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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?