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A Layer 2 bridge connects local network segments and forwards Ethernet frames using MAC addresses. It learns which interface a device’s MAC address is associated with, then uses that information to send frames toward their destination. Ethernet switches commonly perform this transparent-bridging function.
What a Layer 2 bridge does
A Layer 2 bridge joins network segments at the link layer, below the network layer. In transparent Ethernet bridging, the connected segments can function as parts of a larger link-layer network while the bridge forwards frames based on MAC addresses. The term “bridge” can also describe other historical modes, including source-route bridging; the forwarding process below describes transparent bridging. RFC 4188 describes bridge purpose and modes.
How a transparent bridge forwards frames
- It learns the source. When a frame arrives, the bridge records the source MAC address and the interface on which it arrived in a forwarding database.
- It looks up the destination. The bridge checks that database to find the interface associated with the frame’s destination MAC address.
- It forwards or floods. If the destination interface is known, the bridge forwards the frame there. If the destination is unknown, it sends the frame out the other interfaces, except the one on which it arrived.
- It filters local traffic. If the source and destination are on the same interface, the bridge can filter the frame rather than forward it elsewhere.
This learning and forwarding behavior is described in RFC 3518; RFC 1220 also summarizes transparent bridging and its response to redundant paths.
What the forwarding database contains
At its simplest, an entry associates a MAC address with an interface. A VLAN-aware bridge can also include a VLAN ID in the forwarding entry, so forwarding decisions account for VLAN membership. RFC 3518 describes VLAN-aware entries.
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How a bridge differs from a repeater
A repeater relays signals without making forwarding decisions based on learned MAC-address locations. A transparent bridge uses its forwarding database to decide where to send a frame and can filter traffic that does not need to cross to another interface. That address-based decision-making is what makes bridging more than simply repeating a signal.
Layer 2 bridge versus Layer 2 switch
In common Ethernet usage, “Layer 2 switch” is a familiar name for a device that performs transparent bridging. RFC 2674 notes that bridges are often called Layer 2 switches, and RFC 4957 says transparent bridges are usually implemented as switches. RFC 2674 and RFC 4957 support this terminology and implementation relationship.
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The terms describe a common function, not a guarantee that every product sold as a switch has the same features or behavior. Capabilities such as VLAN support, port speeds, and management options depend on the specific device.
Why bridged networks need loop prevention
Redundant links can create a forwarding loop: a frame may circulate through connected segments instead of reaching its destination. Bridged Ethernet networks therefore need a loop-free active topology. Spanning Tree Protocol (STP) or Rapid Spanning Tree Protocol (RSTP) exchanges Bridge Protocol Data Units (BPDUs) and can block ports when needed to prevent loops. As a result, a physical link can be up while a port is not yet forwarding data; spanning-tree convergence and port state matter too. RFC 4957 discusses STP, RSTP, and loop-free topologies.
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