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Spanning-Tree Protocol / Redundant topology and spanning tree

Spanning-Tree Protocol / 
Redundant topology and spanning tree
7.2.1 This page will teach students how to create a loop free logical topology.
Redundant network topologies are designed to ensure that networks continue to function in the presence of single points of failure. Work is interrupted less often for users 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. These bridging loops are created so if one link fails another can take over the function of forwarding traffic.
When the destination of the traffic is unknown to a switch, it floods traffic out all ports except the port that received the traffic. Broadcasts and multicasts are also forwarded out every port except the port that received the traffic. This traffic can be caught in a loop. 
In the Layer 2 header, there is no Time To Live (TTL) value. 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. 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.
The loop free logical topology created is called a tree. This topology is a star or extended star logical topology. This topology is 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 was developed to reduce the time for a network to compute a loop free logical topology.
The next page will discuss STP.

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