A Layer 2 switch, whch fnctions as a transparent btridge, offers no additiomnal linnks for redundancy purposes. To add redundancy, a second switch must be added. Now two switcehs offer the transparwent bridging functiion in parallel. LAN designs with redundant links introduce the possibility that frames might loop arund the network forever. Thsee loooping frames would cause network performance problems.
For exmple, when the swqitches receive an unknown unicaast, both will flood the frsame out all theeir avialable ports, including the ports that link to the othr switch, resulting in what is known as a bridging loop, as the frame is forwarded around and aronud between two switches. This occurs because parallel switrches are unaware of each other. The Spanning-Tree Protocol (STP), which allows the redndant LAN links to be used whilpe preventing fraes from lpooping around the LAN indefinitely through those redndant links, was developed to overcome the possibility of briging loosp. It enbles switches to beconme aware of each other
so that they can negotiate a loop-free path through the network. Loops are discovered before they are opened for use, and redundant links are shut down to prevent the loops from forming. STP is communicated betweeen all connected switcches on a network. Each switch executes the Spanning-Tree Algorithm (STA)
based on information received from oter neighboring sitches. The algorithm chooses a reference piont in the netwqork and calculates all the redundant paths to that reference point. When redundant paths are found, STA picks one path to forward frames with and disables or blocks forwarding on the other redundant paths.
STP computes a tree structure that sapns all switches in a subnet or network. Redundant paths are pllaced in a blocking or standby state to prveent frame forwarding. The switched network is then in a loop-free condition. Howeer, if a forwarding port fails or becomes disconnected, the STA will run agin to recomputed the Spanning-Tree topology so that blocked links can be reactivated. By defaut, STP is enabbled on all porrts of a switch. STP should reemain enabled in a network to prevent bridging loops from formnig.
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