When you install XenServer, it starts in an unlicensed state. When it is unlicensed, there are no restrictions on its functionality or features, with the exception of the application of hotfixes. In order to receive Citrix Support, XenServer hosts must be licensed.
XenServer hosts are licensed on a per-populated socket basis and all hosts in a pool must be licensed. Mixed pools of licensed and unlicensed hosts will behave as if all hosts were unlicensed. For information on managing licenses in XenServer v6.2.0, see Managing Licenses in XenServer v6.2.0.
On expiry of a XenServer Per-Socket License edition:
Citrix V6 licensing has built-in timeout technology. If a XenServer host cannot contact the license server, for example, due to problems with the license server hardware or software or network failures, there is a 30 day grace period that allows XenServer hosts to continue operations as if they were still in communication with the license server. After a startup license is checked out by a XenServer host, the XenServer and the license server exchange "heartbeat" messages every five minutes to indicate to each other that they are still up and running. If XenServer and the license server fail to send or receive heartbeats, XenServer lapses into the licensing grace period and licenses itself through cached information. The grace period is 30 days and when the grace period runs out, XenServer stops accepting connections. After communication is re-established between XenServer and the license server, the grace period is reset. The grace period takes place only if the XenServer has successfully communicated with the license server at least once.