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 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:
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.
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: