The switch provides a flexible and scalable multicast implementation for multimedia applications. Several features are dedicated to multimedia applications and in particular to television distribution.
For TV applications, you can attach several TVs directly to the switch, or through an IGMP-capable Ethernet switch. Base this implementation on IGMP; the set-top boxes use IGMP reports to join a TV channel and IGMP leaves to exit the channel. After a viewer changes channels, the switch issues an IGMPv2 leave for the old channel (multicast group), and sends a membership report for the new channel. If viewers change channels continuously, the number of joins and leaves can become large, particularly if many viewers attach to the switch.
The switch supports more than a thousand joins and leaves per second, which is well adapted to TV applications.
Important
For IGMPv3, ensure a join rate of 1000 per second or less. This ensures the timely processing of join requests.
If you use the IGMP proxy functionality at the receiver edge, you reduce the number of IGMP reports received by switch. This provides better overall performance and scalability.
IGMP Fast Leave supports two modes of operation: single-user mode and multiple-user mode.
In single-user mode, if more than one member of a group is on the port and one of the group members leaves the group, everyone stops receiving traffic for this group. Single-user mode does not send a group-specific query before the effective leave takes place.
Multiple-user mode allows several users on the same port or VLAN. If one user leaves the group and other receivers exist for the same stream, the stream continues. The switch tracks the number of receivers that join a given group. For multiple-user mode to operate properly, do not suppress reports. This ensures that the switch properly tracks the correct number of receivers on an interface.
The Fast Leave feature is particularly useful in IGMP-based TV distribution where only one receiver of a TV channel connects to a port. If a viewer changes channels quickly, you create considerable bandwidth savings if you use Fast Leave.
You can implement Fast Leave on a VLAN and port combination; a port that belongs to two different VLANs can have Fast Leave enabled on one VLAN (but not on the other). Thus, with the Fast Leave feature enabled, you can connect several devices on different VLANs to the same port. This strategy does not affect traffic after one device leaves a group to which another device subscribes. For example, you can use this feature when two TVs connect to a port through two set-top boxes, even if you use the single-user mode.
To use Fast Leave, you must first enable explicit host tracking. IGMP uses explicit host tracking to track all source and group members. Explicit host tracking is disabled by default. For configuration information, see Configuring Fast Leave Mode.
If an IGMPv2 host leaves a group, it notifies the router by using a leave message. Because of the IGMPv2 report suppression mechanism, the router cannot access information of other hosts that require the stream. Thus, the router broadcasts a group-specific query message with a maximum response time equal to the last member query interval (LMQI).
Because this timer affects the latency between the time that the last member leaves and the time the stream actually stops, you must properly tune this parameter. This timer can especially affect TV delivery or other large-scale, high-bandwidth multimedia applications. For instance, if you assign a value that is too low, this can lead to a storm of membership reports if a large number of hosts are subscribed. Similarly, assigning a value that is too high can cause unwanted high-bandwidth stream propagation across the network if users change channels rapidly. Leave latency also depends on the robustness value, so a value of 2 equates to a leave latency of twice the LMQI.
Determine the proper LMQI value for your particular network through testing. If a very large number of users connect to a port, assigning a value of 3 can lead to a storm of report messages after a group-specific query is sent. Conversely, if streams frequently start and stop in short intervals, as in a TV delivery network, assigning a value of 10 can lead to frequent congestion in the core network.
Another performance-affecting factor that you need to be aware of is the error rate of the physical medium. For links that have high packet loss, you can find it necessary to adjust the robustness variable to a higher value to compensate for the possible loss of IGMP queries and reports.
In such cases, leave latency is adversely affected as numerous group-specific queries are unanswered before the stream is pruned. The number of unanswered queries is equal to the robustness variable (default 2). The assignment of a lower LMQI can counterbalance this effect. However, if you configure the LMQI too low, it can actually exacerbate the problem by inducing storms of reports on the network. LMQI values of 3 and 10, with a robustness value of 2, translate to leave latencies of 6/10 of a second and 2 seconds, respectively.
When you choose an LMQI, consider all of these factors to determine the best configuration for the given application and network. Test that value to ensure that it provides the best performance.
Important
In networks that have only one user connected to each port, use the Fast Leave feature instead of LMQI, because no wait is required before the stream stops. Similarly, the robustness variable does not affect the Fast Leave feature, which is an additional benefit for links with high loss.