Avaya 8800 Planning And Engineering, Network Design page 183

General multicast considerations
Figure 84: 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 range from
June 2016 Planning and Engineering — Network Design 183
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