When configure a hybrid deployment, you must configure certificates. You must purchase certificates from a trusted third-party CA. Multiple services, such as AD FS, Exchange 2010 federation, Exchange 2010 services, and Exchange, each require certificates. Depending on your organization, you may decide to do one of the following:

• Use a third-party certificate that's used by all services across multiple servers.
  
  
• Use a third-party certificate for each server that provides services.
  
  

Whether you choose to use the same certificate for all services, or dedicate a certificate for each service, depends on your organization and the service you're implementing. Here are some things to consider about each option:

• Third-party certificate across multiple servers   Third-party certificates that are used by services across multiple servers may be slightly cheaper to obtain, but they may complicate renewal and replacement. The complication occurs because, when a certificate needs replacement, you need to replace the certificate on every server where it's installed.
  
  
• Third-party certificate for each server   Using a dedicated certificate for each server that hosts services allows you to configure the certificate specifically for the services on that server. If you need to replace the certificate or renew it, you only need to replace it on the server where the services are installed. Other servers aren't impacted.
  
  

We recommend that you use a dedicated third-party certificate for the AD FS server, another certificate for the Exchange services on your hybrid servers, and if needed, a certificate on your Exchange server. The on-premises federated trust configured as part of federated delegation uses a self-signed certificate by default. Unless you have specific requirements, there's no need to use a third-party certificate with the federation trust configured as part of federated delegation.

The services that are installed on a single server may require that you configure multiple fully qualified domain names (FQDNs) for the server. Purchase a certificate that allows for the required number of FQDNs. Certificates consistent of the subject, or principal, name, and one or more subject alternative names (SAN). The subject name is the FQDN that the certificate is issued to. SANs are additional FQDNs that can be added to a certificate in addition to the subject name. If you need a certificate to support five FQDNs, purchase a certificate that allows for five domains to be added to the certificate: one subject name and four SANs.