DA Placement and Scope Name Assignment

The placement of DAs and assignment of scope names in a network with a multihomed host, in which routing is disabled but the net.slp.interfaces property is configured, must be done carefully to assure that clients obtain accessible services. Again, if unicast routing is enabled between the interfaces on a multihomed machine, no special DA and scope configuration is necessary, because advertisements 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 can go wrong 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 case, 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 drawback 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 such that the SAs and UAs on each subnet have a different scope. For example, in the above case, scopes could be configured so that UAs and SAs on the 142 net have scope scope142, UAs and SAs on the 143 net have scope scope143, and UAs and SAs on the 144 net have scope 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.