A network switch floods broadcast frames primarily because these frames are intended for all devices located within the same network segment, also known as a broadcast domain. Flooding is the fundamental mechanism switches use to ensure a broadcast message reaches every possible recipient in that domain.
Understanding Broadcast Flooding
When a switch receives a frame with the destination MAC address of FF:FF:FF:FF:FF:FF, it recognizes this as a broadcast frame. Unlike unicast frames, which are sent to a single, specific destination and require the switch to look up the destination MAC address in its forwarding or MAC address table to find the correct outgoing port, broadcast frames are handled differently.
Here's how a switch handles broadcast frames:
- Identification: The switch identifies the frame as a broadcast based on the special destination MAC address (
FF:FF:FF:FF:FF:FF). - Replication and Forwarding: The switch duplicates the broadcast frame.
- Flooding: It then sends a copy of the frame out of all its ports, except for the port on which the frame was originally received. This ensures that every device connected to the switch (and potentially other switches in the same broadcast domain) receives the broadcast.
This flooding behavior is necessary because the switch doesn't know beforehand which devices need to receive the broadcast; the nature of a broadcast is that it's meant for everyone.
The Necessity of Broadcasts (Protocols like ARP)
Broadcast frames are not just random traffic; they are essential for the operation of various network protocols. As noted in information from 08-Sept-2022: "Protocols like ARP depend on Ethernet Broadcast."
The Address Resolution Protocol (ARP), for instance, uses broadcasts extensively. When a device needs to find the MAC address corresponding to a known IP address on the local network, it sends an ARP request as a broadcast. This request asks, "Who has IP address X.X.X.X? Tell me your MAC address." Every device on the network receives this broadcast, but only the device with the matching IP address responds with a unicast frame containing its MAC address. Without the switch flooding this initial ARP broadcast, the requesting device wouldn't be able to discover the necessary MAC address to communicate with other devices on the segment.
How Switches Decide to Flood: Broadcasts vs. Unicast
Switches perform different actions based on the destination MAC address of a frame. For unicast frames, the switch learns the source MAC addresses of devices on its ports and builds a MAC address table (also known as a Content Addressable Memory or CAM table). When it receives a unicast frame, it looks up the destination MAC address in this table to forward the frame only out the specific port where that device is known to be.
However, the handling for broadcasts (and unknown unicast frames) involves flooding. As noted in information from 08-Sept-2022: "The Switch does not check MAC Address Table for any entry. The switch checks the MAC Address Table and executes Frame Flooding if the Destination MAC Address is not in the table."
While the first part of this statement ("The Switch does not check MAC Address Table for any entry") might seem contradictory to the switch's primary function, it can be interpreted in the context of how different frames are handled. For broadcast frames, the switch doesn't need to look up the destination address FF:FF:FF:FF:FF:FF in the unicast forwarding table to find a specific egress port; the broadcast address itself is the signal to flood. For unicast frames, the switch does check the MAC table, and if the destination is unknown (not in the table), it resorts to flooding (this is known as unknown unicast flooding), as described in the second part of the reference quote.
Therefore, a switch floods broadcast frames because the destination address explicitly indicates that the frame is for all devices, and flooding is the method to distribute it universally within the broadcast domain, independent of the unicast MAC address table lookup process.
| Frame Type | Destination MAC | MAC Table Check? (for destination) | Action |
|---|---|---|---|
| Unicast | Specific MAC Address | Yes | Forward out specific port if known; Flood if unknown |
| Broadcast | FF:FF:FF:FF:FF:FF |
No (Special Address) | Always Flood (except ingress port) |
| Multicast | 01:00:5E:xx:xx:xx |
Yes (Multicast Table) | Forward out specific ports if configured; Flood if no multicast groups joined |
Conclusion
In summary, a switch floods broadcast frames out of all ports (except the incoming one) because broadcast frames are inherently intended for all devices within a network segment. This flooding is a necessary function for various network protocols, such as ARP, to operate correctly and allow devices to discover each other.
