands in order that you need to execute to set your switch up to act as a "dumb switch":

enable
write erase
reload
[wait for switch reboot]
enable
configure [terminal]
vtp mode transparent
no cdp run
interface [Fa0/# or Gi0/#]
switchport mode access
exit
exit
write run start

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