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Implementing Spanning Tree

Spanning Tree Protocol Enhancements. Implementing Spanning Tree. STP Standards. 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.

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Implementing Spanning Tree

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  1. Spanning Tree Protocol Enhancements Implementing Spanning Tree

  2. STP Standards • 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+.

  3. Comparison of Spanning-Tree Protocols

  4. About STP

  5. About STP (Cont.)

  6. Spanning-Tree Cost and Priority • Cost determines root port determination. • For ports with equal costs, priority is used.

  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. PVRST+ Configuration • 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.

  9. PVRST+ Configuration (Cont.) • In this example scenario, bridge priority, cost, and priority are manipulated.

  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. Spanning-Tree Port Types and States

  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. RSTP State Transitions (Cont.) • RSTP state transitions: • Because RSTP relies on local negotiations, transition occurs as soon as the negotiation is completed.

  14. RSTP Proposal-Agreement Sequence • 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.

  15. RSTP Link Types

  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. 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. 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 0 1 4096 2 8192 . . 8 (default) 32768 . . F 61440

  19. Verifying PVRST+

  20. Verifying PVRST+ (Cont.)

  21. Introducing MSTP • In some scenarios, many VLANs are spanning several switches. • PVRST+ would need six instances. • Grouping instances simplifies the tree structure.

  22. MST Regions • MST configuration on each switch: • Name • Revision number • VLAN association table

  23. Extended System ID in Bridge ID Field

  24. Implementing MST

  25. Implementing MST (Cont.)

  26. Implementing MST (Cont.)

  27. Verifying MSTP • Displays MSTP configuration switch# show spanning-tree mst 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 • 11,21,31 • 12,22,32 • ------------------------------------------------------------------

  28. Verifying MSTP (Cont.) • Displays configuration information for a specific MSTP instance switch# show spanning-tree mst instance_number 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 • Forces renegotiation with neighboring switches during migration process switch# clear spanning-tree detected-protocols [interface interface-id]

  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.

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