Cisco Systems RJ-45-to-AUX manual Layer 2 Network Loops, Broadcast and Multicast Frames, 200

∙Broadcast frames

∙Multicast frames

∙Unknown unicasts

Broadcast and Multicast Frames

Broadcast and multicast frames are unique in that neither has a specified destination hardware address. The source address is also the hardware address of the device that sent the frame. In the case of broadcasts, the destination address shown in the header is all 1s, indicating that the broadcast goes to all nodes in a network. A multicast specifies a network but changes all the host address bits to all 1s. For example, a broadcast and multicast in binary would appear as shown in Table 10.1.

Table 10.1: Broadcast and multicast destination addresses.

Frame Type	Binary Value	Broadcast Address
Broadcast	11111111.11111111.11111111.11111111	255.255.255.255
Multicast	00001010.00000001.11111111.11111111	10.1.255.255

When a switch receives a broadcast or a multicast, the frame is sent out all the ports of the switch by default, with the exception of the port on which the frame arrived. VLANs can be used to break down the broadcasts into smaller broadcast domains. In the case of a VLAN, if a broadcast arrives on a port assigned to VLAN 5, only ports assigned to VLAN 5 will receive the broadcast.

Unknown Unicasts

An unknown unicast is similar to a broadcast. This type of unicast is sent when a destination address is unknown by the switch. In this case, the bridge forwards the frame just like a broadcast; the frame is sent out all ports except the port on which it arrived.

Layer 2 Network Loops

To sum up STP in one sentence, Spanning Tree Protocol is used to allow the Layer 2 network to have more than one path to a destination by blocking ports that can cause a packet to make a complete circle through the network. This circle, shown in Figure 10.1, is known as a data loop. The data will move in a continuous circle unless some protocol or algorithm is used to stop the data or a maximum time to live is assigned to the data packet. STP reliably monitors the network topology to find all the links and make sure that data loops do not occur.

Figure 10.1: A network with more than one path in the network. Data can return to the source node on a second link, causing a data loop.

Manufacturers needed a consistent standard that they all could use to block redundant paths through a network. Digital Equipment Corporation (DEC) answered the call and submitted to the Institute of Electrical

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