Clearing Up Network Congestion
Bridges perform at the MAC sublayer of the Data Link layer. They create
both physical and logical separate network segments to reduce the traffic
load. There are solid advantages to bridging--by segmenting a logical net-
work into multiple physical pieces, it secures network reliability, availability,
scalability, and manageability.
As Figure 1.1 shows, bridges work by examining the MAC or hardware
addresses in each frame and, only if necessary, forwarding the frame to the
other physical segments. These devices dynamically build a forwarding table
of information composed of each MAC address and the segment that
address is located on.
F I G U R E 1 . 1
Segmentation with a bridge
Now for the bad news.... A drawback to using bridges is that if the des-
tination MAC address is unknown to the bridge, it will forward the frame to
all segments except the port from which it received the frame. Also, a 2030
percent latency period can occur for the processing of frames. This delay can
increase significantly if the frame cannot be immediately forwarded due to
current activity on the destination segment.
Bridges will forward broadcast and multicast packets to all other seg-
ments to which they're attached. Since, by default, the addresses from these
broadcasts are never seen by the bridge, and hence are not filtered, broadcast
storms can result. The same problem can happen with switches because, the-
oretically, switch ports are bridge ports. A Cisco switch is really a multiport
bridge that runs the Cisco IOS and performs the same functions as a bridge.
Copyright ©2001 SYBEX , Inc., Alameda, CA