Use this command to generate RSA key pairs in your Cisco tool (consisting of a router). keys are generated in pairs–one public RSA key and one non-public RSA key.
If your router already has RSA keys when you issue this command, you will be warned and brought on to update the existing keys with new keys.
NOTE: Before issuing this command, make sure that your router has a hostname and IP domain call configured (with the choices hostname and ip domain-call commands).
You can be unable to finish the choices crypto key generate rsa command without a hostname and IP area name. (This situation isn’t always proper whilst you generate simplest a named key pair.)
Here are the stairs to Enable SSH and Crypto Key setup : 2 config ought to requried for SSH
router (Config) # Line VTY 0 15
router (Config-line)# login local
router (Config)# username [loginid] password [cisco]
router (Config)# username loginid1 password cisco1
router (Config)# crypto key generate rsa
what number of bits in the modulus  :1024
router (Config)# ip ssh version2
router (Config)# CTRL Z
Secure Shell (SSH) may also generate an additional RSA key pair if you generate a key pair on a router having no RSA keys. The additional key pair is used handiest by means of SSH and could have a call together with router_FQDN .server.
For instance, if a router name is “router1.cisco.com,” the choices key name is “router1.cisco.com.server.”
This command isn’t always stored inside the router configuration; however, the RSA keys generated by way of this command are saved in the personal configuration in NVRAM (which is never exhibited to the choices user or backed up to every other tool) the following time the configuration is written to NVRAM.
When you generate RSA keys, you may be precipitated to enter a modulus duration. The longer the choices modulus, the stronger the security. However, a longer modules take longer to generate (see the table beneath for sample instances) and takes longer to use.
The size of Key Modulus variety from 360 to 2048. Choosing modulus extra than 512 will take longer time.
Cisco IOS software program does now not aid a modulus greater than 4096 bits. A duration of less than 512 bits is usually no longer recommended. In positive situations, the shorter modulus won’t characteristic properly with IKE, so we recommend the use of a minimum modulus of 2048 bits.
Syntax Description : Optional Strings to embed with SSH Crypto key
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