STP : Spanning tree protocol
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.
This loop avoidance at layer 2 is achieved by Spanning Tree Protocol (STP).
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.
This loop avoidance at layer 2 is achieved by Spanning Tree Protocol (STP).
STP is defined in IEEE 802.1D.
STP Operation:
When the network has stabilized, it has converged and there is one spanning tree per network.
As a result, for every switched network the following elements exist:
=> One root bridge per network
=> One root port per non root bridge
=> One designated port per segment
=> Unused, non-designated ports
Root ports and designated ports are used for forwarding (F) data traffic.
Non-designated ports discard data traffic. These ports are called blocking (B) or discarding ports
STP Operation:
When the network has stabilized, it has converged and there is one spanning tree per network.
As a result, for every switched network the following elements exist:
=> One root bridge per network
=> One root port per non root bridge
=> One designated port per segment
=> Unused, non-designated ports
Root ports and designated ports are used for forwarding (F) data traffic.
Non-designated ports discard data traffic. These ports are called blocking (B) or discarding ports
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