The spanning tree protocol stp is known to have stability problems and poor convergence intervals. The ports in vlans on which stp is disabled are excluded from the single spanning tree. Root port the one switch port on each switch that has the best root path cost to the root. Highavailability networks with spanning tree and rapid. Rfc 7727 spanning tree protocol stp application of the. Spanning tree protocol stp stp often accounts for more than 50 % of the configuration, troubleshooting, and maintenance headaches in real world campus networks especially if they are poorly designed. Bridges will perform the spanning tree algorithm when they are first connected to the network or whenever there is a topology change. The spanningtree protocol uses this information to elect the root switch and root port for the switched network, as well as the root port and designated port for each switched segment. This means that if redundant physical links exist in separate vlans, spanning tree will block all but one of those links. Basic spanning tree protocol spanning tree enabled protocol ieee root id priority 32769 address 0019. Redundant links are found when the switch receives its own bpdu. As the name suggests, it creates a spanning tree within a mesh network of connected layer2 bridges typically ethernet switches, and disables those links that are not part of the spanning tree, leaving a single active path between any two network nodes. Stp overview stp is a layer 2 link management protocol that prov ides path redundancy while preventing loops in the network.
It is quite useful and should be enabled on the switch interfaces. Several things can be learned from thinking about the experience and how to avoid it. There is no way to kill a packet that is stuck in layer 2 loop. In discussions about stp, the term root is used to describe two concepts. Stp feature overview and configuration guide allied telesis. Abcs of spanning tree protocol contemporary controls. Spanning tree protocol and failure domains netcraftsmen. So thats mostly why i got curious about spanning tree protocol, because i primarily use unmanaged switches and i wanted to see if it was a feature i can take advantage of. Spanningtreeinstanceinterfaceshavepriorities36 whatisspanningtreeinstanceinterfacecost.
A redundant link is an additional link between two switches. Spanning tree protocol stp is a layer 2 network protocol used to prevent looping within a network topology. The basic function of stp is to prevent bridge loops and ensuing broadcast radiation. Below is my version generalizing many standard spanning tree algorithms, including depthfirst search, bredthfirst search, minimumweight spanning tree, and shortest path tree also called singlesource shortest path. Furthermore, the administrator can define alternate paths within a spanning tree. Spanning tree also allows a network design to include spare redundant links to provide automatic backup paths if an active link fails. When reading the poem it helps to know that in math terms, a network can be represented as a type of graph called a mesh, and that the goal of the spanning tree protocol is to turn any given network mesh into a tree structure with no loops. Spanning tree protocol algorithm elects root bridge considering the least root id. Portbased vlan affects all ports within the specified portbased vlan stp is a layer 2 protocol. First, it prevents problems caused by loops on a network. To view the information about the stp operation, you can use the show spanning tree command from the privilegedexec mode.
Rootthe port given this role is the selected best path to reach the root switch. Stp works in a broadcast domain therefore each vlan has its own spanning tree. This document assumes that the reader is familiar with rapid stp rstp 802. Introduction to spanning tree protocol contemporary controls. Understanding spanning tree protocols on a qfabric system. Understanding issues related to intervlan bridging pdf. Several protocols and variants exist targeting the replacement of stp variants, most of them proprietary and with limited scope of operation. This table shows the support for mst in various catalyst switches. Once the root switch is elected, each of the ports is given a role depending on its place within the stp topology. Stp allows bridges and switches to communicate with each other so they can create a loop free topology. Spanning tree protocol basics the ieee defines the spanning tree protocol that provides a loop free redundant network topology by placing certain ports in the blocking state.
The inventor of the spanning tree protocol, radia perlman, wrote a poem to describe how it works. To create a loopfree tree, bridges in the network exchange bpdus, and execute the spanning tree protocol as follows. Bpdus enabling a stable spanningtree protocol topology. All switch and bridge ports are in the forwarding state. Whenever two or more switches are connected with each other for redundancy purpose loop can occur. Thats a worthwhile thing to mentionspanning trees job is not to provide redundancy, although it can be used that way, thats not its job, never was, nor even a remote care of the designers. On each bridge, select a root port port with the least cost path to the root bridge. Perlman devised a method by which bridges can obtain layer 2 routing utopia. Cisco ccnp switch spanning tree protocol stp cisco ccnp switch rapid spanning tree protocolrstp root bridge in rstp is elected just like 802. The concept of the spanning tree protocol was devised to address broadcast storming. Overview of spanningtree protocols techlibrary juniper. Understanding spanning tree protocol this section describes how spanningtree features work.
A switch currently has only one vlan configured and is running a single instance of rapid spanning tree. Presentation aqseamanmergedspanningtreeprotocols1108. Spanning tree protocol problems and related design. The spanning tree protocol stp is a network protocol that builds a loopfree logical topology. Thats a worthwhile thing to mention spanning tree s job is not to provide redundancy, although it can be used that way, thats not its job, never was, nor even a remote care of the designers. Even more confusing some whitepapers on ciscos website will define the. Chapter 8 configuring spanning tree protocol stp and. Spanning tree protocol linnbenton community college. Mst was developed to reduce the number of spanning tree instances to reasonable value thus reducing cpu cycles needed to calculate stp for each vlan separately. Information technology telecommunications and information exchange between systems local and metropolitan area networks common specifications part 3.
Fortunately, spanning tree protocol stp can allow you to have redundant links while having a loopfree topology, thus preventing the potential for a broadcast storm. Stp will identify if there are any loops, and then disable or block. While i have posted a couple of blogs regarding spanningtree, the layer 2 loop prevention mechanism that runs on cisco switches, i wanted to post one with a few more switches and a bit more detail on the stepbystep process. The spanning tree algorithm, chooses a single switch as the root bridge and. Introduccion y configuracion del spanning tree protocol stp. The spanning tree protocol stp is a network protocol that ensures a loopfree topology for any bridged ethernet local area network.
Spanning tree protocol stp spanning tree protocol stp was developed to prevent the broadcast storms caused by switching loops. The stp state of a portbased vlan containing these other types of vlans determines the stp state for all the layer 2 broadcasts within the portbased vlan. Stp spanning tree protocol explained with examples. Chapter 8 configuring spanning tree protocol stp and advanced stp features the spanning tree protocol stp eliminates layer 2 loops in networks, by selectively blocking some ports and allowing other ports to forward traffic, based on global bridge and local port parameters you can configure. Sep 25, 2014 this series of tutorial videos on the spanning tree protocol stp dymsytifies and explains some of things that ccna candidates will need to know to pass certification exams. Understanding spanningtree protocol topology changes cisco. Concepts description of the technology stp operation task stepbystep instructions verify troubleshoot stp path cost automatically changes when a port speedduplex is changed troubleshoot commands command summary related information introduction spanning tree protocol stp is a layer 2 protocol that runs on bridges and switches. Even though i know it is not in most cases for me, im glad to have understood the concept better so that i can design smarter and more efficient networks in the future. Example of a bridged network with a loop, and the minimum spanning tree with the loop removed. An stp carrier vlan defines the scope of the stp domain. When too many computers try to send at the same time, overall network performance is affected and can bring all traffic to a near.
Understanding and configuring spanning tree protocol stp. Other parameters of the spanning tree protocol ieee 802. Coworker pete welcher recently helped a customer whose network experienced a spanning tree loop i. Spanning tree protocol stp was developed to prevent the broadcast. Most people will be looking for information on spanning tree, without realizing that in most network rstp is the default version of spanning tree that is used.
Spanning tree protocol, rapid stp port costs and port states are an essential part of the stp algorithm that affect how stp decides to forward or block a port leading to the root bridge. When you monitor spanningtree protocol stp operations, you may be concerned when you see topology change counters that increment in. Both mstp and rspt should be merged into this article as the concept are very similar, and are evolutions on the basic stp protocol. A dependable alternative to the spanning tree protocol. Every switch must implement spanning tree because it is the only way to stop closed loops on switching. Which statement describes a spanningtree network that has converged. Spanning tree protocol, rapid stp port costs port states. The spanning tree algorithm itself is defined by the ieee standard 802. A layer 3 switch is a switch will routing capabilities, but primarily is a switch. Switches running stp will build a map or topology of the entire switching network.
Multiple spanning tree mst is an ieee standard inspired from the cisco proprietary multiple instances spanning tree protocol mistp implementation. The spanning tree protocol is a link management protocol that is designed to support redundant links while at the same time preventing switching loops in the network. For more information, refer to spanning tree protocol. Switch a is performing carrier sense and collision detection, and switch b is not. The person behind the invention of spanning tree protocol is radia perlman. In this document, provider edge pe devices in a redundancy group rg running iccp are used to offer multihomed connectivity to spanning tree protocol stp networks to improve availability of the stp networks. Radia perlman, distinguished engineer at sun microsystems.
Stp was created to avoid the problems that arise when computers compete for the ability to use the shared telecommunications path on a local area network. Stp layer 2 attack manipulating spanning tree protocol. If you enable single stp sstp on the device, the ports in all vlans on which stp is enabled become members of a single spanning tree. Avoids loops by reducing network connectivity reacts to changes in the network topology either good news or bad news is completely transparent to the user stp implements fault tolerance at data link level. The output of this command can be divided into three subsets. Just like every coin has two sides, a redundant link, along with several advantages, has some disadvantages. The node with the smallest id is selected the root bridge 2. Stp spanning tree protocol automatically removes layer 2 switching loops by shutting down the redundant links.
Rush jobs carry a higher risk of problems than wellplanned tasks. Stp uses spanning tree algorithm sta to create a topology database of a network and then search out and disable redundant links. After all switches agree on the root, the following port roles are determined. Rstp is extremely proactive and very quick, and therefore it absolutely needs the 802. With its mechanism of having redundant links, it also helps avoiding the chances of mac address instability. For more information on rstp and mst, refer to these documents. Further, stp can run at multiple link speeds simultaneously. Jul 06, 2015 spanning tree protocol is a layer 2 protocol used to prevent switching loops and the broadcast storm that are resulted when switches are interconnected via multiple paths. We start with the graph where the vertices are the cells and the edges represent the neighbors we can move to in the maze. In addition to the timers that the spanning tree protocol timers section describes, ieee also defines these parameters that relate to stp. Spanning tree also allows a network design to include backup links providing fault tolerance if an active link fails as the name suggests, stp creates a spanning tree that. Rstp speeds the recalculation of the spanning tree when the layer 2 network topology changes.
For an ethernet network to function properly, only one active path can exist between two stations. Bridges designate ports over which they will or will not forward frames by removing ports, extended lan is reduced to a tree 5 spanning tree algorithm root of the spanning tree is elected first the bridge with the lowest identifier. The basic function of stp is to prevent bridge loops and the broadcast radiation that results from them. One of the most confusing things to understand when learning about the switched part of network is the spanning tree protocol stp 802. Understanding spanningtree protocol c1 appendix c understanding spanningtree protocol spanningtree protocol is a link management protocol that provides path redundancy while preventing undesirable loops in the network. The spanning tree protocol stp addresses the need for redundancy while providing mechanisms for loop detection and prevention. In this article, we will learn how stp prevents loops, and how it handles the layer 2 convergence. The interchassis communication protocol iccp supports an inter chassis redundancy mechanism that is used to support high network availability. Aug 04, 2011 spanning tree protocol stp is standardized as ieee 802. Switch b may unblock its port to switch c, thereby creating a loop. Spanning tree protocol configuring spanning tree root bridge.
Second, when redundant loops are planned on a network, stp deals with remediation of network. Lecture notes on spanning trees carnegie mellon school. Thus, you cannot enable or disable stp for individual protocol vlans or for ip subnet vlans. But, too many of them can have an impact on network performances. Follow the steps as we proceed from a blank scenario to one in which all the switches, mac addresses, ports, speeds, and costs are included. Lans in a network are pointtopoint, rstp timers define worstcase. Complex protocol that is generally poorly understood. The carrier vlan must include all the ports that will be protected by the stp domain regardless of their vlan membership, and if configured, the 802. Bridge loops are network loops created by multiple active station paths. Which action will create a second rapid spanning tree instance. Digital equipment corporation created a protocol called spanning tree protocol stp to prevent broadcast storms and network loops at layer 2. Hu china telecom january 2016 spanning tree protocol stp application of the interchassis communication protocol iccp abstract the interchassis communication protocol iccp supports an inter chassis redundancy mechanism that is used to support high. Because stp is a standardsbased solution, it allows interconnection of multiple vendors equipment. Dec 22, 2012 spanning tree protocol stp stp often accounts for more than 50 % of the configuration, troubleshooting, and maintenance headaches in real world campus networks especially if they are poorly designed.
Spanning tree protocol stp is a link management protocol preventing media access control mac bridge loops and broadcast delays on any ethernet local area network lan. The following image shows how the stp changes a physically looped topology into a virtually looped free topology. Rstp is an ieee standard that redefines stp port roles, states, and bpdus. Figure c1 shows how bpdus enable a spanningtree protocol topology. The spanning tree protocol is used to stop switching loops in a layer 2 switched network with redundant paths. Spanning tree protocol overview embed a tree that provides a single unique default path to each destination.
The switch sends bpdus bridge protocol data units out every active port once every 2 seconds so as to find a redundant link. The spanning tree protocol is primarily known today by its three most popular variants. The spanning tree protocol stp is a network protocol that builds a loopfree logical topology for ethernet networks. Mar 12, 2018 spanning tree protocol stp is a link management protocol preventing media access control mac bridge loops and broadcast delays on any ethernet local area network lan. Spanning treeinstanceinterfaceshavepriorities36 whatisspanningtreeinstanceinterfacecost. Spanning tree protocol this document descibes spanning tree protocol stp in a wireless environment. Spanning tree protocol stp was developed before switches were created in order to deal with an issue that occurred with networks that were implementing network bridges. In the graph, all potential neighbors are connected.
Spanning tree protocol the spanning tree protocol stp is an osi layer 2 protocol that is required to create a loop free network topology. When a bridge hears a configuration message, a special type of bpdu bridge protocol data unit, it will begin its disruptive spanning tree algorithm. Stp is a data link layer protocol standardized by the institute of electrical and electronics. A redundant link is usually created for backup purposes. Spanning tree protocol timing protocol is timer driven too short timers can give loops and instabilities too long timers can give long convergence. Spanning tree protocol used by switches to turn a redundant topology into a spanning tree disables unwanted links by blocking ports stp defined by ieee 802.
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