Implementing Spanning Tree

© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-1

Implementing Spanning Tree

Spanning Tree Protocol Enhancements


802.1D-1998: The legacy standard for bridging and STP. CST: Assumes one spanning-tree instance for the entire bridged network,

regardless of the number of VLANs. PVST+: A Cisco enhancement of STP that provides a separate 802.1D

spanning-tree instance for each VLAN configured in the network. 802.1D-2004: An updated bridging and STP standard. 802.1s (MST): Maps multiple VLANs into the same spanning-tree

instance. 802.1w (RSTP): Improves convergence over 1998 STP by adding roles

to ports and enhancing BPDU exchanges. PVRST+: A Cisco enhancement of RSTP using PVST+.

STP Standards


Comparison of Spanning-Tree Protocols

StandardResources

NeededConvergence

CST 802.1D Low Slow All VLANs

PVST+ Cisco High Slow Per VLAN

RSTP 802.1w Medium Fast All VLANs

PVRST+ Cisco Very high Fast Per VLAN

MSTP 802.1s Cisco

Medium or high Fast VLAN list


About STP


About STP (Cont.)


Spanning-Tree Cost and Priority

Cost determines root port determination. For ports with equal costs, priority is used.


Default Spanning-Tree Configuration

Cisco Catalyst switches support three types of spanning tree:

– PVST+ – Cisco default.

– PVRST+.

– MSTP. Default spanning-tree configuration:

– PVST+.

– A separate STP instance for each VLAN.

– Same switch is root bridge for all VLANs.

– No load sharing between links.


Enable PVRST+ globally. Should be configured on all switches in the broadcast domain. Configure a switch as the root bridge for each VLAN. Configure a switch as the secondary root bridge for each VLAN. Load sharing on uplinks.

PVRST+ Configuration


In this example scenario, bridge priority, cost, and priority are manipulated.

PVRST+ Configuration (Cont.)


RSTP Port Roles Ports in forwarding mode:

– Root: chosen path to root bridge

– Designated: port that sends the best BPDU on the segment to which it is connected

Ports in blocking mode:– Alternate: receives more

useful BPDUs from another bridge

– Backup: receives more useful BPDUs from the same bridge


Spanning-Tree Port Types and States

STP Port Role RSTP Port Role STP Port State RSTP Port State

Root port Root port Forwarding Forwarding

Designated port Designated port Forwarding Forwarding

Nondesignated port

Alternative or backup port Blocking Discarding

Disabled Disabled — Discarding

Transition Transition Listening Learning Learning


RSTP State Transitions

Standard spanning-tree state transitions:

Standard spanning tree has no mechanism to determine when network has converged, blocking all transmissions for twice the forward delay.


RSTP state transitions:

Because RSTP relies on local negotiations, transition occurs as soon as the negotiation is completed.

RSTP State Transitions (Cont.)


Ports negotiate locally as soon as different BPDUs are received.

Transition occurs as soon as negotiation is completed.

Negotiation is then immediately started on other ports.

RSTP Proposal-Agreement Sequence


RSTP Link Types


RSTP Edge Ports

Will never have a switch connected to it

Immediately transitions to forwarding

Functions similarly to PortFast

Is configured with spanning-tree portfast command


PortFast Configuration

Configure PortFast on edge ports connected to nonswitch devices.

Ports transition directly to forwarding, bypassing listening and learning states of spanning tree.

Configures PortFast on an interface. Use trunk parameter for trunk interfaces with no switches

connected (e.g., routers or servers with trunks).


Bridge Priority with Extended System ID

Only four high-order bits of the 16-bit Bridge Priority field carry actual priority.

Therefore, priority can be incremented only in steps of 4096, onto which will be added the VLAN number.

Example: For VLAN 11: If the priority is left at default, the 16-bit Priority field will hold 32768 + 11 = 32779.

Priority Values (Hex) Priority Values (Dec) 0 01 40962 8192. .8 (default) 32768. .F

61440


Verifying PVRST+


Verifying PVRST+ (Cont.)


Introducing MSTP

In some scenarios, many VLANs are spanning several switches.

PVRST+ would need six instances.

Grouping instances simplifies the tree structure.


MST Regions

MST configuration on each switch: Name Revision number VLAN association table


Extended System ID in Bridge ID Field


Implementing MST


Implementing MST (Cont.)


Implementing MST (Cont.)


Verifying MSTP

Displays MSTP configuration

switch# show spanning-tree mst configuration

switch# show spanning-tree mst configurationName [XYZ]Revision 1 Instances configured 2

Instance Vlans mapped-------- --------------------------------------------------------0 1-10,13-20,23-30,34-40941 11,21,312 12,22,32------------------------------------------------------------------


Verifying MSTP (Cont.)

Displays configuration information for a specific MSTP instance

Forces renegotiation with neighboring switches during migration process

switch# show spanning-tree mst instance_number

switch# clear spanning-tree detected-protocols [interface interface-id]

switch# show spanning-tree mst 1

##### MST1 vlans mapped: 11,21,31Bridge address 001a.e372.ab00 priority 24577 (24576 sysid 1)Root this switch for MST1

Interface Role Sts Cost Prio.Nbr Type--------------- ---- --- --------- -------- ------------------------Fa0/1 Mstr FWD 200000 128.1 P2pFa0/8 Desg FWD 200000 128.9 P2p


Summary

Enhancements now enable STP to converge more quickly and run more efficiently.

RSTP provides faster convergence than 802.1D STP when topology changes occur.

RSTP configuration includes enabling RSTP and designing the root and secondary root bridges.

RSTP allows several additional port roles to increase the efficiency of the overall mechanism.

show spanning-tree is the main family of commands used to verify RSTP operations.

MSTP reduces the encumbrance of PVST by allowing a single instance of spanning tree to run for multiple VLANs.

MSTP BPDUs carry the MST instance number in the extended system ID field.


Documents

Implementing Spanning Tree