Information Technology Reference
In-Depth Information
Chapter 9
With the exception of traffic corresponding to active voice calls, other traffic that can be transported over telephony networks includes
Music on Hold (MoH), video, fax, and modem traffic.
IP Multicast
IP multicast can be used for a couple of different purposes in an IP telephony network. CUCM MoH can be sent across the network
using multicast transport. In addition, if H.323 gatekeepers are being used in a network, endpoints can use multicast autodiscovery to
locate these gatekeepers.
Multicast can be enabled in a campus-only environment, or it can be enabled in an interdomain environment. Protocols and
mechanisms that are commonly used to enable multicast in a campus environment include Internet Group Management Protocol
(IGMP), IGMP snooping, Cisco Group Management Protocol (CGMP), PIM Sparse-Mode (PIM-SM), and, to a lesser extent, PIM
Dense-Mode (PIM-DM).
Protocols and mechanisms that are commonly used to enable multicast in an interdomain environment include Multiprotocol BGP
(MBGP), Multicast Source Discovery Protocol with PIM Sparse-Mode (often configured in an Anycast-RP architecture), and PIM
Source-Specific Multicast (PIM-SSM).
Protocols and mechanisms such as PIM-SM, PIM-DM, MBGP, MSDP, Anycast RP, Bidirectional PIM, and PIM-SSM help ensure
that multicast traffic is forwarded between the routers closest to multicast sources and the routers closest to the multicast receivers.
IGMP is implemented to ensure that end hosts (multicast receivers) can join multicast groups, maintain membership of multicast
groups, and leave multicast groups. This helps ensure that multicast traffic is delivered to end hosts as and when required. IGMP
operates between end hosts and their local router, but a mechanism called IGMP snooping can be enabled on intervening switches to
ensure optimal distribution of multicast traffic on switch ports. (Multicast traffic is flooded on switch ports, by default.)
