STP Part II

PVST (Per Vlan Spanning Tree):A Vlan field is added to the BPDU header along with Priority & Mac.Priority is 32768, Mac Address is MAC or that Switch & Vlan is the Vlan number used. That is the reason default priority on Cisco switches is 32769 due to default Vlan 1.Advantage of PVST is that we get one Root Bridge per Vlan.For multiple Vlan we will have multiple Root Bridges.

Lets imagine that we have two vlans, Vlan 2 and Vlan 10 in our Network we can easily load balance the traffic by making one switch Root Bridge for Vlan 2 and another for Vlan 10.

STP Part II

The command to Setup a specific switch as Root Bridge for PVST:First Check the commands to verify:Switch(config)# show spanning-tree Check for Root ID and Bridge IDRoot ID is info about the Root BridgeBridge ID is the info about the switch itself.Now make switch as a Root Bridge. (PVST)Switch(config)# spanning-tree vlan 1 root primary NowSwitch(config)# show spanning-tree

Vlan 1 will become Root Bridge

STP Part II

Rapid Spanning Tree:Spanning Tree is very old technology and time to recover involves number of stages ( which are Listening, Learning, Forwarding and Blocking) around 50 sec.

This causes problem with uplink and computers.Solution:Portfast for Access devices and RSTP for switches.PortFast:It basically disables spanning tree.It cannot run on Trunks.Configuration:Switch(config)# int fa 0/2 Switch(config-if)# spanning-tree portfast

STP Part II

Uplink Ports:As Portfast dosen’t work on uplink ports ,so the other solution was RSTP or Rapid Spanning Tree Protocol.Rapid STP :802.1w new standard.It has STP similarities.All switches must support RSTP in Network.There are 3 ports states now.DiscardingLearningForwarding

STP Part II

Rapid STP :Ports roles :Root Port is still thereDesignated Port is still thereDiscarding / Blocking port can be seen as Alternate Port (Basically Switch remembers the port for path so it doesn’t dealy)Edgeport is Portfast. (Enable by portfast command)

Advantages of Rapid Spanning Tree Protocol:

1. Topology change packets (TC packets) inform all the other switches on network

2. RSTP remember the ports3. It is really fast.

STP Part II

Configuration of RSTP: Switch(config)# spanning-tree mode rapid-pvst

Note: mst option will group multiple vlans in single spanning-tree instance.

Thank you ,

Hyp3ri0nwww.itpings.comwww.linuxworld.co