Design Reference

Figure 53: 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. VSP
4000 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 that two Ethernet hosts, receiver A and receiver B, connect 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, also connect to the ports on the same switch as receiver A and B, and
want to receive the stream addressed to 239.129.1.1. If the switch uses 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 transmission 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.
VSP 4000 does not forward IP multicast packets based on multicast MAC addresses—even when
bridging VLANs at Layer 2. Thus, the platform does not encounter this problem. Instead, the
platform 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. While the problem described in the previous example does
not affect the VSP 4000, other switches in the network may be affected. This problem is particularly
true of pure Layer 2 switches.
In a network that includes non-VSP 4000 equipment, the easiest way to ensure that this issue does
not arise is to use only a consecutive range of IP multicast addresses that correspond to the lower-
order 23 bits of that range. For example, use an address range from 239.0.0.0 through
239.127.255.255. A group address range of this size can still easily accommodate the needs of
even the largest private enterprise.
IP multicast network design
108 Network Design Reference for Avaya VSP 4000 Series December 2014
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