Hi baljinder29, as I said in the last post the use of any STP flavor depends on your topology and what you want to accomplish.
In the traditional STP (IEEE 802.1D) we can have a converge time of 50 seconds and even if we have multiple VLANs all of the VLANs use the same instance of STP that is called Common Spanning Tree (CST). Using the same instance of STP for all VLANs may not be optimal because different VLANs may have different traffic pattern. If you have a topology with redundant links could happen that some of those links are not carrying any traffic and are sitting there just waiting for something to fail.
Using Per VLAN Spanning Tree (PVST used over ISL trunks / PVST+ used over IEEE 802.1Q trunks)
we are able to make different switches roots for different VLANs, not only it gives us more optimal pathing for different VLAN traffic patterns but It can also do some traffic balance over those links that were not carrying any traffic if we were using traditional STP.
In this approach, each VLAN is running its own instance of STP. If we do have a lots of VLANs we can define different STP instances using Multiple Spanning Trees Protocol (MSTP - 802.1s) in which we can assing different VLANs to the appropiate instance depending on our logical STP topology.
In addition of those types of STP there is another one called Rapid Spanning Tree Protocol (IEEE 802.1w). Cisco implemented his own variant called Rapid PVST+. The goal for both are fast convergence when there is a topology change on the network.
In traditional STP there is a potential time of 50 seconds delay for a port to go from the blocking state to the forwarding state is there is a topology change in the network, with Rapid PVST+ this time can be in most of cases lower than 5 seconds. How can this approch make it so quickly? Is important to know the terminology used in Rapid PVST+
Port roles:
Root Bridge: The switch in the topology with the lowest Bridge ID (BID)
Designated Port: The port on a network segment that is closest to the root in terms of cost.
Root Port: The port on a non-root bridge, that is closest to the root in terms of cost.
Disable Port: A port that is administratively shut down.
Alternate Port: A port on a switch that is currently discarding data frames, but could provide an alternate path to get to the root bridge.
Backup Port: A port that is currently discardind data frames, although it could be an alternate path to the root bridge, and it is also acting as a redundant link to a shared segment.
Port States:
Discarding: Data is not being forwarded on the port (Alternate, Backup and Disable ports)
Learning: The switch is learning MAC addresses available off of the port (when a port is transitioning to forwarding)
Forwarding: Data is being forwarded o the port (Root & Designated Ports)
**There is not a Listening state**
Link Types:
Point-to-Point: The connected port is running in full-duplex mode and where the link is tipacally connecting one switch to another switch.
Shared: The connecting port is running in half-duplex mode and where the link is tipically connecting a switch to a shared media hub.
Edge: The connected port is not connected to another switch or shared media hub, but instead connects to a network endpoint.( e.g. Laptop, PC, )
In the event of a topology change Rapid PVST+ only considers a topology change to be a port that is not a edge port moving to forwarding state. The switch experincing the topology change does not need to notify the root bridge about the change, It sends a BPDU (with the topology change bit set) to its neighbors which then inform their neighbors of the topology change. This process continues until all switches in the topology know of the change. This can in some cases dramatically speed up convergence time.
In constrast, in traditional STP the switch experincing the topology change sends a Topology Change Notification (TCN) to the root bridge, which the sends a Topology Change Acknoledgment (TCA) to the switch reporting the change, and then the root bridge notifies other switches of the change by setting the Topology Change flag (TC) in its BPDUs.
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The basic concept of Link Aggreation (LAG) is that multiple physical links are combined into one logical bundle. This provides two major benefits, depending on the LAG configuration:
- Increased capacity – traffic may be balanced across the member links to provide aggregated throughput
- Redundancy – the LAG bundle can survive the loss of one or more member links
This layer 2 transparency is achieved by the LAG using a single MAC address for all the device’s ports in the LAG group.
From Spanning Tree Protocol (STP) perspective, no matter how many physical ports are being used to form the LAG, there is going to be only one logical interface representing each LAG bundle. The individual ports are not part of the STP topology, but only the one logical interface. STP is still going to be active on the LAG interface and should not be turned off, so that if there are multiple LAGs configured between two adjacent nodes, STP will block one of them.
HTH
Regards
Leonardo
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