Why would I wish to deploy an SSL certificate on multiple servers?

There are many cases where it is useful to be able to deploy one and the same certificate on multiple servers:

• Your website is behind load balancers or proxies. In that case, SSL termination and offloading would be done by the load balancers, and your certificate should be installed on each of them.
• You want to use a wildcard SSL certificate throughout your internal network to secure multiple machines.
• You've ordered a multi-domain SAN certificate, and want to deploy it on different websites you manage. In most cases, a SAN SSL certificate comes far cheaper than buying individual SSL certificates.
• You've ordered a GlobalSign SSL certificate with free mail subdomains, and want to both secure your web server and your mail server.

Which certificates can I deploy on multiple servers?

Depending on the license issued by the certificate authority, you may or may not be allowed to install your SSL certificate on multiple distinct physical or virtual servers.

Currently, the following certificate authorities allow a single certificate to be used on as many servers as you want:

AlphaSSL

Comodo

GeoTrust

GlobalSign

RapidSSL

Thawte

The following certificate authority requires you to buy additional certificates for each server you wish to deploy on:

Symantec