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Network Access and Layer 2 Multicast

Chapter Description

In this chapter from IP Multicast, Volume I: Cisco IP Multicast Networking, authors Josh Loveless, Ray Blair, and Arvind Durai take an in-depth look at IP multicast messages at Layer 2 and how they are transported in a Layer 2 domain. This chapter covers the basic elements of multicast functionality in Layer 2 domains as well as design considerations for multicast deployments.


The process of communication between devices on an IP network requires the handling or encapsulation of data at each layer of the OSI model. Packets are composed of MAC addresses, IP addresses, port numbers, and other necessary information. Multicast at Layer 2 has unique requirements regarding MAC addresses and the way IP addresses are mapped to them. In the mapping process, 5 bits of the IP address are overwritten by the OUI MAC address, which causes a 32-to-1 IP multicast address-to-multicast MAC address ambiguity. Client devices on the network use Internet Group Management Protocol (IGMP) to signal the intent to receive multicast streams and, in most cases, use IGMP to send leave messages. Modern switches have the capability to “snoop” or listen to IGMP messages and build appropriate forwarding tables. Timely delivery of messages is the most important role of the network and protecting those resources are critical to that function. Storm control can be used to aid in protecting network elements by limiting the types of traffic. Understanding the intricacies of how Layer 2 devices deliver multicast messages internally will help you in building an infrastructure to support your business initiatives.

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