DT211-2Spanning Tree ProtocolNetworking 3

Eoin Irwin, Dylan Kingston, William Leacy

- STP – Brief Overview

○ Spanning Tree Protocol in a nutshell

○Algorithm / Protocol

○Add Links for redundancy

○Prevent packet loops in

multi-interconnected switch networks

(Broadcast storm)

STP – What Does It Do?

○ What are redundant networks?

○Switch interconnection - multiple routes for data flow

○Has an optimal path - lowest metric

○Redundant = Backup, incase optimal path is down

STP – What Does It Do?

○ What are switching loops?

○Switch broadcasts can cause loops

in redundant networks

○Packets sent in circles endlessly,

duplicated at each broadcast

○Packet duplication + circulation

= broadcast storm

STP – Broadcast Storm

STP – Root Bridge and Main Connections

STP – Closed Ports and Sent Data

STP - Root bridge

○ What is the root bridge?

○Place where traffic flows

○How is the root bridge selected?

○Lowest Mac Address

○What happens if STP is left alone?

○One switch acts as the root for every VLAN!

STP – Root Bridge cont.

○How to fix this from happening? ○SW1(config) #spanning-tree vlan 1

○What happens to the ports?

○STP will block unused ports at layer 2

○Set off an election

○What happens when there’s a topology change?

○Spanning tree algorithm

○SW2 (config) #spanning-tree vlan 20 root primary

○SW2 (config) #spanning-tree vlan 30 root primary

STP - BPDU

○What is BPDU?

○Bridge Protocol Data Units

○Final process, root bridge

○BPDU and its configurations

○Contains various information

BPDU - Results

○How are results exchanged?

○One switch in the network being elected

○Forwarding state root ports

○Interfaces in the spanning-tree instance are selected. Root parts and designated ports are put into the forwarding state.

STP - Layer 2 network

○Spanning tree retrieval○Done via BPDU messages

○Message exchanging between switches

○Designated switch for each LAN

○These Message result in

○Removal of redundant loops

○Unique root switch

○Unique designated switch

.

STP - Problems

15 seconds of listening for BPDUs.switch / sends receives BPDUs.

15 seconds of learning mac address. populates the cam table

(bonus) switch will wait up to 20 seconds max before moving a blocked port into listening phase.

Port is forwarding traffic

STP – Problems continued

Problems with PCs - Modern PCs can boot faster than 30 seconds

Problems with uplink ports: 30 seconds of downtime causes big problems

Solution = Rapid Spanning Tree !

Solution = Portfast !

STP – PVST+

Runs an instance of STP per Vlan

Allows different root bridges per Vlan

Root for vlan 10

Root for vlan 20

STP – Rapid STP

○ Rapid STP

○802. 1w

○Proactive system

○Redefined port roles

○Many STP similarities

STP – How RSTP improves Performance

More logical port types

Root port - used to reach the root bridge

Designated port - forwarding port, one per line

Alternative port - discarding port, backup path to root

A

B C

Conclusion

○ Spanning-Tree Protocol (STP) prevents loops from forming when switches or bridges are interconnected via multiple paths.

○ Spanning-Tree Protocol implements the 802.1D IEEE algorithm by exchanging BPDU messages with other switches to detect loops, and then removes the loop by shutting down selected bridge interfaces.

○ Spanning-Tree Protocol guarantees that there is one and only one active path between two network devices.

Any questions?