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| 1.2 |  | Key Characteristics of Various Switching Technologies |
| 1.2.7 |  | Multicast |

Multicast is becoming an integral part of network design although it poses one of the greatest challenges to network engineers. Multicast is configured so that one-to-many and many-to-many communication with voice and video take place. Applications such as Cisco IP/TV depend on multicast functionality in the campus network.

A good multicast network design is fundamentally important for any enterprise that wants to enjoy the many benefits of multicast technology. If the devices in the network are not specifically configured for multicast, in addition to precluding the possibility of end-users receiving multicast streams from multicast servers, the network may be subject to indiscriminate flooding of multicast traffic in portions of the network.

Multicast control is extremely important because of the large amount of traffic involved when several high-bandwidth multicast streams are provided. The multilayer campus design is ideal for the control and distribution of IP multicast traffic. Layer 3 multicast control is provided primarily by the Protocol Independent Multicast (PIM) protocol. Multicast control at the wiring closet is provided by Internet Group Management Protocol (IGMP), and IGMP Snooping or Cisco Group Multicast Protocol (CGMP).

At the wiring closet, IGMP Snooping and CGMP are multilayer services that prune multicast traffic between switch ports and directly connected routers. Without IGMP Snooping or CGMP, multicast traffic would flood all switch ports. IGMP itself prunes unnecessary multicast traffic on router interfaces connected to client workstations.

In the PIM sparse mode, a rendezvous point is used as a broker or proxy for multicast traffic. A design goal of multicast networks is to place the rendezvous point and backup rendezvous point(s) to affect the shortest path. If this is accomplished, there is no potential for suboptimal routing of multicast traffic. Ideally, the rendezvous point and optional backups should be placed on the Layer 3 distribution switch in the server farm close to the multicast sources. If redundant rendezvous points are configured on routers instead of switches, recovery is much closer and more CPU-intensive.

In any case, it is critical that network design take into account multicast functionality. The switches should support IGMP Snooping or CGMP as well as the various PIM modes and Multicast Source Discovery Protocol (MSDP), a relatively new protocol that can, among other things, enhance dynamic rendezvous point selection.