Chapter 15: Rapid Spanning Tree Protocol
This introduction explains why modern LANs need a faster alternative to original STP.
After this chapter you will be able to
- Motivation for Rapid STP. Explain why RSTP is needed despite STP being enabled by default on switches: STP protects LANs from disastrous Layer 2 loops, but the original version can take up to 50 seconds to settle after a change.
- Standard and per-VLAN versions. Distinguish IEEE 802.1D and 802.1w from Cisco’s PVST+ and Rapid PVST+, and explain what a separate spanning tree per VLAN permits.
- MSTP instance grouping benefit. Explain the resource and management costs of one STP instance per VLAN and how MSTP addresses them while retaining separate traffic paths.
- Faster RSTP forwarding. Explain why RSTP can bring ports into the forwarding state faster than STP, without describing the sync mechanism’s internal details.
- Shared topology selection algorithm. Describe how STP and RSTP use the same algorithm to elect a root bridge and select root and designated ports, and distinguish what happens to the remaining ports.
- Short and long port costs. Distinguish the original STP short costs from the long costs introduced with RSTP, and explain how long costs change as port speed changes.
- RSTP states and initial behavior. Describe how RSTP combines STP’s blocking and listening states into discarding, and explain why a newly enabled port starts there and may stay there.
- Two paths to forwarding. Explain the two ways an RSTP root or designated port reaches forwarding, including what happens when the sync mechanism cannot work.
- RSTP port role distinctions. Distinguish RSTP’s root, designated, alternate, and backup ports, including how alternate and backup ports differ when a port is neither root nor designated.
- Shared segment port selection. Explain how the lowest local port ID breaks ties when multiple ports on one switch connect to the same segment, and how this qualifies the usual rule about root-bridge ports.
- Faster RSTP topology recovery. Explain how RSTP reacts to missed BPDUs and moves a next-best port into forwarding, and distinguish that case from a port going down outright.
- Point-to-point and shared links. Distinguish point-to-point and shared RSTP links by duplex mode and how their ports reach the forwarding state.
- Edge ports and PortFast. Explain why an edge port can forward immediately and how a Cisco switch assigns the edge link type.
- RSTP topology-change triggers. Distinguish which port transitions trigger an RSTP topology change and why edge-port activity is excluded.
- Optional feature protocol compatibility. State that PortFast, BPDU Guard, Root Guard, Loop Guard, and BPDU Filter work with both STP and RSTP.
- Root Guard purpose. Explain why a service provider might enable Root Guard on a port connected to a customer network.
- Blocked port and recovery. Describe what happens when a Root Guard-enabled port receives a superior BPDU and how the port returns to normal.
- Loop Guard protection and recovery. Explain why a discarding port that stops receiving BPDUs might incorrectly begin forwarding, and how Loop Guard prevents a loop. Describe what happens when BPDUs resume.
- BPDU Filter activation behaviors. Distinguish BPDU Filter enabled on a specific interface from BPDU Filter enabled globally for PortFast ports, including what happens when each port receives a BPDU.
- BPDU Filter loop risk. Explain why the section recommends generally avoiding BPDU Filter, especially when it is enabled on an individual interface.
What it builds on
- The need for STP (chapter 14) · The need to prevent Layer 2 loops.
- STP timers (chapter 14) · Original STP convergence timing.
- How STP works (chapter 14) · Assumes the purpose of a spanning tree and its active and disabled links.
- Global configuration mode (chapter 5) · Assumes familiarity with entering commands in global configuration mode.
- The STP algorithm (chapter 14) · STP root bridge, root port, and designated port selection
- Root port selection (chapter 14) · STP’s use of path costs in root port selection
- STP port states (chapter 14) · STP port states provide the basis for comparison.
- Designated port selection (chapter 14) · Designated port selection in STP
Section by section
Introduction
This introduction explains why modern LANs need a faster alternative to original STP.
- Motivation for Rapid STP: Explain why RSTP is needed despite STP being enabled by default on switches: STP protects LANs from disastrous Layer 2 loops, but the original version can take up to 50 seconds to settle after a change.
Why it matters: This explains the problem RSTP is intended to address without requiring its mechanics yet.
Key terms: Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol (RSTP), Layer 2 loops, converge
15.1 Spanning Tree Protocol versions
This section distinguishes standard and Cisco per-VLAN spanning-tree versions, shows how to identify and select Cisco modes, and explains why MSTP can be preferable when a LAN has many VLANs.
- Standard and per-VLAN versions: Distinguish IEEE 802.1D and 802.1w from Cisco’s PVST+ and Rapid PVST+, and explain what a separate spanning tree per VLAN permits.
Why it matters: The version determines whether VLANs can use distinct spanning-tree topologies. - Cisco spanning-tree mode commands: Describe how to check and select PVST+ or Rapid PVST+ on a Cisco switch, including the difference between configuration keywords and displayed protocol names.
Why it matters: The displayed IEEE-style protocol name should not be mistaken for the IEEE-standard version running on a Cisco switch. - MSTP instance grouping benefit: Explain the resource and management costs of one STP instance per VLAN and how MSTP addresses them while retaining separate traffic paths.
Why it matters: Grouping VLANs avoids paying for a separate spanning-tree instance when several VLANs can share one.
Key terms: IEEE 802.1D, PVST+, IEEE 802.1w, Rapid PVST+, root bridge, show spanning-tree, spanning-tree mode pvst, spanning-tree mode rapid-pvst, protocol ieee, protocol rstp
15.2 STP and RSTP comparison
Compares STP and RSTP convergence, topology selection, port roles, and BPDU transmission.
- Faster RSTP forwarding: Explain why RSTP can bring ports into the forwarding state faster than STP, without describing the sync mechanism’s internal details.
Why it matters: Speed is the fundamental difference this section identifies between STP and RSTP. - Shared topology selection algorithm: Describe how STP and RSTP use the same algorithm to elect a root bridge and select root and designated ports, and distinguish what happens to the remaining ports.
Why it matters: The familiar STP election rules still apply when identifying an RSTP topology. - BPDU sending behavior: Distinguish which switches send BPDUs in STP and RSTP, and whether sending depends on receiving a root-bridge BPDU.
Why it matters: This is a specific operational difference between the protocols that the section flags as potentially examinable.
Key terms: STP, RSTP, synchronization mechanism, forwarding state, root bridge, bridge ID (BID), root cost, root port, designated port, nondesignated
15.2.1 Port costs
This subsection compares STP’s short port costs with RSTP’s long port costs and shows how to check or change the cost method a switch uses.
- Short and long port costs: Distinguish the original STP short costs from the long costs introduced with RSTP, and explain how long costs change as port speed changes.
Why it matters: The two cost scales cover different speed ranges, so you need to recognize which numbers you are interpreting. - Switch path cost method: Explain why running RSTP does not guarantee that a switch uses long costs, and state how to view or change its cost method.
Why it matters: Checking the switch avoids assuming that RSTP automatically selects long costs.
Key terms: short costs, long costs, port costs, show spanning-tree pathcost method, spanning-tree pathcost method { short | long}, global config mode
15.2.2 Port states
Compares STP and RSTP port states and explains when an RSTP port can begin forwarding immediately rather than waiting for timers.
- RSTP states and initial behavior: Describe how RSTP combines STP’s blocking and listening states into discarding, and explain why a newly enabled port starts there and may stay there.
Why it matters: This explains why an RSTP port may be active without forwarding traffic. - Two paths to forwarding: Explain the two ways an RSTP root or designated port reaches forwarding, including what happens when the sync mechanism cannot work.
Why it matters: The distinction explains why some RSTP ports forward immediately while others behave like STP ports.
Key terms: blocking, listening, discarding, alternate port, backup port, RSTP sync mechanism, forward delay timer, learning, forwarding
15.2.3 Port roles
Explains how RSTP port roles differ from STP roles and how ports sharing a segment are assigned their roles.
- RSTP port role distinctions: Distinguish RSTP’s root, designated, alternate, and backup ports, including how alternate and backup ports differ when a port is neither root nor designated.
Why it matters: The distinction lets you explain why two nonforwarding RSTP ports have different roles. - Shared segment port selection: Explain how the lowest local port ID breaks ties when multiple ports on one switch connect to the same segment, and how this qualifies the usual rule about root-bridge ports.
Why it matters: This tiebreaker explains the port roles in the shared-segment example and an exception to a common root-bridge shortcut.
Key terms: root port, designated port, nondesignated port, alternate port, backup port, designated bridge, lowest local port ID, root port selection, designated port selection, root bridge
15.2.4 RSTP topology changes
Explains how RSTP can restore forwarding faster than STP after a topology change.
- Faster RSTP topology recovery: Explain how RSTP reacts to missed BPDUs and moves a next-best port into forwarding, and distinguish that case from a port going down outright.
Why it matters: This explains why RSTP can restore forwarding much sooner after a failure.
Key terms: RSTP, BPDUs, sync process, forwarding state, max age timer
15.3 RSTP link types
This section explains how RSTP link types affect forwarding, topology-change notifications, configuration, and verification.
- Point-to-point and shared links: Distinguish point-to-point and shared RSTP links by duplex mode and how their ports reach the forwarding state.
Why it matters: Duplex mode helps explain why one RSTP link can reach forwarding rapidly while another cannot. - Edge ports and PortFast: Explain why an edge port can forward immediately and how a Cisco switch assigns the edge link type.
Why it matters: Connecting a host does not by itself make its switch port forward immediately. - RSTP topology-change triggers: Distinguish which port transitions trigger an RSTP topology change and why edge-port activity is excluded.
Why it matters: An edge port coming online need not cause other switches to react to a topology change. - Link-type verification: Describe how to check RSTP link types in Cisco IOS and interpret an edge port’s displayed type.
Why it matters: The command output lets you check whether ports have the link types you expect.
Key terms: point-to-point, shared, full duplex, half duplex, sync mechanism, edge, PortFast, spanning-tree portfast, topology change, non-edge
15.4 Root Guard, Loop Guard, and BPDU Filter
Introduces three additional optional spanning-tree features and states that all five named features work with both STP and RSTP.
- Optional feature protocol compatibility: State that PortFast, BPDU Guard, Root Guard, Loop Guard, and BPDU Filter work with both STP and RSTP.
Why it matters: You can discuss these optional features in the context of either spanning-tree protocol.
Key terms: PortFast, BPDU Guard, Root Guard, Loop Guard, BPDU Filter, STP
15.4.1 Root Guard
Explains how Root Guard keeps an external switch from changing an STP root bridge and what happens when it blocks a port.
- Root Guard purpose: Explain why a service provider might enable Root Guard on a port connected to a customer network.
Why it matters: It explains why a customer connection should not be allowed to change the provider’s root bridge. - Blocked port and recovery: Describe what happens when a Root Guard-enabled port receives a superior BPDU and how the port returns to normal.
Why it matters: It connects the protective action to the customer’s loss of access and shows what allows access to return.
Key terms: Root Guard, root bridge, bridge ID, STP topology, superior BPDU, root-inconsistent state, inferior
15.4.2 Loop Guard
Explains how Loop Guard prevents a blocked switch port from creating a Layer 2 loop when it stops receiving BPDUs.
- Loop Guard protection and recovery: Explain why a discarding port that stops receiving BPDUs might incorrectly begin forwarding, and how Loop Guard prevents a loop. Describe what happens when BPDUs resume.
Why it matters: A port that forwards by mistake can create a Layer 2 loop that disrupts the LAN. - Root Guard and Loop Guard distinction: Distinguish the BPDU conditions that trigger Root Guard and Loop Guard, and state whether both can be enabled on one port.
Why it matters: The guards address different BPDU conditions and cannot be used together on one port.
Key terms: Loop Guard, BPDUs, discarding state, forwarding state, loop-inconsistent state, spanning-tree guard loop, Root Guard, superior BPDUs, mutually exclusive
15.4.3 BPDU Filter
This section explains how the two ways of enabling BPDU Filter differ and why the feature should be used cautiously.
- BPDU Filter activation behaviors: Distinguish BPDU Filter enabled on a specific interface from BPDU Filter enabled globally for PortFast ports, including what happens when each port receives a BPDU.
Why it matters: The two settings respond differently to a received BPDU, so they do not offer the same protection if a port connects to a switch. - BPDU Filter loop risk: Explain why the section recommends generally avoiding BPDU Filter, especially when it is enabled on an individual interface.
Why it matters: Careless interface-level filtering can prevent spanning tree from blocking a loop and bring down a network.
Key terms: BPDU Filter, spanning-tree bpdufilter enable, spanning-tree portfast bpdufilter default, PortFast, RSTP edge ports, Layer 2 loop, discarding state, end hosts
15.5 Summary
Concepts explained in this chapter
- RSTP synchronization mechanismThe RSTP synchronization mechanism is a spanning-tree procedure that coordinates neighboring switch ports so eligible ports can transition rapidly to forwarding without waiting for spanning-tree timers.
- blocking stateThe blocking state, called the discarding state in RSTP, is a spanning-tree port state in which normal data frames are not forwarded or learned while control messages may still be processed.
- backup portA backup port is an RSTP port that provides a redundant path to the same network segment as a designated port on the same switch.
- edge linkAn RSTP edge link is a link connecting a switch port to an end host, allowing the port to transition immediately to forwarding without posing a Layer 2 loop risk.
- point-to-point communicationAn RSTP point-to-point link is a full-duplex link whose connected ports can use the RSTP synchronization mechanism to transition rapidly to forwarding.
- shared linkAn RSTP shared link is a half-duplex link whose ports cannot use the RSTP synchronization mechanism and transition like standard STP ports.
- IEEE 802.1wRapid Spanning Tree Protocol is a faster-converging spanning-tree protocol that reduces the time required to transition ports after topology changes.
- spanning-tree mode commandThe spanning-tree mode command is a Cisco IOS global-configuration command that selects the spanning-tree mode used by a switch.
- spanning tree protocolThe spanning tree algorithm is the procedure for electing a root bridge and selecting root and designated ports to create a loop-free spanning-tree topology.
- BPDU filterBPDU Filter is a spanning-tree feature that suppresses the sending or processing of Bridge Protocol Data Units on a switch port.
- loop guardLoop Guard is a spanning-tree protection feature that places a port into a loop-inconsistent state when expected BPDUs stop arriving, preventing erroneous transition to forwarding.
- root guardRoot Guard is a spanning-tree protection feature that prevents a port from accepting a superior BPDU and thereby prevents an external switch from becoming the root bridge.
- spanning-tree bpdufilter enableThe spanning-tree bpdufilter enable command is a Cisco IOS interface-configuration command that enables BPDU Filter on a specific switch port.
- loop-inconsistent stateThe loop-inconsistent state is a spanning-tree port condition entered when Loop Guard detects that expected BPDUs are no longer being received.
- non-designated portAn alternate port is an RSTP port that provides an alternate path toward the root bridge and can transition to forwarding if the root port fails.
- designated bridgeA designated bridge is the switch that has the designated port for a particular network segment.
- designated portA designated port is an STP port in the forwarding state that provides the preferred path away from the root bridge for a network segment.
- port idA port ID is a unique STP identifier for a switch port, consisting of a port priority and a sequential port number.
- root portRoot port selection is the spanning-tree procedure for choosing the port on a non-root bridge that provides its best path toward the root bridge.
- stp port roleAn STP port role is a spanning-tree assignment that determines how a switch port participates in the loop-free topology.
- link typeAn RSTP link type is a classification of a switch link that determines how its ports transition through RSTP states and react to topology changes.
- bridge identifierA bridge identifier, or bridge ID, is a 64-bit value that identifies a switch within a spanning-tree instance and is advertised in bridge protocol data units.
- network loopA Layer 2 loop is a cyclic forwarding path at the data-link layer that can cause frames to circulate repeatedly through switches.
- spanning-tree link-type commandThe spanning-tree link-type point-to-point command is a Cisco IOS interface-configuration command that manually configures a port's RSTP link type as point-to-point.
- spanning-tree instanceA spanning-tree instance is an independently running spanning-tree protocol instance associated with one VLAN or a group of VLANs.
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