<Jeremy's IT Lab> Day 20: STP (Part 1)

STP (Spanning Tree Protocol)

- classic STP (802.1D - IEEE)

- rapid STP

 

Reason of using STP:

Prob1: Broadcast Storms

- Switches connected redundancy as security, switches forward infinitely broadcasting frames through traffic

Prob2: MAC Address Flapping

- Also switch use source MAC address field to 'learn' MAC and update its table as much as they can

 

STP:

-> Prevents loop in bridged NW

-> all vendor's switches run STP by default

-> types: 

  - blocking state(redundant port): only send STP mesg (BPDUs = Bridge Protocol Data Units), can be used by backup

  - forwarding state(Designated port): send normal traffic

 

 

https://www.firewall.cx/networking/network-protocols/spanning-tree-protocol/spanning-tree-protocol-root-bridge-election.html

-> PVST: implementation of the Spanning Tree Protocol (STP) that operates on a per-VLAN basis

-> To determine the lowest ID, compares bridge priority first, and compare MAC address if previous value is the same

-> Bridge priority can be change uints of '4096' (e.i. 0 - 4096 - 8192 -...)

 

* Hello BPDU: switch send it to ensure every connections go well

 

Steps to STP:

1) powered on, set itself to Root Bridge

2) give up its RB if receive superior BPDU(lower Bridge ID)

3) once topology converged(all agreed RB) only RB send 'Hello BPDU' every 2 sec

 

*one designated port per each connection in collision domain

How to Determine each ports types:

1) Elect Root Bridge (all ports involved in switch become Designated Port) switch

2) other switches, Select Root Port (always across away designated port)

 (1) Lowest Root cost

 (2) Lowest neighbor Bridge ID

 (3) Lowest neighbor Port ID

3) Other switch port would be Non-designated Port (blocking traffic)

 (1) Interface lowest root cost

 (2) Interface lowest bridge ID

 

STP cost (count in only send status)

Speed	STP Cost
10 Mbps	100
100 Mbps	19
1 Gbps	4
10 Gbps	2

 

//CLI

 

SW3#show spanning-tree summary

SW3#show spanning-tree detail

 

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Acronyms:

• STP : Spanning Tree Protocol
• BPDUs : Bridge Protocol Data Units
• PVST : per-VLAN Spanning Tree