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.