Time Servers

Single Reference

Single Reference time servers provide time to Secondary time servers and to their own client workstations.

This server determines the time for the entire network. The network supervisor sets the time on the Single Reference time server. (It is possible for the time to be synchronized to an external clock.)

Because the Single Reference time server is the source of time on the network, all other servers must be able to contact it.

The following figure illustrates a Single Reference time server providing time to Secondary time servers and to its own client workstations. The Secondary time servers, in turn, provide time to their own client workstations.

Figure 5-1 Single Reference Time Server

The Single Reference time server works on networks of any size, but the time synchronization configuration shown in Figure 5-1 is used primarily for small networks that don't include WAN links.

	CAUTION: If you use a Single Reference time server, avoid using Primary or Reference time servers in the same tree because the time references might conflict.

Primary

Primary time servers synchronize the time with at least one other Primary time server or with a Reference time server, and they provide the time to Secondary time servers and directly to client workstations.

Primary time servers "poll" other Primary or Reference time servers and "vote" on a common network time. Primary time servers adjust their internal clocks to synchronize with that common network time. Because all Primary servers adjust their clocks, network time might drift slightly.

The following figure shows Primary time servers in various locations providing time to their respective Secondary time servers. Secondary time servers, in turn, provide time to their client workstations.

Figure 5-2 Primary Time Servers

You should place a Primary time server in each geographically distinct area so that Secondary servers and client workstations can access them without using WAN links.

Use Primary time servers on larger networks to increase Directory fault tolerance by providing redundant paths for Secondary time servers.

If a Primary time server goes down, the Secondary time server can get the time from an alternate Primary time server.

If you use Primary time servers, each one needs to be able to contact another Primary time server or a Reference time server to determine time on the network.

Reference

Reference time servers provide a time that Primary and Secondary time servers and client workstations can synchronize with.

Reference time servers can be synchronized with an external time source, such as a radio atomic clock.

A Reference time server acts as a central point of control for time on the network. Eventually, all Primary time servers adjust their clocks to agree with a Reference time server.

Reference time servers do not adjust their internal clocks; instead, Primary and Secondary servers' internal clocks are adjusted to synchronize with the Reference time server.

The following figure shows a Reference time server synchronized to an external clock. The Reference time server, in turn, provides time to Secondary servers and client workstations, as well as to a Primary time server at another location.

Figure 5-3 Reference Time Server

Use a Reference time server when it is important to have a central point of control for time on the network. Usually, only one Reference time server is installed on a network. If you use more than one Reference time server on a network, you must synchronize each Reference time server with the same external time source, such as a radio atomic clock.

You must have at least one Primary time server that the Reference time server can contact in order to synchronize time on the network.

Whenever Primary and Reference time servers are on a network, they must be able to contact each other for polling and synchronization.

Secondary

Secondary time servers obtain the time from a Single Reference, Primary, or Reference time server. They adjust their internal clocks to synchronize with the network time, and they provide the time to client workstations.

A Secondary time server doesn't participate in determining the correct network time.

Secondary time servers should be close in proximity to Primary or Reference time servers.

For optimal time synchronization, minimize the number of intervening routers and slow LAN segments between Secondary time servers and their Single Reference, Primary, or Reference time server.

Summary

The following table summarizes the types of time servers and their uses.