OSI Troubleshooting
Layer 2: bridges and switches

Bridges
Wireless bridges and APs are data communication devices that operate mainly at Layer 2. Several types of bridging are used as important internetworking devices. Transparent bridging is found primarily in Ethernet environments, while source route bridging occurs primarily in Token Ring environments. Translational bridging provides translation between the formats and transit principles of different media types, usually Ethernet and Token Ring.

Bridges analyze incoming frames, make forwarding decisions based on the information contained in the frames, and forward the frames toward their destinations. Upper-layer protocol transparency is a primary advantage of bridging. Because the device operates at the data link layer, it is not required to examine upper-layer information. This allows it to rapidly forward traffic representing any network-layer protocol. It is not uncommon for a bridge to move AppleTalk, DECnet, TCP/IP, XNS, and other traffic between two or more networks.

Bridges are capable of filtering frames based on any Layer 2 fields. A wireless bridge, for example, can be programmed to reject all frames from a particular network. Because data-link layer information often includes a reference to an upper-layer protocol, bridges can usually filter on this parameter. Furthermore, filters can be helpful in dealing with unnecessary broadcasts and multicast packets.

By dividing large networks into self-contained units, wireless bridges provide several advantages. The bridge will act as a firewall for some potentially damaging network errors, and can support communication between a larger number of devices than a single LAN connected to the bridge can support. Bridges extend the effective length of a LAN, permitting the attachment of distant stations that were not previously permitted.

Switches
Switching is a technology that alleviates congestion in Ethernet LANs by reducing traffic and increasing bandwidth. Common switch characteristics include Ethernet or Fiber ports to provide connectivity between network devices such as workstations, printers, servers, and internetworking devices such as routers, switches, and hubs. Typical switches are shown in Figure .

In data communications, all switching and routing equipment perform two basic operations:

  • Switching data frames – The process by which a frame is received on an input medium and then transmitted to an output medium.
  • Maintenance of switching operations – Switches build and maintain switching tables and search for loops. Routers build and maintain both routing tables and service tables.

Like bridges, switches connect LAN segments, use a table of MAC addresses to determine the segment on which a datagram needs to be transmitted, and reduce traffic. Switches operate at much higher speeds than bridges, and can support new functionality, such as virtual LANs (VLANs). If VLANs have been configured on a switch, this may affect connectivity to other devices on the LAN, depending on the router configuration.

Switches determine the segmentation of a network by building address tables that contain the address of each network device and identify which segment must be used to reach that device. While the learning occurs, traffic will not be forwarded.

If traffic does not pass after the learning phase and if VLANs are set correctly, another common issue may be port security configurations that block traffic from unauthorized host devices. Check the switch configuration to verify security settings.