pvst+

[Manny003]

is there a maximum to the number of root ports allowed per vlan in rapid pvst+ spanning tree ?

[Deepak Kumar]

Hi,

Per VLAN it will one single root port on a switch. 

[Manny003]

I think you misunderstood my question.  which was  Is there a maximum number of root ports allowed per vlan  not per switch

[Reza Sharifi]

Rules of Operation

This section lists rules for how STP works. When the switches first come up, they start the root switch selection process. Each switch transmits a BPDU to the directly connected switch on a per-VLAN basis.

As the BPDU goes out through the network, each switch compares the BPDU that the switch sends to the BPDU that the switch receives from the neighbors. The switches then agree on which switch is the root switch. The switch with the lowest bridge ID in the network wins this election process.

Note: Remember that one root switch is identified per-VLAN. After the root switch identification, the switches adhere to these rules:

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html

HTH

[Giuseppe Larosa]

Hello @Manny003 ,

a single VLAN PVST instance can have only one root port in the direction to the root bridge for the VLAN.

Different VLANs can have different root bridge and so a single switch can be using different root ports for different VLANs achieving some load balancing that is one of the advatanges of PVST+ or Rapid PVST.

However, within a single topology only one port is selected as root port.

In other terms STP does not support load balancing on parallel links like routing protocols like OSPF or EIGRP do.

The only way to use multiple links is to have them in a ether bundle, that is considered as a single logical link by STP.

LACP is recommended to build and mantain the the ethernet channels.

Hope to help

Giuseppe

[Deepak Kumar]

Hi,

There is no such limit. If you have 100 switches and connected to Root Bridge then there will be 100 root ports. To understand the concept of this root port limited, you have to understand how STP is working. How to design STP? 

There are many limitations such as Message Age and Hop Count etc. I hope you will get more details https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/spantree.html#wp1174727