Spanning Tree Protocol – PVST and RSTP Explained

PVST+ (Per VLAN Spanning Tree Protocol)

The original IEEE 802.1D standard defines only one spanning-tree instance for the entire switched network, regardless of the number of VLANs. A network running 802.1D has these characteristics:

  • No load sharing is possible. One uplink must block for all VLANs.
  • The CPU is spared. Only one instance of spanning tree must be computed.


Cisco developed PVST+ so that a network can run an independent instance of the Cisco implementation of IEEE 802.1D for each VLAN in the network.

This implementation dictates that the Priority field now contains two parts:

  • Priority: which can be configured to affect the RBID of the BPDUs being sent influencing the BPDU to be superior or inferior manually
  • System ID Extension field is also included which allows the VLAN ID to be placed into what was formerly the last 12 bits of the priority field.


A BPDU is sent for each VLAN (can be seen in System ID Extension) and they are processed separately by the switch as it runs STP instance per VLAN:

VLAN Load Balancing with PVST+:

With PVST+, it is possible for one trunk port on a switch to block for a VLAN while forwarding for other VLANs. PVST+ can be used to manually implement Layer 2 load balancing. The switches in a PVST+ environment require greater CPU process and BPDU bandwidth consumption than a traditional STP because each VLAN runs a separate instance of STP.


Note: PVST and PVST+

PVST and PVST+ are Cisco proprietary implementations of STP, difference is related to compatibility with standard STP:
* PVST will only send it own proprietary BPDU.
* PVST+ will send 802.1D (standard STP) BPDUs (with its own proprietary BPDU), which make it backward compatible with the standard STP.

In simple words, 2 BPDU are send with PVST+:
– PVST proprietary BPDU (includes, Bridge ID extension)
– Standard STP BPDU


RSTP (Rapid Spanning Tree Protocol)

RSTP provides significantly faster spanning tree convergence after a topology change, introducing new convergence behaviors and bridge port roles to accomplish this. While STP can take 30 to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3 × hello times (default: 3  ×  2 seconds) or within a few milliseconds of a physical link failure.


RSTP Port states

RSTP reduces the port states types to three (no listening state), in order to speed convergence following a link failure:

  • Discarding: No user data is sent over the port.
  • Learning: The port is not forwarding frames yet, but is populating its MAC-address-table.
  • Forwarding: The port is fully operational.


RSTP Port Roles

Also RSTP modifies the port roles Roles:

  • Root: A forwarding port that is the best port from non-root bridge to root bridge
  • Designated: A forwarding port for every LAN segment
  • Alternate: An alternate path to the root bridge. This path is different from using the root port
  • Backup: A backup/redundant path to a segment where another bridge port already connects
  • Disabled: Not strictly part of STP, a network administrator can manually disable a port


So, The New Port Roles are:

  • Alternate:

An alternate port receives BPDUs from another switch but remains in a blocked state. For example, let’s say a switch has two paths to the root bridge. It will elect one of the two ports as a root port and the other will become an alternate port. If at any time the root port fails, this redundant path—the alternate port—will become the new root port.

  • Backup:

A backup port receives BPDUs from its own switch but remains in a blocked state. For example, If a switch has two ports connecting to different switches, then one port will be elected as a root port and the other will become the backup port.



RSTP Initial synchronization Process:

The same process will happen if SW-2 have non EDGE ports, as a result of the Sync Operation (step-3), the non-EDGE ports on SW-2 are in Designated discarding state, and they will start sending Proposal (which include SW-1 ID as Root).


Let’s look at a similar example:

RSTP improves convergence on point-to-point links by reducing the Max-Age time to 3 times Hello interval, removing the STP listening state, and exchanging a handshake between two switches to quickly transition the port to forwarding state. RSTP does not do anything differently from STP on shared links.


RSTP Topology Change synchronization Process:



Reference:

bethepacketsite.wordpress.com/2016/02/24/spanning-tree-protocol-ieee-802-1d-bridge-id/

Bilel

Bilel

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