Avaya 8800 Planning And Engineering page 157

Figure 66: Multicast IP address to MAC address mapping
Most Ethernet switches handle Ethernet multicast by mapping a multicast MAC address to
multiple switch ports in the MAC address table. Therefore, when you design the group
addresses for multicast applications, take care to efficiently distribute streams only to hosts
that are receivers. The Avaya Ethernet Routing Switch 8800/8600 switches IP multicast data
based on the IP multicast address, not the MAC address, and thus, does not have this issue.
As an example, consider two active multicast streams using addresses 239.1.1.1 and
239.129.1.1. Suppose two Ethernet hosts, receiver A and receiver B, are connected to ports
on the same switch and only want the stream addressed to 239.1.1.1. Suppose also that two
other Ethernet hosts, receiver C and receiver D, are also connected to the ports on the same
switch as receiver A and B and wish to receive the stream addressed to 239.129.1.1. If the
switch utilizes the Ethernet multicast MAC address to make forwarding decisions, then all four
receivers receive both streams—even though each host only wants one stream. This increases
the load on both the hosts and the switch. To avoid this extra load, Avaya recommends that
you manage the IP multicast group addresses used on the network.
The switch does not forward IP multicast packets based on multicast MAC addresses—even
when bridging VLANs at Layer 2. Thus, the switch does not encounter this problem. Instead,
it internally maps IP multicast group addresses to the ports that contain group members.
When an IP multicast packet is received, the lookup is based on the IP group address,
regardless of whether the VLAN is bridged or routed. Be aware that while the Avaya Ethernet
Routing Switch 8800/8600 does not suffer from the problem described in the previous example,
other switches in the network, particularly pure Layer 2 switches, can.
In a network that includes non-Ethernet Routing Switch 8800/8600 equipment, the easiest way
to ensure that this issue does not arise is to use only a consecutive range of IP multicast
addresses corresponding to the lower order 23 bits of that range. For example, use an address
Planning and Engineering — Network Design
General multicast considerations
November 2010 157