The primary reason to deploy DAs is to reduce the amount of multicast traffic and the delays that are associated with gathering unicast replies. In a large network with many UAs and SAs, the amount of multicast traffic that is involved in service discovery can become so large that network performance degrades. By deploying one or more DAs, UAs must unicast to DAs for service and SAs must register with DAs by using unicast. The only SLP-registered multicast in a network with DAs is for active and passive DA discovery.
SAs register automatically with any DAs they discover within a set of common scopes, rather than accepting multicast service requests. Multicast requests in scopes that are not supported by the DA are still answered directly by the SA, however.
Service requests from UAs are unicast to DAs rather than multicast onto the network when a DA is deployed within the UA's scopes. Consequently, DAs within the UA's scopes reduce multicast. By eliminating multicast for normal UA requests, the time that is required to obtain replies to queries is greatly reduced (from seconds to milliseconds).
DAs act as a focal point for SA and UA activity. Deploying one or several DAs for a collection of scopes provides a centralized point for monitoring SLP activity. By turning on DA logging, it is easier to monitor registrations and requests than by checking the logs from multiple SAs that are scattered around the network. You can deploy any number of DAs for a particular scope or scopes, depending on the need to balance the load.
In networks without multicast routing enabled, you can configure SLP to use broadcast. However, broadcast is very inefficient, because it requires each host to process the message. Broadcast also does not normally propagate across routers. As a result, in a network without multicast routing support, services can be discovered only on the same subnet. Partial support for multicast routing leads to inconsistent ability to discover services on a network. Multicast messages are used to discover DAs. Partial support for multicast routing, therefore, implies that UAs and SAs register services with all known DAs in the SA's scope. For example, if a UA queries a DA that is called DA1 and the SA has registered services with DA2, the UA will fail to discover a service. See Configuring Broadcast-Only Routing for more information on how to deploy SLP on networks without multicast enabled.
On a network with inconsistent site-wide support for multicast routing, you must configure the SLP UAs and SAs with a consistent list of DA locations by using the net.slp.DAAdresseses property.
Finally, the Solaris SLPv2 DA supports interoperability with SLPv1. SLPv1 interoperability is enabled by default in the Solaris DA. If your network contains SLPv1 devices, such as printers, or you need to interoperate with Novell Netware 5, which uses SLPv1 for service discovery, you should deploy a DA. Without a DA, the Solaris SLP UAs are unable to find SLPv1 advertised services.