![]() 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. ![]() 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. Many networks have melted down due to large multicast streams. 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. These methods will be discussed later in this chapter. The switch may use Cisco Group Management Protocol (CGMP) or Internet Group Management Protocol (IGMP) snooping for IPv4 multicast. There are several ways in which a Layer 2 switch can dynamically learn where the destinations are located. 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. The list of destination interfaces includes only those interfaces connected to a device subscribed to the specific multicast flow. Multicast MAC frames may have several destination interfaces, depending upon which devices have requested content from the associated IP multicast stream.īefore the Layer 2 switch can forward multicast frames, it must know the destination interfaces on which those messages should be sent. ![]() 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. Layer 2 switches send frames to a physical or logical interface based on the destination MAC address.
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