Spanning Tree

Allows ethernet LANs to have redundant links, without having switching loops.

If frame arrives on an access interface, switch will use that interface's access VLAN to determine which ports to send out on.

If frame arrives on a trunk interface, the switch will use the VLAN listed in the frames trunking header.

Access ports belong to one VLAN

Trunk ports can forward traffic to all VLANs known by the switch

IEEE 802.1D

STP stops: - Broacast Storms - a frame or frames that are sent to all switches in an infinite loop - MAC table instability - continual updating of MAC tables with incorrect entries - Multiple Frame Transmission - multiple copies of a frame are delivered to the intended host

Criteria for putting a port into Forwarding State: - Root switch - all ports are forwarding - Each non-root switch has a port that has the least admin cost to the root switch, this is the root port - forwarding - Switch with lowest root cost in a segment to another switch is the designated switch. This port is the designated port. Forwarding. - all other ports are BLOCKING

STP Bridge ID (BID)8 byte value - 2 bytes are the priority field - 6 bytes are the system ID (MAC address)

BPDU = Bridge Protocol Data UnitsBPDUs are used to exchange info between switches

Hello BPDU contains: - Root Bridge ID (the BID of the switch this switch believes to be the root) - Senders BID - Senders Root Cost - STP cost between switch and root switch - Timer Values on the root switch - Hello Timer, MaxAge Timer, Forward delay Timer

Electing the root switch:Switches elect the root switch based on the BIDs received in BDPUs.The switch with the lowest BID is elected the root.First, the lowest priority, followed by the lowest MAC address if there is a tie

Superior BPDU - a BPDU that has a root BID better than the current rootInferior BPDU - a BPDU that has a root BID higher than current root BID

Electing Root Ports:Each switch sends hello BPDUs with its own root cost.All switches determine their root port based on the info received in the hello BPDUs.The lowest cost to the root switch will be the root port.

Electing Designated Ports:Port that advertises the lowest root cost on a LAN segment will be made the designated port.In a tie, switches choose the switch with the lowest BID

Default STP Port Costs10MBPS = 100100MBPS = 191GBPS = 410GBPS = 2

Default Hello timer = 2 secondsDefault MaxAge timer = 20 seconds

1. Root switch sends out a Hello BPDU out all ports2. Non-root switches receive hello BPDU on root port. They then alter this BPDU with their own BID as the sender BID and the root cost.Steps 1 and 2 continue until something changes.

When a switch doesn't receive a hello BPDU in the hello time (default 2 seconds) the switch continues as normal.If switch still doesn't receive the BPDU within the MaxAge timer (default 20 seconds) the switch reacts by taking steps to change the STP topology

After MaxAge timer has expired, switches elect another root and restart STP convergence process

Interface States:Forwarding - Port is sending traffic out this interfaceBlocking - Port is not sending traffic out of this interfaceListening - Interface does not forward frames. Any unused MAC addresses on port are discardedLearning - Interface does not forward frames. Switch starts learning MAC addresses on this port

EtherchannelCombines parallel segments of equal speed (up to 8) between same pair of switches into 1 Etherchannel.

EtherchannelSwitch treats all ports as one interface with regards to STP

EtherchannelIf one port fails, the connection remains and STP convergence is not required

EtherchannelUses load balancing to determine which port in an Etherchannel to send data out on

PortFastAllows a switch to go straight from Blocking to Forwarding (skipping learning and listening)

PortFastCan only be safely use on ports that a switch will not be connected on - eg end user devices.

BPDU GuardDisables any port that receives a BDPU.This can be used in conjunction with PortFast

Rapid STP = IEEE 802.1W

Rapid STPHas faster convergence than STP.STP convergence default is 50 seconds.RSTP converges in 3x Hello BPDU timers

BID Priority by default is 32768. This can be changed to a different value (multiple of 4096) using:spanning-tree vlan vlan-id root {primary|secondary}spanning-tree clan vlan-id priority priority

Default Interface costs:10mb: 100100mb: 191gb: 410gb: 2

To change the interface cost on an interface:spanning-tree vlan vlan-id cost cost

Portfast is not enabled by default. To enable it on an interface, use:spanning-tree portfast

show spanning-tree vlan vlanxThis shows all the spanning tree info for vlanx.It shows:- root bridge info (priority, mac and will say if this switch is the root)- spanning tree info for the local switch (priority (priority+vlan), address and timers)- spanning tree info for all ports (root ports, designated ports)

show spanning-tree root- this lists all the roots BID for each vlan- it also shows the root port to get to the root, and the root cost

To change spanning tree port costs:spanning-tree vlan cost cost

To change the priority of the switch (global conf):spanning-tree vlan vlan-id priority value

To force the switch to either choose a priority that will make it the root, or a backup, use:spanning-tree vlan vlan-id root primary|secondaryIf the current root has a priority higher than 24,576 (ie the default of 32,768) the switch will set it's priority to 24,576.If the current roots priority is equal to or lower than 24,576 the switch will set it's priority to as close to 24,576 as it can

Switching Fundamentals

stp fundamentals

STP convergence

STp features

STP Settings