Multicast Scoping Overview

You use multicast scoping to limit multicast traffic by configuring it to an administratively defined topological region. Multicast scoping controls the propagation of multicast messages—both multicast group joins upstream toward a source and data forwarding downstream. Scoping can relieve stress on scarce resources, such as bandwidth, and improve privacy or scaling properties.

IP multicast implementations can achieve some level of scoping by using the time-to-live (TTL) field in the IP header. However, TTL scoping has proven difficult to implement reliably, and the resulting schemes often are complex and difficult to understand.

Administratively scoped IP multicast provides clearer and simpler semantics for multicast scoping. Packets addressed to administratively scoped multicast addresses do not cross configured administrative boundaries. Administratively scoped multicast addresses are locally assigned, and hence are not required to be unique across administrative boundaries.

The administratively scoped IP version 4 (IPv4) multicast address space is the range from 239.0.0.0 through 239.255.255.255.

The structure of the IPv4 administratively scoped multicast space is based loosely on the IP version 6 (IPv6) addressing architecture described in RFC 1884, IP Version 6 Addressing Architecture.

There are two well-known scopes:

• IPv4 local scope—This scope comprises addresses in the range 239.255.0.0/16. The local scope is the minimal enclosing scope and is not further divisible. Although the exact extent of a local scope is site-dependent, locally scoped regions must not span any other scope boundary and must be contained completely within or be equal to any larger scope. If scope regions overlap in an area, the area of overlap must be within the local scope.
• IPv4 organization local scope—This scope comprises 239.192.0.0/14. It is the space from which an organization allocates subranges when defining scopes for private use.

The ranges 239.0.0.0/10, 239.64.0.0/10, and 239.128.0.0/10 are unassigned and available for expansion of this space.

Two other scope classes already exist in IPv4 multicast space: the statically assigned link-local scope, which is 224.0.0.0/24, and the static global scope allocations, which contain various addresses.

All scoping is inherently bidirectional in the sense that join messages and data forwarding are controlled in both directions on the scoped interface.

You can configure multicast scoping either by creating a named scope associated with a set of routing device interfaces and an address range, or by referencing a scope policy that specifies the interfaces and configures the address range as a series of filters. You cannot combine the two methods (the commit operation fails for a configuration that includes both). The methods differ somewhat in their requirements and result in different output from the show multicast scope command. For details and configuration instructions, see Creating a Named Scope for Multicast Scoping and Using a Scope Policy for Multicast Scoping.

For information about supported standards for multicast scoping, see IP Multicast Specifications.