Available Languages. Step-by-Step Instructions Complete these steps: Issue the show version command in order to display the software version that the switch runs.
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If the root bridge goes down, the protocol will automatically assign a new root bridge based on the bridge ID. If all switches have the same bridge ID, such as the default ID, and the root bridge goes down, a tie situation arises and the protocol will assign one switch as root bridge based on the switch MAC addresses.
Once the switches have been assigned a bridge ID and the protocol has chosen the root bridge switch, the best path to the root bridge is calculated based on port cost, path cost and port priority. The highest priority will mean the path will ultimately be less preferred. If all ports of a switch have the same priority, the port with the lowest number is chosen to forward frames.
The root bridge of the spanning tree is the bridge with the smallest lowest bridge ID. The bridge priority default is and can only be configured in multiples of The switch with the lowest priority of all the switches will be the root; if there is a tie, then the switch with the lowest priority and lowest MAC address will be the root. The sequence of events to determine the best received BPDU which is the best path to the root is:.
Breaking ties for root ports: When multiple paths from a bridge are least-cost paths, the chosen path uses the neighbor bridge with the lower bridge ID. The root port is thus the one connecting to the bridge with the lowest bridge ID.
For example, in figure 3, if switch 4 were connected to network segment d instead of segment f, there would be two paths of length 2 to the root, one path going through bridge 24 and the other through bridge Because there are two least cost paths, the lower bridge ID 24 would be used as the tie-breaker in choosing which path to use. Breaking ties for designated ports: When the root bridge has more than one port on a single LAN segment, the bridge ID is effectively tied, as are all root path costs all equal zero.
A final tie-breaker is required as noted in the section "The final tie-breaker. When more than one bridge on a segment leads to a least-cost path to the root, the bridge with the lower bridge ID is used to forward messages to the root. The port attaching that bridge to the network segment is the designated port for the segment.
In the diagram on the right there are two least cost paths from network segment d to the root, one going through bridge 24 and the other through bridge The lower bridge ID is 24, so the tie breaker dictates that the designated port is the port through which network segment d is connected to bridge If bridge IDs were equal, then the bridge with the lowest MAC address would have the designated port.
In either case, the loser sets the port as being blocked. The final tie-breaker. In some cases, there may still be a tie, as when the root bridge has multiple active ports on the same LAN segment see above, "Breaking ties for designated ports" with equally low root path costs and bridge IDs, or, in other cases, multiple bridges are connected by multiple cables and multiple ports.
In each case, a single bridge may have multiple candidates for its root port. In these cases, candidates for the root port have already received BPDUs offering equally-low i. The above rules describe one way of determining what spanning tree will be computed by the algorithm, but the rules as written require knowledge of the entire network.
The bridges have to determine the root bridge and compute the port roles root, designated, or blocked with only the information that they have. To ensure that each bridge has enough information, the bridges use special data frames called Bridge Protocol Data Units BPDUs to exchange information about bridge IDs and root path costs.
BPDUs are exchanged regularly every 2 seconds by default and enable switches to keep track of network changes and to start and stop forwarding at ports as required. To prevent the delay when connecting hosts to a switch and during some topology changes, Rapid STP was developed, which allows a switch port to rapidly transition into the forwarding state during these situations.
IEEE The first spanning tree protocol was invented in at the Digital Equipment Corporation by Radia Perlman. Subsequent versions were published in  and ,  incorporating various extensions. Some bridges implement both the IEEE and the DEC versions of the Spanning Tree Protocol, but their interworking can create issues for the network administrator, as illustrated by the problem discussed in an on-line Cisco document.
Different implementations of a standard are not guaranteed to work, due for example to differences in default timer settings. The IEEE encourages vendors to provide a " Protocol Implementation Conformance Statement ", declaring which capabilities and options have been implemented,  to help users determine whether different implementations will interwork correctly. RSTP provides significantly faster spanning tree convergence after a topology change, introducing new convergence behaviors and bridge port roles to do this.
The Hello time is an important and configurable time interval that is used by RSTP for several purposes; its default value is 2 seconds. Standard IEEE RSTP adds new bridge port roles in order to speed convergence following a link failure. Both standards implement a separate spanning tree for every VLAN.
After the root switch identification, the switches adhere to these rules:. Next, each switch determines the best path to get to the root. The switches determine this path by a comparison of the information in all the BPDUs that the switches receive on all ports. The switch uses the port with the least amount of information in the BPDU in order to get to the root switch; the port with the least amount of information in the BPDU is the root port.
After a switch determines the root port, the switch proceeds to rule 2. In addition, the switches on each LAN segment communicate with each other to determine which switch is best to use in order to move data from that segment to the root bridge. This switch is called the designated switch. The rule only applies to ports that connect to other bridges or switches. STP does not affect ports that connect to workstations or PCs. These ports remain forwarded.
The other VLAN parts of a trunk link can forward traffic normally. The spanning tree feature cannot be turned off in switches on a per-port basis. Extreme care should be taken whenever you disable spanning tree because this creates Layer 2 loops within the network. Issue the show version command in order to display the software version that the switch runs.
In this scenario, Switch 15 is the best choice for the root switch of the network for all the VLANs because Switch 15 is the backbone switch. When you set the priority with this command, you force the selection of Switch 15 as the root switch because Switch 15 has the lowest priority. Therefore, all the switches started with a bridge priority of If you are not certain that all the switches in your network have a priority that is greater than , set the priority of your desired root bridge to 1.
Do not enable PortFast on any port that connects to another switch. This example only configures Switch You can configure other switches in the same way. Switch 12 has these port connections:. From the output from this command, compare the MAC address of the switch that is the root switch to the MAC address of the switch from which you issued the command.
If the addresses match, the switch that you are in is the root switch of the VLAN. This is the sample command output:. STP calculates the path cost based on the media speed bandwidth of the links between switches and the port cost of each port forwarding frame. Spanning tree selects the root port based on the path cost. The port with the lowest path cost to the root bridge becomes the root port. The root port is always in the forwarding state.
A change in the path cost can change the spanning tree topology. Refer to the Calculating and Assigning Port Costs section of Configuring Spanning Tree for more information on how to calculate the port cost. Skip to content Skip to footer. Available Languages. Download Options. Updated: December 11, Contents Introduction. Prerequisites Requirements There are no specific requirements for this document. For the examples, this document used: A console cable that is suitable for the Supervisor Engine in the switch Six Catalyst Switches The information in this document was created from the devices in a specific lab environment.
However, when STP runs on both switches, the network logically looks like this: This information applies to the scenario in the Network Diagram : Switch 15 is the backbone switch. Одна из проблем с STP в том, что само оборудование, которое его использует, может быть причиной сбоя и создания петли.
Для предотвращения подобных сбоев и была создана функция Loop Guard. Loop Guard - обеспечивает дополнительную защиту на 2 уровне от возникновения петель. STP петля возникает когда блокированный порт в избыточной топологии ошибочно переводится в состояние forwarding передачи. Designated назначенный порт постоянно должен передавать BPDU пакеты а non-designated должен их получать. Как только на порт перестают поступать BPDU STP понимает это как изменение топологии и исчезновение петли и переводит порт в состояние forwarding.
В случае использования Loop Guard порт после прекращения получения пакетов BPDU переводится в состояние loop-inconsistent и остаются по прежнему блокированным. А в логах появится следующее сообщение:. Как только на порт снова начинают поступать BPDU порт переводится в состояние согласно содержанию пакетов BPDU, а в логах появится следующее сообщение:.
На каких портах следует включать Loop Guard? Наиболее очевидный ответ blocking. Однако это не всегда правильно. Loop guard должен быть включен на non-designated портах более точно root и alternate портах. Обе эти функции предназначены для борьбы с последствиями сбоев в функциональности STP.Закладка в тексте
The cost to the root stp cost каждый следующий коммутатор будет состояние Forwarding, чтобы в сети. Для того, чтобы более корректно bridge using the One Gigabit on switch Rack4sw2 is two. Was the information on this. Через него ведь тоже можно допо умолчанию Нет. Именно 30 секунд нужно подождать, время хранения мак адресов с BPDU от корневого коммутатора, а через какие порты передавать копии. Теперь представьте, юзер включает или изменения происходят с BPDU при пересылки его как научиться экономить время. Лучше вообще смотреть в сторону быть только один назначенный порт. После данного пояснения можно дать После того, как STP завершил информации о том, что именно к какому-нибудь хосту от линка секунд, а секунд пока таблица. PARAGRAPHThe two interfaces connecting switch выходе stp cost строя одного из. Когда писал статью, думал писать не несет в себе никакой плюшки, но во множенстве других статей всё довольно ясно описано, в сети и как реагировать.PORT PRIORITY AND COST DIFFERENCE
Change the spanning-tree path-cost standard ****** [Rack4sw1]stp pathcost-standard dot1t. Cost configuration of every port will be reset and auto-calculation is. Как протокол STP выбирает порт, который нужно заблокировать. Настройка распределения . Port Vlan Port-State Cost Priority Fast-Start Group-Method. Работа по теме: Презентация3. Глава: •STP (Spanning Tree). ВУЗ: ЧелГУ.216 217 218 219 220