What is frame flooding?
Network switches use “Frame flooding,” which sends an incoming data frame out every port except the one it was received on. Layer 2 switches, which distribute broadcasts and discover networks, must behave this way.
The following provides a thorough description of frame flooding’s operation, occurrence, possible effects, and management:
How Frame Flooding Works
- Frame Arrival: Devices send Ethernet frames with destination MAC addresses.
- MAC Address Table Check: When it gets the frame, the switch examines its filter table or CAM table for the destination MAC address. MAC addresses are assigned to switch ports in this table.
- Unknown Destination: If the destination MAC address is not in its MAC address database, the switch does not know its port.
- Flooding Action: The switch copies and transmits the frame from all open ports except the one it was received on to ensure delivery. The frame will reach the destination device regardless of network segment location with this approach.
When Frame Flooding Occurs
There are several main causes of frame flooding:
Unknown Unicast Frames
The most frequent cause of flooding is Unknown Unicast Frames. A switch floods a unicast frame (a one-to-one connection) when it gets a destination MAC address that is not in its MAC address database because it has not yet been learnt.
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- MAC Learning: The switch “learns” its MAC address and the corresponding port once the destination device receives the flooded frame and responds, updating its MAC address table. Subsequent frames intended for that device will not be flooded after that; instead, they will be routed straight to the appropriate port (filtered). As a result, frame flooding is a transient behaviour that ends when the destination MAC is discovered.
Broadcast Frames
Frames for broadcasting Broadcast frames have the destination MAC address FF:FF:FF:FF:FF and are intended for all network devices. To ensure that all devices receive the message, a switch must flood a broadcast frame out of all its ports (inside the same VLAN, excluding the incoming port). Frame flooding is needed for broadcast ARP and DHCP.
Multicast Frames
The switch floods multicast frames by default if multicast filtering is off.
Frame Flooding vs Ethernet Broadcast
The genesis of frame flooding and the switch’s decision-making process are different, even though they can have an effect that is comparable to an Ethernet broadcast:
| Feature | Ethernet Broadcast | Frame Flooding |
| Destination MAC Address | Must be FF:FF:FF:FF:FF:FF | May be any MAC address other than the broadcast address. |
| MAC Table Check | The switch does not check its MAC address table for this address; it inherently knows to send it to all active ports (except incoming). | The switch checks its MAC address table for the destination MAC address. Flooding occurs only if no entry is found for that specific destination. |
| Purpose | Deliberately intended for all devices on the network. | A mechanism to discover the location of an unknown device or deliver multicast traffic when a specific port is unknown. It’s a temporary measure until the switch learns the destination’s port. |
| Dependencies | Protocols like ARP depend on Ethernet broadcasts. | No protocols inherently depend on frame flooding itself, but rather on the initial discovery process it facilitates. |
Consequences of Excessive Flooding
Uncontrolled flooding can cause serious network issues, especially in redundant network topologies without appropriate loop avoidance techniques, even while it is required for initial device discovery and broadcast delivery:
- Broadcast Storms: A single broadcast, multicast, or unknown unicast Ethernet frame loops endlessly throughout a local area network (LAN). Requiring CPUs to process too many frames might overload network links, blocking valid traffic and degrading end-user device performance.
- MAC Table Instability (Thrashing): Looping frames can lead switches’ MAC address tables to become unstable. The switch will continuously update its table entries if frames with the same source MAC address arrive on different ports as a result of a loop. Switch resources are used up by this “thrashing” of the MAC table, which can hinder effective frame forwarding.
- Several Frame Transmissions: Looping frames may also send the same frame to the destination host in several copies, confusing the host and perhaps leading to application problems.
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Loop Avoidance
In network topologies with redundant links, Spanning Tree Protocol (STP) or Rapid Spanning Tree Protocol (RSTP) is crucial to reducing the negative impact of looping frames brought on by floods. These protocols function by:
- Putting some switch ports in a blocking mode. Except for STP/RSTP messages, ports in a blocking state do not process received user frames, forward user frames, or learn MAC addresses from received frames.
- Creating a loop-free topology by guaranteeing that there is only one active logical path connecting any two LAN segments.
Frame Flooding vs Hubs
It’s critical to differentiate between hub behavior and frame flooding caused by a switch. All incoming frames are broadcast blindly to all ports by a hub, which is a simple device. A switch, on the other hand, is a clever mechanism that only floods frames under certain conditions, as when managing broadcast traffic or when a destination is unknown. A switch is far more efficient than a hub because of this selective procedure, which cuts down on pointless network traffic.