Spanning Tree Protocol (STP) What STP is, why do we need it, and how to configure STP (properly) Frank Olshansky – Frank_Olshansky@iu13.org

Introduction to the Spanning Tree Protocol Also known as STP Developed in 1990 IEEE 802.1D standard

Why do we need STP STP was developed to allow for redundancy between switches and to prevent Layer 2 loops No Layer 2 equivalent of Time To Live (TTL) If there was no STP: Broadcast Storms Unstable MAC address table Multiple copies of same frame

Without STP 2 1 3

Without STP 2 1 3

Without STP 2 1 3

Without STP 2 1 3

Without STP 2 1 3

Without STP 2 1 3

What STP does 2 1 3

What STP does 2 1 3

What STP does 2 1 3

Root bridge STP elects a single root bridge for the network The root bridge is selected through an election process, and the switch that wins is the switch with the lowest Bridge ID (BID) The bridge ID is an 8 byte field 2 byte bridge priority 0-61440 32768 is the default 6 byte MAC address of the switch

Bridge Protocol Data Unit (BPDU) Frames that contain information about STP Sent as multicast to 01:80:C2:00:00:00 Configuration BPDUs are sent every 2 seconds by default Three types Configuration BPDU: Provides information to all switches. TCN (Topology Change Notification): Lets switches know about changes in the STP topology. TCA (Topology Change Acknowledgment): Acknowledges TCN.

Bridge Protocol Data Unit (BPDU)

STP port states Root Port – Port on a non-root bridge that is closest to the root bridge, according to STP port cost Designated Port – The port on a network link that is closest to the root bridge, according to STP port cost Note: Every network link has a designated port Non-designated port – Port that is blocked by STP, so that a loop-free Layer 2 topology is maintained Disabled port – A port that is administratively down

STP port costs 10 Mbps = 100 100 Mbps = 19 1 Gbps = 4 10 Gbps = 2

Bridge Protocol Data Unit (BPDU)

STP port states 2 1 3

STP convergence times When primary link to root bridge down, it takes 50 seconds for the switch to transition to the backup link The switch links go through the following states: Blocking – Switch listens for BPDUs from root 20 seconds Listening – Listens to any BPDUs arriving on any interfaces 15 seconds Learning – Populating MAC address table Forwarding

STP Flavors Common Spanning Tree (CST) – One spanning tree topology for all of the VLANs Per-VLAN Spanning Tree plus (PVST+) – One spanning tree topology per VLAN; Cisco proprietary PVST – Over ISL trunk PVST+ – Over an 802.1Q trunk Multiple Spanning Trees (MST) – One spanning tree topology for one or multiple vlans IEEE 802.1s

STP Flavors The bridge ID is an 8 byte field 2 byte bridge priority 0-61440 32768 is the default 6 byte MAC address of the switch With PVST+ and MST, the bridge priority field is split 4 bit bridge priority Increments of 4096 12 bit extended system id 1 to 4095 corresponding to the VLAN

Rapid Spanning Tree Protocol Spanning tree topology convergence is much quicker Convergence time goes from 50 seconds to 3 seconds

Rapid Spanning Tree Port Roles Root port – Same as root STP port state in STP Port on a non-root bridge that is closest to the root bridge, according to STP port cost Designated Port – Same as designated port state in STP The port on a network link that is closest to the root bridge, according to STP port cost Note: Every network link has a designated port Disabled Port – Same as disabled port state in STP A port that is administratively down

Rapid Spanning Tree Port Roles No such thing as non-designated ports Alternate Port – Port that STP blocks, so that a loop-free Layer 2 topology is maintained Can be changed to a designated port very quickly Backup port – Only occurs when there are two connections going into a hub

Rapid Spanning Tree Port States Disabled – Data is not forwarded on port Disabled ports, Backup ports, and Alternate ports Learning – Switch is learning MAC addresses through port Seen when port is transitioning from disabled to forwarding Forwarding – Data is being forwarded on port Root and Designated ports

Rapid Spanning Tree Link Types Point-to-Point – Inter-switch connections Shared – Link going from a switch to a hub Edge – Link going from switch to an edge device

Spanning Tree “Optional” Features PortFast Allows a switch port to go into forwarding state almost immediately Configured on ports connecting to endpoints Can set it up by default or on a per-interface basis Turned off on trunk ports

Spanning Tree “Optional” Features BPDU Guard Causes a port to go into an error disabled state if a BPDU is received on that port Can be enabled by default or on a per interface basis Should be configured on any port that should not be connecting to other switches

Questions?