A network bridge is a Layer 2 device that connects LAN segments and forwards Ethernet frames according to MAC addresses. By filtering unnecessary traffic, a bridge separates collision domains and helps a local network operate more efficiently.
Bridge operation is a useful foundation for understanding modern Ethernet switches. If you are preparing for CCNA, review the concepts below and practise reading MAC address tables. For additional revision, see the 200-301 CCNA Premium PDF with the latest available questions and answers.
What Is a Network Bridge?
A bridge connects two or more network segments that use the same Layer 2 protocol. It examines the source and destination MAC address in each Ethernet frame, learns where devices are located, and decides whether the frame should be forwarded, filtered, or flooded.
A bridge does not normally examine application data or make routing decisions based on IP networks. Those functions belong to higher-layer devices such as routers.
How a Bridge Works
1. Learning source MAC addresses
When a frame enters a bridge port, the bridge records the source MAC address and the incoming port in its MAC address table. This process lets the bridge learn which devices are reachable through each segment.
2. Forwarding known traffic
If the destination MAC address is already in the table and it is located on another port, the bridge forwards the frame only through that port. This reduces unnecessary traffic on the remaining segment.
3. Filtering local traffic
If both the source and destination are on the same segment, the bridge does not forward the frame to the other segment. This decision is called filtering.
4. Flooding unknown and broadcast frames
If the destination is unknown, the bridge floods the frame through all eligible ports except the port on which it arrived. Broadcast frames are also flooded within the broadcast domain.
Bridge vs Switch vs Router
| Device | Typical OSI layer | Forwarding information | Main purpose |
|---|---|---|---|
| Bridge | Layer 2 | MAC address | Connect and filter a small number of LAN segments |
| Switch | Layer 2, or Layer 3 for multilayer switches | MAC address or IP route | Provide multiport LAN connectivity and segmentation |
| Router | Layer 3 | IP address and routing table | Connect different IP networks and select paths |
A traditional bridge usually has fewer ports and performs its work in software. A modern Ethernet switch is essentially a high-speed multiport bridge implemented with specialized hardware. Both learn MAC addresses and separate collision domains, but neither automatically separates broadcast domains unless VLANs or routing are used.
Transparent Bridging and Spanning Tree
Ethernet bridges are called transparent because end devices do not need to know that the bridge exists. Redundant Layer 2 links can improve availability, but they can also create switching loops. A loop may cause broadcast storms, duplicate frames, and an unstable MAC address table.
Spanning Tree Protocol prevents Layer 2 loops by placing selected redundant paths into a non-forwarding state. The network can then activate an alternate path when the active topology changes.
Important CCNA Points
- A bridge operates primarily at OSI Layer 2.
- It learns entries from source MAC addresses.
- Known unicast traffic is sent toward the learned destination port.
- Unknown unicast and broadcast traffic is flooded.
- Each bridge or switch port creates a separate collision domain.
- VLANs separate broadcast domains on a switch.
- Spanning Tree prevents loops in redundant Layer 2 topologies.
Study Approach
Build a small switched topology, generate traffic between hosts, and use show commands to observe how the MAC address table changes. Compare the behavior of known unicast, unknown unicast, and broadcast frames. You can also review our Cisco CCNA notes and the official Cisco learning material for the current objectives.
After learning the concept, use the Cisco 200-301 Premium PDF for structured practice with the latest available questions and answers.