With the growing popularity of IPTV applications, multicast services are more widely deployed than ever. When multicast services are deployed on a Layer 2 network, a number of problems may arise:
The following concepts are involved in multicast CAC.
Layer 2 multicast CAC constrains the generation of multicast forwarding entries. When a preset threshold is reached, no more forwarding entries can be generated. This ensures that devices have adequate processing capabilities and controls link bandwidth.
Layer 2 multicast CAC can restrict the following items:
Restriction on the number and bandwidth of multicast groups
The number of multicast groups allowed can be limited when a device creates Layer 2 multicast forwarding entries. This protects device and network performance by limiting the number of groups available for users to join. After IGMP Report messages are received from downstream user hosts, the device checks CAC statistics to determine whether the threshold for the number of multicast groups has been reached. If the threshold has not been reached, a forwarding entry is generated and CAC statistics are updated to show the increase in groups. If the threshold has been reached, no entry is generated. When IGMP Leave messages are received or entries age, the entries are deleted and CAC statistics are updated.
If the bandwidth of each multicast group is fixed and each group uses approximately the same amount of bandwidth, the total bandwidth for multicast traffic is basically fixed. For example, if there are 20 multicast groups and each multicast group has 4 kbit/s of bandwidth, the total bandwidth for multicast traffic is 80 kbit/s. If there are 20 multicast groups and the bandwidth values of the multicast groups are different, some being 4 kbit/s and the others being 18 kbit/s, the total bandwidth for multicast traffic cannot be determined. In a case like this, setting a limit on the number of multicast groups is not adequate to control bandwidth. Bandwidth usage must be limited.
Restriction on the number and bandwidth of multicast groups in a channel
If a network offers channels for different content providers, the number of multicast groups and the amount of bandwidth must be limited based on channels.
Before a Layer 2 multicast entry is generated, the multicast group address must be checked to determine which channel's address range to which this address belongs. Whether CAC is configured for the address range needs to be checked also. If CAC is configured for the address range and the number or bandwidth of member multicast groups exceeds the upper threshold, the Layer 2 entry will not be generated. The Layer 2 entry will be generated only if the number or bandwidth of member multicast groups is below the upper threshold.
Layer 2 multicast CAC applies to VPLS networks.
The Layer 2 multicast CAC feature provides the following benefits: