Yes, switches absolutely forward broadcast frames.
According to all documentation relating to switching states, a switch will forward a broadcast on all ports within the same broadcast domain (typically a VLAN), except for the port on which the broadcast was received.
Why Switches Forward Broadcasts
Ethernet switches operate primarily at Layer 2 of the OSI model, using MAC addresses to make forwarding decisions. When a switch receives a frame destined for a specific MAC address (a unicast), it looks up that address in its MAC address table (sometimes called a CAM table). If the destination MAC is found, the switch forwards the frame only out the specific port where that MAC address is known to reside.
However, a broadcast frame is intended for all devices on the local network segment. The destination MAC address for an Ethernet broadcast is FF:FF:FF:FF:FF:FF. When a switch receives a frame with this destination address, it knows it cannot simply forward it to a single port based on a MAC table lookup.
- Key Action: A switch floods broadcast frames out all ports belonging to the same broadcast domain as the receiving port.
- Exception: The frame is not sent back out the port it was received on.
This flooding mechanism ensures that all devices within the broadcast domain receive the broadcast message.
Broadcast Domain Scope
It's important to understand the scope of broadcast forwarding. Switches forward broadcasts only within the broadcast domain they belong to.
- Default: By default, a single switch represents a single broadcast domain (all ports are in the default VLAN).
- VLANs: When Virtual Local Area Networks (VLANs) are configured, each VLAN creates a separate broadcast domain. A broadcast frame originating in one VLAN will not be forwarded by the switch into another VLAN.
- Routers: To send a broadcast from one broadcast domain (e.g., VLAN 10) to another (e.g., VLAN 20), a Layer 3 device like a router is required. Routers typically do not forward broadcasts by default; they act as boundaries between broadcast domains.
Practical Examples of Broadcasts
Broadcasts are fundamental to many network functions. Common examples include:
- ARP (Address Resolution Protocol): Devices use ARP broadcasts to discover the MAC address associated with a known IP address on the local network.
- DHCP (Dynamic Host Configuration Protocol): Clients use DHCP broadcasts to find a DHCP server to obtain an IP address.
- Network Discovery: Protocols used by operating systems for discovering nearby computers and services often use broadcasts.
How Switches Handle Different Frame Types
Switches handle different types of Ethernet frames based on their destination MAC address:
| Frame Type | Destination MAC | Switch Action (within broadcast domain) |
|---|---|---|
| Unicast | Specific MAC (e.g., 00:1A:2B:3C:4D:5E) | Look up in MAC table; forward out specific port. If unknown, flood. |
| Broadcast | FF:FF:FF:FF:FF:FF | Flood out all ports (except ingress). |
| Multicast | Starts with 01:00:5E or 33:33 | Flood out all ports by default, unless IGMP snooping is enabled (then forward only to interested ports). |
As the table shows, while unicasts are forwarded intelligently based on the MAC table, and multicasts can sometimes be optimized, broadcasts are always flooded across the relevant broadcast domain by the switch.
In conclusion, switches are designed to propagate broadcast messages to all connected devices within the same logical network segment, making them essential for the proper functioning of many network protocols.
