1. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-1 Implementing Spanning Tree Spanning Tree Protocol Enhancements

2. © 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

3. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-3 Comparison of Spanning-Tree Protocols Standard Resources Needed Convergence CST802.1DLowSlowAll VLANs PVST+CiscoHighSlowPer VLAN RSTP802.1wMediumFastAll VLANs PVRST+CiscoVery highFastPer VLAN MSTP 802.1s Cisco Medium or high FastVLAN list

4. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-4 About STP

5. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-5 About STP (Cont.)

6. © 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.

7. © 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.

8. © 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

9. © 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.)

10. © 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

11. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-11 Spanning-Tree Port Types and States STP Port RoleRSTP Port RoleSTP Port StateRSTP Port State Root port Forwarding Designated port Forwarding Nondesignated port Alternative or backup port BlockingDiscarding Disabled —Discarding Transition Listening Learning Learning

12. © 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.

13. © 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.)

14. © 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

15. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-15 RSTP Link Types

16. © 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

17. © 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).

18. © 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 1 4096 2 8192. 8 (default) 32768. F 61440

19. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-19 Verifying PVRST+

20. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-20 Verifying PVRST+ (Cont.)

21. © 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.

22. © 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

23. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-23 Extended System ID in Bridge ID Field

24. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-24 Implementing MST

25. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-25 Implementing MST (Cont.)

26. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-26 Implementing MST (Cont.)

27. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-27 Verifying MSTP  Displays MSTP configuration switch# show spanning-tree mst configuration Name [XYZ] Revision 1 Instances configured 2 Instance Vlans mapped -------- -------------------------------------------------------- 0 1-10,13-20,23-30,34-4094 111,21,31 212,22,32 ------------------------------------------------------------------

28. © 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: 11,21,31 Bridge 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 P2p Fa0/8 Desg FWD 200000 128.9 P2p

29. © 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.

30. © 2009 Cisco Systems, Inc. All rights reserved. SWITCH v1.0—3-30