System Administration Guide: Network Services

Configuring the Multicast Time-to-Live Property

The multicast time–to-live property (net.slp.multicastTTL) determines the range over which a multicast packet is propagated on your intranet. The multicast TTL is configured by setting the net.slp.multicastTTL property to an integer between 1 and 255. The default value of the multicast TTL is 255, which means, theoretically, that the packet routing is unrestricted. However, a TTL of 255 causes a multicast packet to penetrate the intranet to the border routers on the edge of your administrative domain. Correct configuration of multicast on border routers is required to prevent multicast packets from leaking into the Internet's multicast backbone, or to your ISP.

Multicast TTL scoping is similar to standard IP TTL, with the exception that a TTL comparison is made. Each interface on a router that is multicast enabled is assigned a TTL value. When a multicast packet arrives, the router compares the TTL of the packet with the TTL of the interface. If the TTL of the packet is greater than or equal to the TTL of the interface, the packet TTL is reduced by one, as with the standard IP TTL. If the TTL becomes zero, the packet is discarded. When you use TTL scoping for SLP multicasting, your routers must be properly configured to limit packets to a particular subsection of your intranet.