From CCIE Study Wiki

Rapid Spanning Tree Protocol is a STP standard designed to improve convergence time.

Theory

Rapid Spanning Tree Protocol or RSTP is defined by standard 802.1w. The key components of RSTP are:

• Only waits for three missed hellos before assuming a topology change and reacting.
• New convergence process that bypasses the listening state of 802.1d.
• Standardization of advanced STP features like PortFast, UplinkFast, and BackboneFast
• Allows to have a backup Designated Port when a switch has multiple ports connected to the same shared LAN segment

RSTP utilizes the same Hello BPDUs, but uses the (previously) undefined bits to create new features.

RSTP categorizes ports into one of three port types

• point-to-point - Connects a switch to another switch
• shared - Connects a switch to a hub - it assumes that other switches are reachable off of that hub
• edge - end-user device port (treated like PortFast)

Port states are: Discarding - Learning - Forwarding

The term port role is used to describe whether a port is acting as a RP or DP. RSTP adds two additional roles:

• Alternate Port - Same as the alternate port concept in UplinkFast; an alternate root port.
• Backup Port - A port attached to the same link-type shared link as another port on the switch, but the other port is the DP for the segment. Ready to take over if the DP fails.

RSTP provides for rapid convergence by different methods according to port type:

1. Edge ports - enabled with PortFast configuration and will immediately transition to forwarding state.
2. Root ports - if a new root port is selected, RSTP blocks the old port and immediately transitions the new root port to the forwarding state
3. Point-to-point - If you connect a port to another PTP link and the local port becomes a designated port, it negotiates a rapid transition with that port by using a proposal-agreement-handshake to ensure a loop-free topology
   • two connected switches, A+B are connected via a PTP link. Switch A, with a smaller priority, sends a proposal message (a config BPDU with proposal flag set) to switch B, proposing itself as the designated switch
   • after receiving the proposal from A, B selects its new root port from the port where the proposal was received, forces all of its non-edge ports to the blocking state, and sends an agreement message through it's new root port
   • after receiving the reply from B, A now transitions its designated port to the forwarding state. No loops in the network are formed because B has set all of it's non-edge ports to blocking
   • switch B now performs a similar handshake with any switches downstream to it.

Commands

• spanning-tree mode rapid-pvst (global) - enables RSTP on a per-vlan basis. Can also enable MST, which by definition uses RSTP

Default Settings

• maxage - three missed hellos or 6 secs by default

Verification

• Nothing listed yet

Troubleshooting, Tips, and Tricks

• Nothing listed yet

Online Resources

• Excellent Cisco White Paper on RSTP.

• RSTP and fast convergence from Petr of Internetwork Expert.

• Aragoen from Route My World! shares his notes on Rapid Spanning Tree.

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