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7.2 | |
Spanning-Tree Protocol | |
| 7.2.1 | |
Redundant topology and spanning tree |
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Redundant networking topologies are
designed to ensure that networks continue to function in the presence of
single points of failure. Users have less chance of interruption to
their work, because the network continues to function. Any interruptions
that are caused by a failure should be as short as possible.
Reliability is increased by redundancy.
A network that is based on switches or bridges will introduce
redundant links between those switches or bridges to overcome the
failure of a single link. These connections introduce physical loops into the network.
Switches operate at Layer 2 of the OSI model and forwarding decisions are made at this layer. As a result of this process, switched networks must not have loops. Switches flood traffic out all ports when the traffic is sent to a
destination that is not yet known. Broadcast and multicast
traffic is forwarded out every port, except the port on which the
traffic arrived. This traffic can be caught in a loop.
In the Layer 2 header there is no Time To Live (TTL). If a frame is sent into a Layer 2 looped topology of switches, it can loop forever. This wastes bandwidth and makes the network unusable. At Layer 3 the TTL is decremented and the packet is discarded when the TTL reaches 0. This creates a dilemma. A physical topology that contains switching or bridging loops is necessary for reliability, yet a switched network cannot have loops. The solution is to allow physical loops,
but create a loop free logical topology.
The loop free logical topology created is called a tree. This topology is a star or extended star logical topology, the spanning tree of the network. It is a spanning tree because all devices in the network are reachable or spanned. The algorithm used to create this loop free logical topology is the spanning-tree algorithm. This algorithm can take a relatively long time to converge. A new algorithm called the rapid spanning-tree algorithm is being introduced to reduce the time for a network to compute a loop free logical topology.
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These
bridging loops are created so if one link fails another can take over
the function of forwarding traffic.
For this
logical topology, traffic destined for the server farm attached to Cat-5
from any user workstation attached to Cat-4 will travel through Cat-1
and Cat-2. This will happen even though there is a direct physical
connection between Cat-5 and Cat-4.
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