why a foreward delay is introduced in listening and learning state in STP before reaching to forewarding state

[binojbaby1984]

i have doubt regarding STP , in listening state and learning state ,of stp port sates why its needed to introduce a delay of default of 15 seconds before moving to forewarding state . please let me know what ths use of it

[Giuseppe Larosa]

Hello Binoj,

the reason is to allow enough time to STP BDPU to propagate in all the network: STP BPDUs are sent every 2 seconds and 7 switch hops are supported by default in STP so 2 times 7 + 1 = 15 seconds.

STP target is to prevent bridging loops so before moving from one STP state to another  one on a specific port this wait time is required in classic 802.1D STP.

Hope to help

Giuseppe