Spanning Tree Protocol

STP helps to prevent loops in switched network. It works by selecting one root bridge, every non root bridge will select one root port and each network segment will select one designated port.  For selecting the root bridge it has some criteria’s first it will check the priority, if the priority is same it will check the Lowest MAC address and it will elected as a root bridge.

Root Port: Lowest path cost to root bridge, if cost is same lowest sender bridge ID (MAC)

Designated Port: Lowest path cost to root bridge, if cost is same lowest sender bridge ID (MAC)

 1. Lowest Bridge ID

 2. Lowest path cost to root bridge

 3.Lowest sender bridge ID

 4.lowest port priority

 5.Lowest Port ID.

Default Priority is 32768

STP and RSTP Difference

States:

STP                              RSTP

Disabled                              

Blocking                               Discarding

Listening                             

Learning                               Learning

Forwarding                           Forwarding              

RSTP will wait for 3 missed BPDU’s then it will go to learning to forwarding state

STP Convergence:

(Hello = 2 Sec)
Max Age (Blocking) = 20 secs
Listening = 15 secs
Learning = 15 secs
2*forward Delay +Max Age (2*15+20) = 50 secs to convergence

RSTP Convergence:

3 missed BDPUs @ 2 sec each = 6 secs
Learning (no listening) = 15 secs
= 21 secs to convergence