When attempting to SSH into older Cisco switches like the Catalyst 3750 series, you might encounter the frustrating "Invalid key length" error. This typically occurs when modern SSH clients enforce stricter security standards than what the legacy device supports.

ssh_dispatch_run_fatal: Connection to 192.168.7.6 port 22: Invalid key length

The root cause stems from cryptographic algorithm compatibility. Modern OpenSSH versions (7.4+) disable certain weak algorithms by default, while older Cisco IOS versions rely on these deprecated protocols.

Key factors to check:

• Switch SSH server configuration
• Client SSH version and supported algorithms
• Key exchange (KEX) methods available
• Host key types supported

Create or modify your SSH client config file (~/.ssh/config) with these parameters:

Host 192.168.7.*
    HostkeyAlgorithms ssh-rsa,ssh-dss
    KexAlgorithms diffie-hellman-group1-sha1,diffie-hellman-group14-sha1
    Ciphers aes128-ctr,aes192-ctr,aes256-ctr,aes128-cbc,3des-cbc
    MACs hmac-sha1,hmac-md5

On the Cisco switch, ensure proper SSH configuration:

! Generate RSA key (minimum 768 bits, recommended 2048)
crypto key generate rsa modulus 2048

! Configure SSH parameters
ip ssh version 2
ip ssh rsa keypair-name SSHKEY
ip ssh time-out 60
ip ssh authentication-retries 3

If you still encounter issues, consider these workarounds:

1. Use a dedicated SSH client like PuTTY with explicit algorithm selection
2. Create a Docker container with older SSH client versions
3. Temporarily enable telnet for configuration changes

Check your SSH configuration on the switch:

show ip ssh
show ssh
show crypto key mypubkey rsa

While these solutions get you connected, be aware they weaken cryptographic security. For production environments, consider:

• Upgrading to newer IOS versions that support modern crypto
• Implementing network segmentation for management access
• Using jump hosts with stricter security controls

When attempting to SSH into a Cisco Catalyst 3750 series switch with OpenSSH 7.6+, many administrators encounter the frustrating error:

ssh_dispatch_run_fatal: Connection to [IP] port 22: Invalid key length

Cisco's older IOS versions (particularly on 3750 switches) use cryptographic algorithms that modern OpenSSH clients consider insecure:

• Default RSA key sizes below 2048 bits
• Outdated key exchange (KEX) methods
• Disabled SHA-1 hashing algorithms

Option 1: Client-Side Workaround

Create or modify your SSH config file (~/.ssh/config) with these parameters:

Host 192.168.7.6
    HostkeyAlgorithms ssh-rsa,ssh-dss
    KexAlgorithms diffie-hellman-group1-sha1
    Ciphers aes128-ctr,aes192-ctr,aes256-ctr
    MACs hmac-sha1

Option 2: Switch Configuration

On the Cisco switch CLI:

configure terminal
ip ssh version 2
crypto key generate rsa modulus 2048
end
write memory

After making changes, verify with:

show ip ssh
SSH Enabled - version 2.0
Authentication timeout: 120 secs; Authentication retries: 3
Minimum expected Diffie Hellman key size : 2048 bits
IOS Keys in SECSH format(ssh-rsa, base64 encoded):

While these solutions work, they involve cryptographic downgrades. For production environments:

• Consider upgrading switch IOS if possible
• Limit SSH access to management VLANs
• Implement additional network segmentation

If SSH remains problematic:

# Telnet (insecure fallback)
telnet 192.168.7.6

# Serial console connection
screen /dev/ttyUSB0 9600