Broadcast domains reflect how far multicast and/or broadcast traffic are forwarded throughout an internetwork. Network operation relies on frames sent to a multicast or broadcast destination being forwarded throughout a local area network. Network operating system (NOS) services are often located by sending queries to a broadcast address.
Broadcast domains are defined by looking at where a Layer 2 broadcast (i.e., the all ones address or 0xFFFFFFFFFFFF) propagates throughout a LAN.
What does a repeater do if it receives a broadcast? Repeaters have no concept of broadcasts, frames or even addresses. Repeaters work at Layer 1 and whatever comes in one port is retransmitted out all other ports. A repeater propagates a broadcast (as it propagates or forwards everything).
Bridges (or Layer 2 switches) understand frames and MAC addresses. Bridges will realize that a broadcast is destined for all devices on a network, and wanting to preserve the LAN functionality, the bridge MUST forward the broadcast out all ports. A broadcast is propagated across a bridge.
Routers understand frames, but they do not retransmit the broadcast. The router can listen to the broadcast and take note of the information inside the frame (i.e., it could contain a routing update or be advertising the presence of an IPX NetWare server), but it will not retransmit the broadcast to any other port. Routers define the boundaries of the broadcast domain.
If a network had excess broadcasts causing network delays, a router is the only device that can alleviate this problem. If we segment the LAN into separate broadcast domains, we create separate collision domains (because routers do not propagate broadcasts or collisions).
There are some switch solutions that “throttle” or discard broadcasts once they reach a certain percentage of traffic. This is a dangerous and unpredictable practice because broadcasts are essential to the correct operation of every network operating system (NOS). For example, IP needs Address Resolution Protocol (ARP), and Internetwork Packet Exchange (IPX) devices need Service Advertising Protocol (SAP).
The broadcast domain also defines the subnet (i.e., different broadcast domains require different network identifiers (NETID) in the Layer 3 address). This makes sense when a device compares its own NETID with a destination. If they are the same, the two devices are directly connected (i.e., in the same broadcast domain so IP can perform ARP for the Layer 2 address). If they are different, a router is required.