A key chain is a series of keys that can be created to help ensure secure communication between routers in a network. Authentication occurs whenever neighboring routers exchange information. Plain text authentication sends a plain text key with each message, and plain text is vulnerable to snooping. Key chains allow a rotating series of keys to be used for limited periods of time to decrease the likelihood of a compromise.
To configure a key chain, an administrator defines a series of keys, and the router software rotates through them. Each key also has an associated time interval, or "lifetime," for which it will be activated. The authentication code included in each key is called the key string.