A Layer 2 bridge connects local network segments and forwards Ethernet frames using MAC addresses. A transparent bridge learns which interface has seen each source MAC address, then uses that forwarding information to send frames toward known destinations. This lets connected segments operate as part of a larger link-layer network without the bridge making the forwarding decision at the network layer.
What a Layer 2 bridge does
A Layer 2 bridge operates below the network layer. It connects LAN segments and makes forwarding decisions from link-layer addresses rather than routing packets between IP networks. The term “bridge” also has historical breadth: transparent bridging is the mode described here, while other modes, including source-route bridging, have existed. RFC 4188 describes bridges as devices that connect LAN segments and distinguishes transparent and source-route bridging.
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How a transparent bridge forwards frames
It learns source MAC addresses
When a frame arrives, the bridge notes the source MAC address and the interface on which it arrived, then records that association in a forwarding database. In a VLAN-aware bridge, an entry can also be associated with a VLAN ID. RFC 3518 describes this learning and forwarding behavior.
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It checks the destination
- Known destination: The bridge forwards the frame through the interface associated with the destination MAC address.
- Unknown destination: It sends the frame out its other interfaces, excluding the interface on which the frame arrived. This is called flooding.
- Source and destination on the same interface: The bridge can filter the frame rather than send it back out that interface.
This address-based decision is what makes a bridge more selective than a repeater, which does not use a learned MAC-address forwarding database to choose where traffic goes.
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How a bridge differs from a router
A bridge forwards link-layer frames within a bridged network using MAC-address information. A router operates at the network layer and forwards packets between IP networks using network-layer addressing. A device can include both bridging and routing capabilities, so identify the function being discussed rather than relying only on the device’s name.
Why switches are often called Layer 2 bridges
In common Ethernet usage, “Layer 2 bridge” and “Layer 2 switch” refer to closely related concepts. RFC 2674 notes that bridges are often known as Layer 2 switches, and RFC 4957 says transparent bridges are usually implemented as switches. This describes common terminology and implementation, not a guarantee that every product marketed as a switch has identical features or behavior.
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Why bridges need loop prevention
Redundant links between bridged segments can create forwarding loops, in which frames circulate through the network. Ethernet bridged topologies therefore need a loop-free active path. Spanning Tree Protocol (STP) or Rapid Spanning Tree Protocol (RSTP) exchanges Bridge Protocol Data Units and may place ports in a state that blocks forwarding to prevent loops. As RFC 4957 explains, a physical link being up does not necessarily mean it is forwarding data; spanning-tree convergence and port state also affect traffic.
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- A Layer 2 bridge connects LAN segments and forwards link-layer frames.
- A transparent bridge learns source MAC addresses and their incoming interfaces.
- Its forwarding database directs frames for known destinations; unknown destinations are flooded out other interfaces.
- Ethernet switches commonly implement transparent bridge behavior.
- Redundant paths require loop prevention, which STP or RSTP can provide.
RFC 1220 also gives a concise account of transparent bridge forwarding and spanning-tree handling of redundant paths.
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