The next key bit to understand is the timers. In the original STP it can take a long time for interfaces to enter a forwarding state. Typically up to 50 seconds. This is called STP convergence and there are a few timed processes we need to know, these are:

 

• Hello - This is the interval that BPDUs are sent by the switches (root switch in a converged network). The default interval is 2 seconds

• Forward Delay - The is the delay for the transitory states of listening and learning. Each delay is 15 seconds so a total of 30 seconds for original STP is seen

• MaxAge - This is the timeout for each switch to not receive a BPDU from the root, which is usually 10x the Hello timer, so 20 seconds

 

Example Output

When looking at the original STP (PVST+) output of a Cisco appliance, you'll notice that the port role states "Altn". This is used in Rapid-STP which we will review shortly. When original STP (PVST+) is in use, ignore this and deem it a designated port. Let's look at an example output below. (The topology for this output is two PCs linked to Fa0/1 & Fa0/2, with Gi0/1 & Gi0/2 linked to other switches.)

 

Switch3#sh spanning-tree

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    32769

             Address     0002.1687.8C70

             Cost        4

             Port        25(GigabitEthernet0/1)

             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)

             Address     00D0.D3E6.DDBA

             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time  20

Interface        Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/1            Desg FWD 19        128.1    P2p

Fa0/2            Desg FWD 19        128.2    P2p

Gi0/1            Root FWD 4         128.25   P2p

Gi0/2            Altn BLK 4         128.26   P2p  

 

Note that only on Cisco devices the port role here is listed incorrectly. It should be listed as Designated.

​

Looking at the output, we can see that this switch isn't the root switch, and port Gi0/2 is in a blocking state. Gi0/1 is listed as a root port which Fa0/1 and Fa0/2 are in a designated forwarding state. You can see how the output lists the Root Bridge IDs, then it's own Bridge IDs which contains information such as timers, the bridge priority and burnt-in MAC address. Remember that the extended system ID is the VLAN number, which in this case is VLAN1.

 

Optional STP Features

 

There are some important features that we need to understand that can improve the stability of the network and further prevent loops, these are:

 

• PortFast - this can be used against edge ports (where the device connected to the switch is for example a PC, a phone or a server etc. By configured a switchport as PortFast, this tells the switch to place the port directly into a forwarding state, bypassing the listening/learning states

• BPDU Guard - this can be used against edge ports as well as PortFast to further secure the network. When BPDU Guard is enabled, BPDUs can still be transmitted however should a BPDU be received, the port is shut down until it is manually re-enabled. This avoids possible malicious attacks by users attempting to introduce a switch with a lower priority than the legitimate root in the topology. Typically, you would configure BDPU Guard if PortFast is enabled. You would not want to configure this on ports linking switches together.

 

 

The problem with original STP (PVST+)

 

STP (or PVST+) takes a relatively long time to converge following a network change. Which when it was implemented wasn't an issue, however in modern networks it is unacceptable and should be avoided where possible. The default values of the Hello, MaxAge and Forward Delay timers cause a convergence delay of up to 50 seconds. Once a link goes down, the MaxAge count down begins from the last time a BPDU frame was received. 20 seconds later the port is put back into a listening state for 15 seconds, then into a learning state for 15 seconds. Once these stages are completed the network will have reconverged however with how critical modern networks are to today's society, that is too long. So, how was the delay issue fixed? See below:

 

• Removing some port states (Disabled and blocking combined into Discarding, and Listening state removed)

• Adding some port roles (alternate and backup) which allows for immediate interface transition in the event of a link failure

• All switches now send their own BPDUs, rather than just the root switch

• MaxAge timer much shorter (typically 3x hello time)

 

The above changes are used in what's known as 802.1W RSTP (Rapid Spanning Tree). Again Cisco has it's own implementation called Rapid-PVST+. In fact, most network devices implemented today used RSTP (Rapid-PVST+) by default.

 

RSTP (Rapid Spanning Tree)

 

RSTP (Rapid PVST+) Concepts

RSTP is very similar to STP (so similar they can actually be used on the same network). The only real differences are the port states, roles and timers.  The process for RSTP is the same as STP:

 

• Elect a root switch

• Assign root ports

• Calculate designated and blocking ports

 

There is one key difference though and that is the implementation of the alternate port. This port is an alternate path to the root and will be in a blocking state while the main root port is up and active. There is also the addition of a backup port which is used when the switch is connected to an hub. Because hubs are rarely seen in modern networks it's unlikely you'll see a backup port in the RSTP topology.

 

 

Let's go through the stages again on how STP converges the topology, and then how the new RSTP alternate role fits into this process.