IP Multicast Support
IP Multicast Support |
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Multicast boundaries are configurable administrative barriers that limit the extent of the IP internetwork over which multicast traffic can be forwarded. Without boundaries, an IP multicast router forwards all appropriate IP multicast traffic. With a Windows 2000–based router, you can define multicast boundaries by a range of IP addresses known as a multicast scope, by the value of the TTL field in the IP header, or by the rate of multicast traffic.
Multicast boundaries are configured per interface from the Multicast Boundaries tab from the properties of an interface in the General node under IP Routing in the Routing and Remote Access snap-in.
The 239.0.0.0/8 range of IP multicast addresses is defined as the administratively scoped IP multicast address space. Multicast addresses in this range can be prevented from propagating in either direction (send or receive) through the use of scope-based boundaries. A scope-based boundary defines the edge or boundary beyond which a multicast packet for a specified range is not forwarded.
To configure a scope (a range of IP multicast addresses) for address-based boundaries, you must first add the scope from:
You must enter the address range corresponding to the scope as an IP address and mask. However, the Local Scope of 239.255.0.0/16 is excluded. Therefore, configured scopes must be in the range of 239.0.0.0 to 239.254.255.255. For a range of IP multicast addresses, determine the appropriate IP address and mask that define the range. For a single group address, the IP address is the group address being scoped and the mask is 255.255.255.255.
Once the scopes are created, scope-based boundaries are configured per interface.
For more information about administrative scoping for IP multicast traffic, see RFC 2356.
TTL-based boundaries prevent the forwarding of IP multicast traffic with a TTL less than a specified value. TTL-based boundaries apply to all multicast packets regardless of the multicast group. Typically used TTL thresholds are listed in Table 4.5.
Table 4.5 TTL Thresholds and Their Scope
| TTL Threshold | Scope |
|---|---|
| 0 | Restricted to the same host. |
| 1 | Restricted to the same subnet. |
| 15 | Restricted to the same site. |
| 63 | Restricted to the same region. |
| 127 | Worldwide. |
| 191 | Worldwide; limited bandwidth. |
| 255 | Unrestricted in scope. |
Therefore, setting a TTL scope of 15 on an interface prevents the forwarding of IP multicast traffic that is intended to be restricted to the site. Only regional or beyond traffic is forwarded.
TTL-based boundaries are less effective than scope-based boundaries due to interactions with multicast routing protocols. For more information, see RFC 2365.
With multicast rate limiting, you can restrict multicast traffic forwarding for traffic beyond a specified rate in kilobytes per second.