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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.

Switching Multicast Frames

Layer 2 switches send frames to a physical or logical interface based on the destination MAC address. Multicast MAC addresses are a different animal than unicast MAC addresses, because a unicast MAC address should be unique and have only a single destination interface. Multicast MAC frames may have several destination interfaces, depending upon which devices have requested content from the associated IP multicast stream.

Before the Layer 2 switch can forward multicast frames, it must know the destination interfaces on which those messages should be sent. The list of destination interfaces includes only those interfaces connected to a device subscribed to the specific multicast flow. The destination can be added as static entries that bind a port to a multicast group, or the switch can use a dynamic way of learning and updating the ports that need to receive the flow.

There are several ways in which a Layer 2 switch can dynamically learn where the destinations are located. The switch may use Cisco Group Management Protocol (CGMP) or Internet Group Management Protocol (IGMP) snooping for IPv4 multicast. These methods will be discussed later in this chapter.

If a Layer 2 switch does not have a mechanism to learn about where to send multicast messages, it treats all multicast frames as broadcast, which is to say it floods the packet on every port or VLAN port! As you can imagine, this is a very bad thing. Many networks have melted down due to large multicast streams. For example, when sending computer operating system image files, a tremendous amount of data is sent to every device in the broadcast domain, every computer, router, printer, and so on. The unfortunate side effect of these messages is that network performance may be affected in locations on the network that do not need the multicast stream. How could this happen if these are broadcast messages and will not go beyond the local network? These messages will not go beyond any local Layer 3 devices, but local Layer 3 devices must process each one of the broadcast messages. While the Layer 3 device is inundated processing these messages, it may not have the available cycles to process other more important messages, such as routing updates or spanning-tree messages. As you can imagine, or may have already experienced, this can impact or “melt-down” the entire network.

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