Avaya CS 1000, Fundamentals

Page: 260 / 270

Multicast configuration
Configuration of multicast groups depends on the scope, or reach, the multicast traffic must
have. This is reflected in the multicast address and the Time To Live (TTL) of the IPv4 multicast
packet. The following sections provide an overview of the involved parameters for the DAP
and network components.
DAP configuration
SIP DECT configuration contains the definition of the IPv4 multicast address the SIP DECT
system is to use. A default of 239.192.49.49 is proposed, and usually accepted. This address
is considered to have Organization-Local scope, which means, according to RFC4291,
“Organization-Local scope is intended to span multiple sites belonging to a single
organization.” Even though this is not a set rule, it most closely matches the intended scope
of the SIP DECT multicast traffic. More detailed scope is achieved by configuring the TTL
parameter.
Network configuration
Many different network topologies are possible, but the most common elements are switches
and routers.
When routers are deployed in the network and multicast traffic needs to pass through them,
the routers must be multicast aware. This means the routers must know what multicast group
topology is needed to connect all members of the multicast group(s). Various different routing
technologies are defined for these purposes. The main issue is that the router must know on
which networks the various multicast group members reside.
Internet Group Management Protocol (IGMP) was designed for this purpose. The DAP uses
IGMP version 2 (RFC2236) and acts as a multicast host. The DAP indicates that it is a member
of a multicast group so that the routers can take appropriate measures to forward the multicast
traffic. This group membership is indicated during startup of the DAP and when asked for, by
a Querier, defined in RFC2236 as being the router with the lowest IP address.
When switches are deployed in the network, the switches do not have the same knowledge
as routers to distribute multicast traffic to specific network ports. Unlike IPv4 unicast of
broadcast, with multicast the switches do not have the option to learn and filter on the Ethernet
MAC address. This results in multicast traffic being flooded to all network ports of the switch
(usually restricted to the VoIP VLAN), so that the individual connected hosts can process the
multicast packets.
With the development of more advanced switches (Layer 3 switches, which take information
from the Layer 3 Network layer and use that to make decisions on the Layer 2 Link layer), more
intelligence came into the distribution of network packets, including multicast packets. In order
to make intelligent decisions on the distribution of multicast packets, the Layer 3 switch listens
to the IGMP traffic and uses that information to distribute the multicast traffic on network ports
on which it knows multicast members are present. This feature is called
IGMP snooping .
DAP multicast group membership
260 SIP DECT Fundamentals October 2012
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