© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-1 Implementing Spanning Tree...
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Transcript of © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-1 Implementing Spanning Tree...
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-1
Implementing Spanning Tree
Spanning Tree Protocol Enhancements
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-2
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
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-3
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
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-4
About STP
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-5
About STP (Cont.)
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-6
Spanning-Tree Cost and Priority
Cost determines root port determination. For ports with equal costs, priority is used.
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-7
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.
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-8
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
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-9
In this example scenario, bridge priority, cost, and priority are manipulated.
PVRST+ Configuration (Cont.)
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-10
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
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-11
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
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-12
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.
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-13
RSTP state transitions:
Because RSTP relies on local negotiations, transition occurs as soon as the negotiation is completed.
RSTP State Transitions (Cont.)
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-14
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
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-15
RSTP Link Types
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-16
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
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-17
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).
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-18
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
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-19
Verifying PVRST+
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-20
Verifying PVRST+ (Cont.)
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-21
Introducing MSTP
In some scenarios, many VLANs are spanning several switches.
PVRST+ would need six instances.
Grouping instances simplifies the tree structure.
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-22
MST Regions
MST configuration on each switch: Name Revision number VLAN association table
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-23
Extended System ID in Bridge ID Field
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-24
Implementing MST
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-25
Implementing MST (Cont.)
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-26
Implementing MST (Cont.)
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-27
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------------------------------------------------------------------
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-28
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
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-29
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.
© 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-30