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Rapid spanning tree protocol (RSTP)

Rapid Spanning Tree Protocol (RSTP), which was designed to take over the duties of STP was standardized in IEEE 802.1w and 802.1D. Compare to STP, RSTP has shorter converge time both on startup and failure recovery.

(R)STP attempts to avoid this endless loop problem by removing the redundant path. Both RTP and RSTP need to elect a root bridge (or switch) based on priority and bridge ID.

These priorities and bridge IDs are relayed through the exchange of Bridge Protocol Data Units (BPDUs), which are sent by RSTP every “hello” interval—by default, every two seconds. The bridge that has the superior priority or bridge ID is elected the root bridge.

A big difference between the original STP and RSTP is in how they converge. With the original STP, all of the switches must wait as the interfaces go through blocking (at startup), listening, learning port states (total 50 seconds) before transition to forwarding state. With RSTP, a proposal and agreement (P/A) process is put in place, which is automatically used as soon as an interface comes up. This P/A process drastically shortens the time needed for an interface to move into a forwarding state. However, this process only happens on what RSTP calls point-to-point (p2p) links.

Cisco switches use the duplex setting of an interface to determine the link type. Interfaces with a full-duplex setting follow the P/A process; interfaces with a half-duplex setting go through the slower STP convergence process (30 seconds total) before converging. An exception to this rule is root ports, which are automatically pushed into a forwarding state when a switch is brought up. In RSTP, these half-duplex links are referred to as shared (Shr) links. RSTP also defines a third type of link, edge links; these ports operate like older Cisco PortFast ports and are automatically put into a forwarding state.

RSTP has a 4 types of ports. The alternative and backup ports does not exit for for STP. These states are a method of indicating preferred secondary paths. The alternative port state indicates that an interface will be the next potential root port if the current root port fails. The backup port state indicates that an interface will be the next potential designated port; this only happens when a switch has multiple links into the same Ethernet segment.

  1. root port: a forwarding port that forwards frame from Non-root bridge to Root bridge.
  2. designated port: the frame exiting port for every LAN segment.
  3. alternate port: as it name implies, is an alternate path to the root bridge which does not use the root port.
  4. backup port: a redundant path to a segment where another bridge port already connects.

The port states are also referenced differently in RSTP. STP has five port states: disabled, blocking, listening, learning, and forwarding. RSTP combines those states into a total of three: discarding (disabled, blocking, listening), learning, and forwarding.

  1. Discarding – wherein a port discards information received on the interface, discards frames switched from another interface for forwarding, does not learn MAC addresses, and listens for BPDUs.
  2. Learning – a situation where the switch creates a switching table that will map MAC addresses to a port number. It also happens when a port discards frames received on the interface, discards frames switched from another interface for forwarding, learns MAC addresses, and listens for BPDUs.
  3. Forwarding – wherein a port receives and forwards the frames received on the interface, forwards frames switched from another interface, learns MAC addresses, and listens for BPDUs.

PVST -- Cisco’s RSTP Implementation

Cisco’s default implementation of STP differs from the standard. Instead of using a single STP instance that works over all of the network, including over all configured VLANs, Cisco implements an STP instance for each specific VLAN, which is referred to as Per-VLAN Spanning Tree (PVST).

It is based on the 802.1D standard and uses Cisco proprietary ISL trunking protocol. It prevents creating a loop by forwarding some VLANs on another trunk. It is the default spanning-tree mode used on all Ethernet port-based VLANs.

PVST is succeeding by Cisco proprietary extensions like BackboneFast, UplinkFast, and PortFast.

Cisco Lab

Here is an video demonstrating how to configure RSTP on sisco switches.

[ICND1 and ICND2 break down]

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