On a legacy Ethernet network using thick coaxial cables as a transmission medium, multiple nodes on a shared medium share the bandwidth on the link and compete for the right to use the link. A network collision occurs when more than one node attempts to send a packet on this link at the same time. The carrier sense multiple access with collision detection (CSMA/CD) mechanism is used to solve the problem of collisions. Once a collision occurs on a link, the CSMA/CD mechanism prevents data transmission on this link within a specified time. Collisions are inevitable on an Ethernet network, and the probability that collision occurs increases when more nodes are deployed on a shared medium. All nodes on a shared medium constitute a collision domain. All the nodes in a collision domain compete for bandwidth. Packets sent from a node, including unicast, multicast, and broadcast packets, can reach all the other nodes in the collision domain.
Packets are broadcast in a collision domain, which results in a low bandwidth efficiency and degrades packet processing performance of network devices. Therefore, broadcasting of packets must be restricted. For example, the ARP protocol sends broadcast packets to obtain MAC addresses mapping specified IP addresses. The all 1s MAC address FFFF-FFFF-FFFF is the broadcast MAC address. All nodes must process data frames with this MAC address as the destination MAC address. A broadcast domain is a group of nodes, among which broadcast packet from one node can reach all the other nodes. A network bridge forwards unicast packets according to its MAC address table and forwards broadcast packets to all its ports. Therefore, nodes connected to all ports of a bridge belong to a broadcast domain, but each port belongs to a different collision domain.