Rapid PVST+

Rapid Spanning Tree Protocol (RSTP)  and   Rapid-PVST+ are enhancement of the original STP 802.1D protocol because they can switch ports from blocking to forwarding without relying on timers, execute spanning tree calculations and converge the network faster than STP..

The RSTP 802.1w protocol is an IEEE open implementation. Cisco has its own proprietary implementation of RSTP, that includes the benefits of its Per-VLAN spanning tree protocols, called Rapid-PVST+.


In STP, network convergence can take up to 50 seconds, with RSTP and Rapid-PVST+ network convergence can happen in just over 6 seconds.

In RSTP and Rapid-PVST+, switchports transition from discarding (blocking) to forwarding based solely on a switch-to-switch proposal and agreement process, using BID priority, instead of having to also wait for timers. That result in a much faster network convergence.

RSTP includes many of Cisco's earlier proprietary 802.1D enhancements. It is also backwards compatible with STP and can work with legacy switches running STP. RSTP and its variants are widely implemented so it is beneficial to understand a little about how RSTP works.

The main advantage is that it runs the rapid spanning-tree protocol for each VLAN

RSTP Characteristics

BPDU

- RSTP messages or advertisements are called version 2, BPDUs, and has all the same fields, making it backwards compatible with 802.1D STP BPDUs.

- As with STP, RSTP version 2 BPDUs  are sent every 2 seconds.

- With RSTP BPDUs, the version field is set to 2, and Flag field is used for RSTP control information, including the Proposal and Agreement bits.

Edge Ports and Non-Edge Ports

- In RSTP, ports are defined as either edge ports and non-edge ports.
  
Edge ports are ports that are not intended to be connected to other switches. Edge ports can go immediately into forwarding mode since they do not need to negotiate BPDU messages with other switches.

Edge ports, the equivalent of PortFast-enabled ports.The edge port concept is well known to Cisco spanning-tree users because it corresponds to the PortFast feature

Unlike PortFast, an edge port that receives a BPDU immediately loses its edge port status and becomes a normal spanning-tree port. When an edge port receives a BPDU, it generates a topology change notification (TCN).

- Non-edge ports can have either a point-to-point or shared link type. Non-edge ports are temporarily blocked during the proposal and agreement process.

Link Types - Point-to-Point and Shared

- With RSTP, non-edge ports can be configured as either a point-to-point link type or a shared link type. The shared link type is for a port that is connected to other switches by way of a shared device like a hub.
- Point-to-point link types can switch to forwarding state quickly depending on its port role

Port Roles and Port States

Root bridge: The switch in the topology with the lowest bridge ID (BID)

 

Root Port - port that is closest to the root bridge in term of cost. It goes directly to the forwarding state.

Designated Port - The port on a network segment that is closer to the root bridge in term of cost.
 Only one designated port allowed per network segment (switch-to-switch link), this can either be an alternate port and a designated port, or a root port and a designated port.

When there are two designated ports in the same segment the proposal and agreement process will result in one of them becoming an alternate (discarding) port. In this way designated ports make the most use of the proposal and agreement process. A designated port will eventually be a forwarding port though it can temporarily transition to the listening (discarding) state during the proposal and agreement process.

Alternate Port - available to transition to designated and forwarding if needed, otherwise discarding
(blocking).It could provide an alternate path to get to the root bridge( that's an alternate to the root port)


Backup Port - used with a redundant switch-to-switch port link. Is in the discarding state.

Disable port  : A port that is administratively shut down and wont forward any data

 

Port state

In IEEE 802.1D STP, when a port has been selected by spanning tree to become a designated port, it must wait two times the forward delay before transitioning the port to the forwarding state. RSTP significantly speeds up the recalculation process after a topology change, because it converges on a link-by-link basis and does not rely on timers expiring before ports can transition. Rapid transition to the forwarding state can only be achieved on edge ports and point-to-point links. In RSTP, this condition corresponds to a designated port in the discarding state.

Let's review to get some concepts clear:

A point to point is a link type where the connected port is running a full duplex mode , usually a switch connected to another switch

Shared is a link type where the connected port is running a half duplex mode  and where the link is typically connecting a switch to a shared media  hub

Edge Port:  A link type where the connected port is not connected to another switch or shared media hub, but instead connected to a network endpoit (E.G a pc)

If you connect a port configured with the spanning-tree link-type point-to-point command to a remote port through a point-to-point link and the local port becomes a designated port, the switch negotiates with the remote port and rapidly changes the local port to the forwarding state.

RSTP Topology change:A non-edge port transition to the forwarding state

Comparison of topology change with STP and RSTP

Topology change notification with STP : The switch sending a topology change send a Topology Change Notification (TCN) to the root bridge, which them send a Topology Change acknowledge (TCA) to the switch reporting the change and the  the root switch notifies the other switches by sending a Topology Change (TC) flags in its BPDUs

Topology changes with RSTP : The switch experiencing topology change send a BPDU ( with the  topology change  (TC) to it's neighbor, which them infor their neighbor of the topology change.This process goes on and on till all switch know the topology change

So with rstp it drastically speed the conversion time

 

We will work with this topology 

 1.- SWITCH A

2.-We did the same on switch B and C


3.-Now let's go back to Switch A
 We will configure 50 and 80  as the primary and VLAN 110 as secondary

4.-Now let's go to Switch C

We will configure 110 as the primary and VLAN 50 and 80 as secondary  and we will verify that information

 5.- Now we have this

We will check now on Switch A how Vlan 50 looks like

But we will added a phone , int fa0/14

And our lab in real life  will look like this:

6.-Let go to Switch A and check VLAN 50

We see that for Vlan 50 switch A is the root port (primary) , we see the connections as designated ports,  that they are in forwarding mode  and the link  type P2P ( automatically Point to Point  can tell they are connected to another switch or router through full duplex mode )

If we would be in a half duplex mode it will assume we are connected to a share media segment

We will move to switch C and manually add the Fa0/14 of the  phone and we will check spanning-tree of VLAN 110

7.-We see that this is the root for vlan 110,  the designated ports, the forwarding state and the link type point to point.

Now let's try to make fa0/6 (the one to switch B )  a share link. There are two ways..
First way

Check that we didnt have the option to make it an edge port


Second way  (with verification)

8.-We will make a port an edge port and doing that we are letting that port know that we are connecting to an end device , so don't need to wait for a learning and listening state

You have the warning about edge port


So we must delete the trunking ports from interface fa0/14:
..

Switch(config)#interface f0/14
Switch(config-if)#no switchport trunk encapsulation dot1q
Switch(config-if)#no switchport mode dynamic desirable
Switch(config-if)#switchport mode access

the output of "show run"

On fa0/14 we can see the results of vlan 450 and vlan 460

We could see the edge ports...So the exercise is done and it was successful!

If you like this lab comment and share..Thank you!