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When designing a switched network,one of the biggest issues that needsto be dealt with is loop prevention. Ifa loop were to develop in a switchednetwork, the amount of traffic thatcould be passed between switcheswould quickly utilize the entirebandwidth available within each ofthe switches affected. One method toprevent loops is to only provide asingle path between switches andensure that there is no path redundancy across the entire switched network. While thismay work in small deployments, when dealing with switched networks that are a bit largerthe deployment of redundancy is a required element. To be able to deal with thesepotential loops that could be caused by these redundant paths the Spanning Tree Protocol(STP) can be deployed. STP is tasked with preventing loops throughout the switchednetwork. It does this by temporarily blocking redundant paths between switches; in thiscase, if the primary forwarding path is disrupted, this blocking would be removed allowingtraffic to be passed. This article takes a look at some of the basic STP concepts andreviews the configuration required to deploy STP on a switch.

When designing a switched network, one of the biggest issues that must be dealt with is loopprevention. If a loop were to develop in a switched network, the amount of traffic that could bepassed between switches would quickly utilize the entire bandwidth available within each of theswitches affected. One method to prevent loops is to only provide a single path between switchesand ensure that there is no path redundancy across the entire switched network. While this maywork in small deployments, when dealing with switched networks that are a bit larger thedeployment of redundancy is a required element.

To be able to deal with these potential loops that could be caused by these redundant paths theSpanning Tree Protocol (STP) can be deployed. STP is tasked with preventing loops throughout theswitched network. It does this by temporarily blocking redundant paths between switches; in thiscase, if the primary forwarding path is disrupted, this blocking would be removed allowing traffic tobe passed. This article takes a look at some of the based STP concepts and reviews theconfiguration required to deploy STP on a switch.

Spanning Tree Protocol ConceptsThe first thing to point out is that this article focuses on the original version of STP that is covered inIEEE 802.1D; some of the material about the newer Rapid STP (RSTPIEEE 802.1w) will also bebriefly discussed.

The basic function of STP is to provide a loop free switched network; this is done by creating atopology of all participating STP switches. The best loop free path through the switched network isthen determined from this topology information. The initial step taken by each STP is to elect a rootswitch; the root switch is used as a central point in a switched network to determine the best routethrough the switched network. Initially, all switches act as if they are the root switch and do thisuntil they receive traffic from another superior switch (as determined by switch priority); this isreferred to as a root switch election.

Another thing that must be understood is that multiple root switches can exist in the networkdepending on what STP mode is being used. By default, on Cisco switching equipment, each VLANhas its own STP instance and a root switch is elected for each VLAN; this mode is called PerVLAN Spanning Tree Plus (PVST+). If implementing RSTP, Rapid PVST+ is used.

Spanning Tree Protocol Port RolesOnce the root switch is elected, each of the ports is given a role depending on its place within theSTP topology; the available port roles when using 802.1D spanning tree are shown below:

Root—The port given this role is the selected best path to reach the root switch

Spanning Tree Protocol Concepts and ConfigurationBy Sean Wilkins

Jul 13, 2011

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Designated—The port given this role is selected with the best path to a specific switchedsegment; there is only one designated port per switched segment.

Alternate—The port given this role is selected as a backup to the root port; if the root portshould have a problem, this port would take over the root port role.

Backup—The port given this role is selected as a back to the designated port; if the designatedport should have a problem this port would take over the designated port role.

Once the best path is calculated and each of the ports has been given a role, all ports with thealternate or backup STP roles will be blocked to prevent loops.

Spanning Tree Protocol Interface StatesEach of the ports on a switch that are enabled participates in STP; each of these ports goesthrough a process of interface states before they are allowed to forward traffic. The sequence of802.1D interface states is shown in Figure 1.

Figure 1 Spanning Tree States (802.1D)

As shown in Figure 1, there are five different states that a port can be in, described below:

Blocking State—Ports that are in the blocking state do not forward traffic; they simply listen tothe network to ensure that they should continue to block traffic. Should the state of theswitched network change, the port could transition to listening state. All ports start in blockingstate after initial switch initialization.

Listening State—Ports that are in the listening state do not forward traffic. While in this state,the port will only listen to traffic as they did when in blocking state. This is the first state thatcomes after the blocking state after the port is set to start frame forwarding. The default time inthe listening state is 15 seconds.

Learning State—Ports that are in the learning state do not forward traffic; while in this state theport will listen to traffic and begin to learn addresses from the connected devices on a segment.The default time in the learning state is 15 seconds.

Forwarding State—Ports that are in the forwarding state forward traffic as well as continue tolearn addresses from the segment.

Disabled State—Ports that are in the disabled state do not forward traffic or listen to thenetwork traffic.

When implementing the RSTP, the time that a port takes to transition and the method used totransition has changed. This provides the ability for a switched network to begin forwarding trafficsooner without unneeded delays; these delays are a common complaint about the 802.1D version ofSTP.

Spanning Tree Protocol ConfigurationBy default, STP is enabled on VLAN 1 and all newly created VLAN’s; because of this there are nocommands required to enabled STP on a newly initialized switch. If for some reason an older switchhas STP disabled on a specific VLAN, the commands shown in Table 1 are used to reenable STP.

Table 1: Enabling STP

When initially setting up STP, it is best to determine which of the switches on the network willbecome the root switch. While it is possible for the network to determine this by itself, the electionwill simply come down to a question of who has the lowest MAC address. By default, each switchbegins with a priority of 32768; this priority is then combined with the MAC address of the switch tocreate the bridge ID. During a root switch election, the switch with the lowest bridge ID will beelected the root switch.

The commands to determine the root switch are shown in Table 2.

Step 1 Enter privileged mode. router>enable

Step 2 Enter global configurationmode.

router#configure terminal

Step 3 Enable STP on a VLAN. router(config)#spanningtree vlan vlanid

Step 4 Exit configuration mode. router(config)#end

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Table 2: Root Switch Selection

If the default Spanning Tree mode needs to be changed from the default of PVST+, use thecommands shown in Table 3.

Table 3: Spanning Tree Mode

SummarySTP is one of those protocols that are used by most people without them even knowing that itexists, but without it modern switched networks could not operate. Hopefully the contents of thisarticle have been able to introduce the concepts used by STP to eliminate switch loops and howSTP can be configured to get the best performance out of the network.

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Step 1 Enter privileged mode. router>enable

Step 2 Enter global configurationmode.

router#configure terminal

Step 3 Set the switch to become theroot switch.

This command determines theswitch priority required tomake the switch root andchanges the switch priority tothis number.

router(config)#spanningtree vlan vlanid rootprimary

Step 3 Set the switch to become thesecondary root switch.

This command changes thepriority of the switch to 28672.

router(config)#spanningtree vlan vlanid rootsecondary

Step 3 Set the switch priority, as thedefault switch priority is 32768any value less then this willmake the switch root.

It is recommended that thiscommand not be used in favorof the earlier commands.

router(config)#spanningtree vlan vlanid prioritypriority

Step 4 Exit configuration mode router(config)#end

Step 1 Enter privileged mode. router>enable

Step 2 Enter global configurationmode.

router#configure terminal

Step 3 Configure the Spanning Treemode to use.

router(config)#spanningtree mode pvst | rapidpvst

Step 4 Exit configuration mode. router(config)#end

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So nice lecture thanks from ur teachRiazullah Arab • a year ago

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