Information Technology Reference
In-Depth Information
hubs operate on the physical layer of the OSI model and perform the same functions as
basic repeaters. The difference is that hubs have more ports than basic repeaters.
Bridges
Bridges connect separate segments of a network. They differ from repeaters in that
bridges are intelligent devices that operate in the data link layer of the OSI model. Bridges
control the collision domains on the network. Bridges also learn the MAC layer addresses
of each node on each segment and on which interface they are located. For any incoming
frame, bridges forward the frame only if the destination MAC address is on another port
or if the bridge is unaware of its location. The latter is called flooding . Bridges filter any
incoming frames with destination MAC addresses that are on the same segment from
where the frame arrives; they do not forward these frames.
Bridges are store-and-forward devices. They store the entire frame and verify the cyclic re-
dundancy check (CRC) before forwarding. If the bridges detect a CRC error, they discard
the frame. Bridges are protocol transparent; they are unaware of the upper-layer protocols
such as IP, IPX, and AppleTalk. Bridges are designed to flood all unknown and broadcast
traffic.
Bridges implement Spanning Tree Protocol (STP) to build a loop-free network topology.
Bridges communicate with each other, exchanging information such as priority and bridge
interface MAC addresses. They select a root bridge and then implement STP. Some inter-
faces are in a blocking state, whereas other bridges have interfaces in for warding mode.
Figure 3-3 shows a network with bridges. STP has no load sharing or dual paths, as there is
in routing. STP provides recovery of bridge failure by changing blocked interfaces to a for-
warding state if a primary link fails. Although DEC and IBM versions are available, the
IEEE 802.1d standard is the STP most commonly used.
STP elects a root bridge as the tree's root. It places all ports that are not needed to reach
the root bridge in blocking mode. The selection of the root bridge is based on the lowest
numerical bridge priority. The bridge priority ranges from 0 to 65,535. If all bridges have
the same bridge priority, the bridge with the lowest MAC address becomes the root. The
concatenation of the bridge priority and the MAC address is the bridge identification
(BID). Physical changes to the network force spanning-tree recalculation.
Switches
Switches use specialized integrated circuits to reduce the latency common to regular
bridges. Switches are the evolution of bridges. Some switches can run in cut-through
mode, where the switch does not wait for the entire frame to enter its buffer; instead, it
begins to forward the frame as soon as it finishes reading the destination MAC address.
Cut-through operation increases the probability that frames with errors are propagated on
the network, because it forwards the frame before the entire frame is buffered and
checked for errors. Because of these problems, most switches today perform store-and-
for ward operat ion as br idge s do. As shown in Figure 3-4, switches are exactly the same as
bridges with respect to collision-domain and broadcast-domain characteristics. Each port
broadcast domain. Assignment to different VLANs changes that behavior.
 
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