cisco ssh legacy Unable to negotiate with no matching key exchange method found. Their offer: diffie-hellman-group1-sha1

Error in legacy ssh switches:

Unable to negotiate with 192.168.1.1: no matching key exchange method found. Their offer: diffie-hellman-group1-sha1

The OpenSSH website has a page dedicated to legacy issues such as this one. It suggests the following approach, on the client:

ssh -oKexAlgorithms=+diffie-hellman-group1-sha1 123.123.123.123

or more permanently, adding

Host 123.123.123.123
    KexAlgorithms +diffie-hellman-group1-sha1

to ~/.ssh/config.

This will enable the old algorithms on the client, allowing it to connect to the server.