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Ip ssh version 2

As explained in Acing the CCNA Exam: Volume 2 Advanced Networking and Security

ip ssh version 2 is a Cisco IOS global configuration command that restricts the device's SSH service to SSH version 2.

Restricts a Cisco IOS device's SSH service to SSHv2 connections. Cisco IOS enables SSHv1.99 by default, allowing both SSHv1 and SSHv2; SSHv2 is the recommended, more secure choice. section 5.3.2 section 5.4

Remember these configuration facts:

  • Enter the command in global configuration mode after generating an RSA key pair. section 5.3.2
  • Use an RSA modulus of at least 768 bits for SSHv2; newer software may require at least 2,048 bits. section 5.3.2
  • Combine it with transport input ssh on the VTY lines to exclude Telnet; SSH uses TCP port 22. section 5.3.2

How Acing the CCNA Exam: Volume 2 Advanced Networking and Security covers it

How deep it goes
Explains the command, default protocol behavior, RSA-size limitation, and related VTY configuration, but not SSH internals.
Examples
Uses an annotated SW1 configuration and SSH login from a workstation or Cisco router.
What it assumes
Assumes Cisco IOS global configuration and RSA key generation, while defining the SSHv1, SSHv2, and SSHv1.99 distinction.
What you can do after
Operate/configure Cisco IOS SSH service for SSHv2-only access and verify its supporting VTY requirements.
How it connects
Connects SSHv2 with SSHv1, RSA keys, VTY lines, Telnet, local authentication, and access control lists.
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