Cisco Catalyst 3750 Series Switch Configuration

Official documentation

• Cisco Catalyst 3750 Series Switches - Cisco
• Getting Started Guide
• Software Configuration Guide

Command aliases

Command	Alias
configure terminal	conf t
show	sho
show running-config	sho ru

Factory reset switch

Press and hold the ‘MODE’ button for ~10 seconds.

Lights should start blinking after 3 seconds, and stop blinking 7 seconds later. Release button once they stop blinking.

---

Create a maximum privilege user

! Turn on privileged commands if your shell is 'Host>' rather than 'Host#'.
Host> enable

! Enter configuration mode
Host# configure terminal

! Create user 'admin' and define privilege level '15' and plaintext secret 'insecure'
Host(config)# username admin privilege 15 secret 0 insecure

Test running-config changes

Show running config. We use do to access the upper-menu command show. If you exit the ‘config’ mode/menu with exit, you no longer need to use do.

Host(config)# do show running-config

Try logging in on another session with your new user. You should go directly into privileged mode with #:

Host(config)# exit
Host# exit
$ ssh admin@10.20.200.1
Password: insecure
Host#

Changes currently won’t persist upon a power-cycle or reload as they are not yet in the non-volatile ‘startup-config’:

Host(config)# do show startup-config

Undo any running-config mistakes

To disable a setting, use no. To reset a setting to factory defaults, use default. For example, to remove the admin user:

Host(config)# default username admin

Alternatively, to reset the running-config back to its startup by completely overwriting it with the contents of config.txt, which contains the ‘startup-config’. List the directory with dir, and read the files present with more.

Host(config)# exit
Host# configure replace config.txt

Don’t use copy startup-config running-config as this performs a merge, where running-config options will be overwritten only if they exist in startup-config. This is a copy paste, not a replace.

Write changes to startup-config

Write ‘running-config’ to non-volatile memory:

Host(config)# do write

Finalise your changes

The password for the ‘admin’ user will currently be in the config in plaintext. To hash the stored password in the config, power cycle the switch:

Host(config)# do reload

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Setup SSH

Source: Configuring Switch-Based Authentication

hostname# configure terminal
hostname(config)# hostname Switch  ! Should already have a hostname by default
Switch(config)# ip domain name Cisco
Switch(config)# crypto key generate rsa general-keys modulus 2048
Switch(config)# line vty 0 4  ! select virtual terminal lines 0-4
Switch(config-line)# login local  ! Disable tacacs authentication so local passwords/secrets work

Hardening

Prevent telnet connections on vty 0-4. Only allow ssh:

Switch(config-line)# transport input ssh

Prevent telnet and ssh connections on vty 5-15

Switch(config-line)# exit  ! exit vty conf 0-4
Switch(config)# line vty 5 15
Switch(config-line)# transport input none
Switch(config-line)# end  ! return to privileged exec mode

Save changes to startup-config

Switch# write  ! save to startup-config

Deprecated cryptology

OpenSSH has deprecated the algorithms and ciphers which this switch tries to use. To resolve this without changing any OpenSSH configuration files, I use these command line arguments:

$ ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 -oHostKeyAlgorithms=+ssh-rsa -oPubkeyAcceptedKeyTypes=+ssh-rsa -oCiphers=+aes256-cbc cisco@10.20.200.1

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Change switch IP

Source: Assigning the Switch IP Address and Default Gateway

Switch# configure terminal
Switch(config)# interface Vlan1

! Static
Switch(config-if)# ip address 10.20.30.40 255.255.255.0

! DHCP
Switch(config-if)# ip address dhcp

Clear err-disabled

Check interface status

Switch# show interfaces status
...
Gi1/0/50                     err-disabled 1            auto   auto unknown
Gi1/0/51                     err-disabled 1            auto   auto unknown
Gi1/0/52                     err-disabled 1            auto   auto unknown

Clear error and restart interface

Switch# clear errdisable interface gigabitEthernet 1/0/52 vlan 1
Switch# configure terminal
Switch(config)# interface GigabitEthernet 1/0/52
Switch(config-if)# shutdown  ! interface down
Switch(config-if)# no shutdown  ! interface up

Check interface status

Switch(config-if)# end
Switch# show interfaces status
...
Gi1/0/50                     err-disabled 1            auto   auto unknown
Gi1/0/51                     err-disabled 1            auto   auto unknown
Gi1/0/52                     notconnect   1            auto   auto unknown