How to Enable SSH on Cisco IOS

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You can enable secure shell (SSH) on Cisco routers both with and without AAA (Authentication, Authorization, and Accounting)

If you wish to use SSH on a Cisco router without AAA in IOS release 12.4 and above, you can configure local username and password authentication and use enhanced password security.

Here are the two methods for enabling SSH.

Configuring SSH with AAA

This configuration will only allow connections from the 10.2.1.0/24 subnet via access list 99.

hostname CISCO877
aaa new-model

 

username nick privilege 15 secret 5 XXX

access-list 99 permit 10.2.1.0 0.0.0.255

line vty 0 4
 access-class 99 in
 privilege level 2
 login
 transport input ssh
 transport output none

Configuring SSH without AAA

This is the minimum configuration needed to support inbound SSH sessions on a router:

hostname CISCO877
ip domain-name xxx.xxx.com
crypto key generate rsa
! define local usernames, use passwords or secrets
username a password b
username x secret y
!
ip ssh version 2
!
line vty 0 4
 login local

I hope this post helps you get SSH set up on Cisco IOS for your router.

Here's another way to do it:

It is recommended that Secure Shell (SSH) is used for remote administration of Cisco Routers and Switches.

To see if SSH is already enabled

CISCO877# show ip ssh
%SSH has not been enabled

To enable SSH on your Cisco Switch or Router, do the following from the global configuration mode:

Configure the Hostname on the Switch or Router

Router(config)# hostname CISCO877

Configure the Domain name for the Cisco Switch or Router

CISCO877(config)# ip domain-name my.domain.com

Generate a RSA Key Pair

CISCO877(config)# crypto key generate rsa

This enables SSH on the Cisco switch or the router.

The following optional commands are recommended but are not mandatory:

Set the SSH Negotiation phase timeout interval (in seconds)

CISCO877(config)# ip ssh time-out 120

This sets the time period for the Cisco Router or Switch to wait for 120 seconds before timing out the client during the SSH Negotiation phase.

Set the Maximum retry attempts

CISCO877(config)# ip ssh authetication-retries 3

This sets the maximum number of the Authentication retires to be 3 times before the interface (vty) is reset.

To change the default port for SSH (default is 22) connection

CISCO877(config)# ip ssh port 3536

This sets the port number to listen for SSH connections to be 3536.

Try to logon and logoff the Cisco IOS Router or switch to ensure it works OK and then disable Telnet access to the switch. This can be done by making SSH as the only transport agent.

CISCO877(config)# line vty 0 4
CISCO877(config)# transport input ssh

This makes SSH as the only way to connect to the Cisco Routers or Switches remotely.

Write the config changes to the startup-config:

CISCO877# wr mem

To see the status of the SSH on the Cisco switch or the router

CISCO877# show ip ssh
SSH Enabled – version 1.5

 

Authentication timeout: 120 secs; Authentication retries: 3

To view the status of SSH connections

CISCO877# show ssh

Connection      Version     Encryption State   Username

 0                1.5       3DES    Session Started  guest

If at any time, you want to disable SSH on the Cisco Router or Switch, from the Global Configuration

CISCO877(config)# crypto key zeroize rsa

This deletes the RSA key-pair. Once you delete the RSA key-pair, it disables the SSH server.

In this case, if you had ssh as the only transport mode then this needs to be changed to default

CISCO877(config)# line vty 0 4
CISCO877(config)# transport input telnet
 

 

Comments

  1. Hi Nick, I have a similar setup to you and since 28 October 2009 I have been plagued by ATM interface drop outs and high numbers of errors. Are you still with BE Broadband? Would you be willing to share a little information about your configuration? All best, Chris Bailes.
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