DA Placement and Scope Name Assignment

The placement of DAs and assignment of scope names on a network with a multihomed host must be done carefully to assure that clients obtain accessible services. Be particularly cautious when routing is disabled and the net.slp.interfaces property is configured. Again, if unicast routing is enabled between the interfaces on a multihomed machine, no special DA and scope configuration is necessary. The advertisements are cached with the DA identify services that are accessible from any of the subnets. However, if unicast routing is disabled, poor placement of DAs can result in problems.

To see what problems can result in the previous example, consider what would happen if bigguy runs a DA, and clients on all subnets have the same scopes. SAs on the 143 subnet register their service advertisements with the DA. UAs on the 144 subnet can obtain those service advertisements, even though hosts on the 143 subnet are unreachable.

One solution to this problem is to run a DA on each subnet and not on the multihomed host. In this situation, the net.slp.interfaces property on the multihomed hosts should be configured with a single interface host name or address, or it should be left unconfigured, forcing the default interface to be used. A disadvantage of this solution is that multihomed hosts are often large machines that could better handle the computational load of a DA.

Another solution is to run a DA on the multihomed host, but configure scopes so that the SAs and UAs on each subnet have a different scope. For example, in the previous situation, UAs and SAs on the 142 subnet might have a scope that is called scope142. UAs and SAs on the 143 subnet might have another scope that is called scope143 and UAs and SAs on the 144 subnet could have third scope that is called scope144. You can configure the net.slp.interfaces property on bigguy with the three interfaces so that the DA serves three scopes on the three subnets.