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CCNP (switch) – STP – Basics (BPDU)

Basic words in STP:

Root Bridge (RB)

Non Root Bridge   (NRB)

Forwarding Ports

  • Designated Port (DP)
  • Root Port   (RP)

Non Designated Port   (Blocking Port)

 

Three steps to remember for STP:

  1. 1 Root Bridge / Network
  2. 1 Designated Port / Segment
  3. 1 Root Port / Non Root Bridge

 

BPDU (Bridge Data Protocol Unit):

Root Bridge generates BPDU in every two seconds. When root bridge generates BPDU, this is processor consuming task. So, you have to use best switch as Root Bridge.

  • Designated Port: A port which sends best BPDU is known as Designated Port.
  • Root Port: A port which receives best BPDU is known as Root Port.

BPDU

Four fields in BPDU:

  • Root Bridge ID [Priority(32768) + MAC ID]
  • Cost
  • Sender Bridge ID [Priority(32768) + MAC ID]
  • Sender Port ID [Port Priority(64 or 128) + Port Number]

 

bpdu

Now match the fields of BPDU one by one, where you will find the least value, election will be final there. Either election will final in 1st, 2nd , 3rd or any field, after that field of BPDU you need not to see further fields for election. If you see the above two BPDUs, election will be final in last field of BPDU.

Example:

bpdu-exmp

 

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