Which statement is true about the spanning tree protocol stp. Bpdus are now used as a keepalive mechanism similar to what routing protocols like ospf or eigrp use. Spanningtree protection uplinkfast and backbone fast are cisco proprietary means to optimize spanningtree. The max age timer controls the maximum length of time interval that a spanning tree protocol stp switch port saves its configuration bridge protocol data. Spanning tree enabled protocol mstp root id priority 4096 address e05f. The default maxage timer is 20 seconds which is used to determine if a self originated bpdu should be considered when determining a loop free topology. The classic stp network recomputation reduces from 50 seconds to 30 seconds. Upon expiration of max age timer, the port will transition to designated role and will relay the bpdus that it receives on its root port. The spanning tree protocol stp is a network protocol that builds a loopfree logical topology for ethernet networks. Mst does it similarly but in addition to messageage in the ist part of the bpdu configured using spanningtree mst maxage that gets incremented, there is also a cist remaining hops field in the mst extension part of the bpdu which is set to 20 configured by spanningtree mst maxhops that gets decremented, and if it drops to 0, the bpdu. Ive been reading up on the spanning tree protocol and understand why its necessary in a network it prevents loops and i understand that. There is some room, however, to adjust the maxage timer.
After configuring backbonefast on cisco switches, we can save 20 seconds by skipping the max age timer. The age timer starts at the message age that is received in that configuration bpdu. Why do we use stp spanning tree protocol or rstp rapid spanning tree protocol when packets are not routed optimally. The ports on a switch with enabled spanning tree protocol stp are in one of the following five port states. Many people struggle with stp so in this video, we try to break it down as much as possible. Configure spanning tree timers sun ethernet fabric. This document descibes spanning tree protocol stp in a wireless environment. Other parameters of the spanning tree protocol ieee 802. Pvst, rapid spanning tree protocolrstp and multi spanning tree protocol mstp. 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.
Stp is a network protocol that builds a logical loopfree topology for ethernet networks. The following example shows how to set stp timers on a device and check. Switches connected in an actual loop topology in todays networks are rare. Spanning tree protocol stp is a big topic on the ccna exam. Max age typically 20 seconds minimally 6 seconds hello time typically 2 seconds forward delay typically 15 seconds minimally 4 seconds rapid spanning tree 802. It is desirable to have redundant links in a switched lan so that a single link failure cannot interrupt normal operation of the network. As the spanning tree protocol timers section mentions, each bpdu includes the hello, forward delay, and max age stp timers.
Stp is a layer 2 link management protocol that provides path redundancy while preventing loops in the network. What is rapid spanning tree protocol sysnettech solutions. Ppt spanning tree protocol powerpoint presentation. In addition to the timers that the spanning tree protocol timers section describes, ieee also defines these parameters that relate to stp.
If this age timer reaches max age before another bpdu is received that refreshes the timer, the information is aged out for that port. Stp timers hello timer, forward delay timer, max age timer and their. Understanding and tuning spanning tree protocol timers cisco. If the device sending this best bpdu fails, it may take 20 seconds before a switch transitions the connected port to listening.
Spanning tree protocol ccna exploration semester 3 chapter 5 topics redundancy in a converged network how spanning tree protocol stp eliminates layer 2 loops the. Internally, stp assigns to each bridge or switch port a specific role. Then, we will see how the spanning tree protocol creates a loopfree logical topology in a network with physical loops, so that a broadcast storm cannot occur. The portbased vlan that contains this spanning tree instance of stp. When edge ports and pointtopoint links are properly configured, rstp network convergence does not require forwarddelay and maxage timers. Radia perlman, distinguished engineer at sun microsystems. However, ciscos curricula treats this protocol in much more detail than its newer and faster version rapid spanning tree protocol rapidstp. As the spanning tree protocol timers section states, cisco switches and probably all switches add 1 sec to the message age before the switches forward a bpdu. I am new to networking so this probably a naive question. In a previous lesson i explained the differences between classic and rapid spanningtree and how rapid spanningtree works. Root id the id assigned by stp to the root bridge for this spanning tree. Autoedge detection converts ports into edge ports when they do not receive a new bpdu before the current bpdu expires, as measured by the maxage timer. Instead, it is recommended that you set the network diameter so that the spanning tree protocol automatically adjusts these timers in accordance. Generally, you are not advised to directly change these timers.
Maximum time taken for a switch to save the bpdu configuration. Download pdf stp spanning tree protocol step by step configuration tutorial when sw3 start receiving inferior bpdus at its interface from sw2, switch3 would ignore the inferior information until the bpdus stored with the blocked port expires max age timer 20 seconds. For a layer 2 network to function properly, only one active path can exist between any two stations. Cisco spanning tree protocol guide stp examples and.
The slowness of the stp protocol is due to the max age timer 20 seconds and forward delay timer 15 seconds periods, and you can change these values if you wish. The maximum age timer should be longer than the configured hello timer. Understanding spanning tree protocol timers huawei. Spanning tree protocol stp ensures a loopfree topology in a local area network lan made up of switches. Spanning tree protocol in layer 2layer 3 environments. Spanning tree protocolcisco ccna icnd2 certification guide.
Configure the spanning tree timer 11 in this example. Ccnp switch chapter 4 exam answers version 7 score 100%. The max age timer controls the maximum length of time interval that a spanning tree protocol stp switch port saves its configuration bridge protocol data unit bpdu information. Bridge program users guide this guide accompanies the ibm tokenring network bridge program, version 2. The max age timer and the stable bpdu information is stored on a per port basis and not on a per bridge basis. 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 was developed to prevent the broadcast storms caused by. The switch uses the maxage timer to age out entries in the mac address table. Rstp draft3 will continue to be supported on ruckus devices for backward compatibility. Spanning tree protocol overview the plugandplay spanning tree protocol stp, a layer 2 protocol that ensures a loopfree topology in bridged lans, has been around for decades. The classic spanningtree uses a max age timer 20 seconds for bpdus before they are discarded. Sonos, cisco, stp, mst, and spanning trees sonos community. Stp timers hello timer, forward delay timer, max age timer. Perlman devised a method by which bridges can obtain layer 2 routing utopia.
Hello time for root bridge to transmit hello bpdus. But stp can save you from consequences of a potential human error, such as port channel misconfiguration. Root cost the cumulative cost from this bridge to the root bridge. Notice that the instant it heard the root on a different port it went to listening mode. This time is 20 sec by default, but you can tune the time to be between 6. The spanning tree protocol run my experiment on geni. A device determines that a received configuration bpdu times out when the max age expires. There are also a couple of options to protect spanningtree that you should be aware of root guard. The switch recomputes the spanning tree topology if it does not receive another bpdu before the maxage timer expires. Spanning tree protocol stp helps to prevent network topology loops and is highly recommended for use in any network. If you have not configured portbased vlans on this device, all stp information is for vlan 1. An ieee bridge is not concerned about the local configuration of the timers value. Cisco catalyst 5000 series switches, use bdpus bridge protocol data units to determine the spanning tree topology.
A switch does not enter any of these port states immediately except the blocking state. A free powerpoint ppt presentation displayed as a flash slide show on. Spanningtree protocol includes several timers that control various aspects regarding how frequently bpdu packets are sent and. Bpdu protection for spanningtree protocols techlibrary. The max age timer indicates how long a switch will retain bpdu. Configuring rapid spanning tree protocol rstp junos. Setting rstp timers cloudengine 8800, 7800, 6800, and. The spanningtree portfast auto command enables autoedge detection on the configuration mode interface, superseding the spanningtree portfast command. Spanning tree protocol timing protocol is timer driven too short timers can give loops and instabilities too long timers can give long convergence times until network reaches a stable spanning tree configuration proto id 2 root id 8 ver 1 bpdu type 1 flags 1 root path cost 4 bridge id 8 port id 2 msg age 2 max age 2 hello time 2. If the switching device does not receive any bpdus within an interval of hello time x 3 x timer factor, the device recalculates the spanning tree.
Core layer switches shorten the expiration of the max age timer when an indirect link fails. This section describes how spanningtree features work. Switches running stp will build a map or topology of the entire switching network. When the spanning tree protocol stp is enabled, every switch in the network starts in the blocking state and later changes to the listening and. Spanning treeenabled switches go through a root election process based on bridge p rotocol. Rstp linux rapid spanning tree protocol rstp implementation for linux. The basic function of stp is to prevent bridge loops and the broadcast radiation that results from them. Rstp provides significantly faster spanning tree convergence after a topology change, introducing new convergence behaviors and bridge port roles to do this. Stp timersmax age timer max age is the time that a bridge stores a bpdu before discarding it. If you havent seen it before, i would recommend to look at it first before diving in the configuration having said that, lets look at the configuration. In a rstp or mst enviroment, bpdus are used as keepalives in pointtopoint links.
Understanding and tuning spanning tree protocol timers. Which set of statements about spanning tree protocol default timers is true. Spanning tree protocol explained step by step youtube. If a switch misses three bpdus from a neighbor switch it will assume connectivity to this switch has been. Stp spanning tree protocol step by step configuration. Spanningtree protocols user guide techlibrary juniper. Spanning tree enabled protocol ieee root id priority 0 address c200. Understanding stp max age the legacy spanning tree protocol stp which was standardized by ieee in 802. For additional information about source routing and the spanningtree protocol, we recommend the following reference material the ibm titles are available from an ibm representative or ibm branch office. Spanningtree convergence issues have been a challenge for network administrators to. We will also see how the spanning tree protocol reacts when the topology changes. By default, the max age timer is 2000 centiseconds 20 seconds. Correct so if three consecutives bpdus are lost, we can assume the path between the two switches is down. Earlier implementation by ruckus of rapid spanning tree protocol rstp, which was 802.
C waits for 20 sec before deciding which is max age timer and after 20 sec it start converge the topology. What does it mean that if the bpdu info is equal to or better. With each network change, the stp algorithm is run on all switches that have a root port. Because of the possibility of these delays, preventing the spanningtree topology from converging until all switches have had time to receive accurate information is important. Stp uses a tree algorithm sta to prevent loops, resulting in a stable network topology. The spanning tree protocol stp is a network protocol that builds a loopfree logical topology. Each port saves a copy of the best bpdu it has received.
497 231 640 659 1145 1343 315 1069 626 670 1326 1219 1065 878 615 755 1142 48 347 1097 1029 955 599 122 932 294 85 1387 188 1348 1089 1126 1346