STP Convergence

STP Covergence

Imagine there is some failure in one of the links in your network.How long that blocked port will transition into forwarding state?

This is the time for the convergence: Traditional Convergence Time
MaxAge (20 sec) + Listening (15 sec) + Learning (15 sec) = 50 seconds

This 50 seconds protocol is the IEEE 802.1D protocol

A bridge protocol data unit (BPDU) is a data message transmitted across a local area network to detect loops in network topologies.
 A BPDU contains information regarding ports, switches, port priority and addresses. BPDUs contain the information necessary to configure and maintain spanning tree topology.

CST

 Common Spanning-Tree (CST) assumes one spanning-tree instance for the entire bridged network, regardless of the number of VLANs. 
This implementation reduces CPU load since only one Spanning Tree instance is maintained for the entire network. This version of the STP   can be used when only one Layer 2 topology is needed in the network

Another version of spanning-tree is able to calculate a topology for each VLAN. This version is called PVST (Per VLAN Spanning-Tree) and it’s the default on Cisco switches.

 PVST

Two types of PVST:
Per Vlan Spanning Tree (PVST) used over ISL trunks
Per Vlan Spanning Tree Plus (PVST+) used over IEEE802.1Q trunk

They are designed to give us different root bridges to different VLANs, and each VLAN is running their own instance of spanning tree protocol.

Multiple Instance Spanning Tree Protocol

MISTP (802.1s) is an IEEE standard which allow several VLANs to be mapped to a reduced number of spanning-tree instances. This is possible since most networks do not need more than a few logical topologies. Each instance handles multiple VLANs that have the same Layer 2 topology.

Rapid Spanning Tree Protocol (RSTP)

One big disadvantage of STP is the low convergence which is very important in switched network. To overcome this problem, in 2001, the IEEE with document 802.1w introduced an evolution of the Spanning Tree Protocol: Rapid Spanning Tree Protocol (RSTP), which significantly reduces the convergence time after a topology change occurs in the network.

RSTP is faster than STP: while STP can take 30 to 50 seconds to transit from a blocking state to a forwarding state, RSTP is typically able to respond less than 10 seconds of a physical link failure.

RSTP works by adding an alternative port and a backup port compared to STP. These ports are allowed to immediately enter the forwarding state rather than passively wait for the network to converge.Look  at here for example..

PVST+ (Per Vlan Spanning Tree Plus) : This feature (by cisco) is used to create an instace of

STP for each VLAN.

e.g.If you have a switch with 3 vlans you will have a 3 instances of VLANs that you could modify the parameters (like to make root bridge for a specific VLAN) independently accord to your design goals.


The goal of PVST and RSTP is fast convergence.
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.

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.