Preventing Loops with Spanning Tree Protocol
Bridging loops form because parallel switches (or bridges) are unaware of each other. STP
was developed to overcome the possibility of bridging loops so that redundant switches
and switch paths could be used for their benefits. Basically, the protocol enables switches
to become aware of each other so they can negotiate a loop-free path through the network.
Note: Because STP is involved in loop detection, many people refer to the catastrophic
loops as “spanning-tree loops.” This is technically incorrect because the Spanning Tree
Protocol’s entire function is to prevent bridging loops. The correct terminology for this
condition is a bridging loop.
Loops are discovered before they are made available for use, and redundant links are effectively
shut down to prevent the loops from forming. In the case of redundant links,
switches can be made aware that a link shut down for loop prevention should be brought
up quickly in case of a link failure. The section “Redundant Link Convergence” in Chapter
8 provides more information.
STP is communicated among all connected switches on a network. Each switch executes
the spanning-tree algorithm based on information received from other neighboring
switches. The algorithm chooses a reference point in the network and calculates all the redundant
paths to that reference point. When redundant paths are found, the spanning-tree
algorithm picks one path by which to forward frames and disables, or blocks, forwarding
on the other redundant paths.
As its name implies, STP computes a tree structure that spans all switches in a subnet or
network. Redundant paths are placed in a Blocking or Standby state to prevent frame forwarding.
The switched network is then in a loop-free condition. However, if a forwarding
port fails or becomes disconnected, the spanning-tree algorithm recomputes the spanningtree
topology so that the appropriate blocked links can be reactivated.
was developed to overcome the possibility of bridging loops so that redundant switches
and switch paths could be used for their benefits. Basically, the protocol enables switches
to become aware of each other so they can negotiate a loop-free path through the network.
Note: Because STP is involved in loop detection, many people refer to the catastrophic
loops as “spanning-tree loops.” This is technically incorrect because the Spanning Tree
Protocol’s entire function is to prevent bridging loops. The correct terminology for this
condition is a bridging loop.
Loops are discovered before they are made available for use, and redundant links are effectively
shut down to prevent the loops from forming. In the case of redundant links,
switches can be made aware that a link shut down for loop prevention should be brought
up quickly in case of a link failure. The section “Redundant Link Convergence” in Chapter
8 provides more information.
STP is communicated among all connected switches on a network. Each switch executes
the spanning-tree algorithm based on information received from other neighboring
switches. The algorithm chooses a reference point in the network and calculates all the redundant
paths to that reference point. When redundant paths are found, the spanning-tree
algorithm picks one path by which to forward frames and disables, or blocks, forwarding
on the other redundant paths.
As its name implies, STP computes a tree structure that spans all switches in a subnet or
network. Redundant paths are placed in a Blocking or Standby state to prevent frame forwarding.
The switched network is then in a loop-free condition. However, if a forwarding
port fails or becomes disconnected, the spanning-tree algorithm recomputes the spanningtree
topology so that the appropriate blocked links can be reactivated.
