STP was originally developed to operate in a bridged environment, basically supporting a single LAN (or one VLAN). Implementing STP into a switched environment has required additional consideration and modification to support multiple VLANs.
Common Spanning Tree
The IEEE 802.1Q standard specifies how VLANs are to be trunked between switches. It also specifies only a single instance of STP that encompasses all VLANs. This instance is referred to as the Common Spanning Tree (CST). All CST BPDUs are transmitted over trunk links using the native VLAN with untagged frames.
Having a single STP for many VLANs simplifies switch configuration and reduces switch CPU load during STP calculations. However, having only one STP instance can cause limitations, too. Redundant links between switches will be blocked with no capability for load balancing. Conditions also can occur that would cause CST to mistakenly enable forwarding on a link that does not carry a specific VLAN, whereas other links would be blocked.
Per-VLAN Spanning Tree
Cisco has a proprietary version of STP that offers more flexibility than the CST version.
Per-VLAN Spanning Tree (PVST) operates a separate instance of STP for each individual VLAN. This allows the STP on each VLAN to be configured independently, offering better performance and tuning for specific conditions. Multiple spanning trees also make load balancing possible over redundant links when the links are assigned to different VLANs. One link might forward one set of VLANs, while another redundant link might forward a different set.
Because of its proprietary nature, PVST requires the use of Cisco Inter-Switch Link (ISL) trunking encapsulation between switches. In networks where PVST and CST coexist, interoperability problems occur. Each requires a different trunking method, so BPDUs are never exchanged between STP types.
Per-VLAN Spanning Tree Plus
Cisco has a second proprietary version of STP that allows devices to interoperate with both PVST and CST. Per-VLAN Spanning Tree Plus (PVST+) effectively supports three groups of STP operating in the same campus network:
■ Catalyst switches running PVST
■ Catalyst switches running PVST+
■ Switches running CST over 802.1Q
To do this, PVST+ acts as a translator between groups of CST switches and groups of PVST switches. PVST+ can communicate directly with PVST by using ISL trunks. To communicate with CST, however, PVST+ exchanges BPDUs with CST as untagged frames over the native VLAN. BPDUs from other instances of STP (other VLANs) are propagated across the CST portions of the network by tunneling. PVST+ sends these BPDUs by using a unique multicast address so that the CST switches forward them on to downstream neighbors without interpreting them first. Eventually, the tunneled BPDUs reach other PVST+ switches where they are understood.
STP port state
STP Timers
Two types of BPDU exist:
■ Configuration BPDU, used for spanning-tree computation
■ Topology Change Notification (TCN) BPDU, used to announce changes in the net-
work topology
Topology Changes
To announce a change in the active network topology, switches send a TCN BPDU.
A topology change occurs when a switch either moves a port into the Forwarding state or moves a port from the Forwarding or Learning states into the Blocking state. In other words, a port on an active switch comes up or goes down. The switch sends a TCN BPDU out its root port so that, ultimately, the root bridge receives news of the topology change. Notice that the TCN BPDU carries no data about the change but informs recipients only that a change has occurred. Also notice that the switch will not send TCN BPDUs if the port has been configured with PortFast enabled.
Bridge ID
STP path cost
Sequence of four criteria that STP uses to choose a path:
1. Lowest bridge ID
2. Lowest root path cost
3. Lowest sender bridge ID
4. Lowest sender port ID
Tuning the Port ID
As an example, you can use the following command sequence to change the port priority of Gigabit Ethernet 3/16 from 128 (the default) to 64 for VLANs 10 and 100:
Switch(config)# interface gigabitethernet 3/16
Switch(config-if)# spanning-tree vlan 10,100 port-priority 64
Tuning the Root Path Cost
Switch (config-if)# spanning-tree [vlan vlan-id] cost cost
PortFast
Catalyst switches offer the PortFast feature, which shortens the Listening and Learning states to a negligible amount of time. When a workstation link comes up, the switch immediately moves the PortFast port into the Forwarding state. Spanning-tree loop detection is still in operation, however, and the port moves into the Blocking state if a loop is ever detected on the port.
UplinkFast: Access-Layer Uplinks
Consider an access-layer switch that has redundant uplink connections to two distribution-layer switches. Normally, one uplink would be in the Forwarding state and the other would be in the Blocking state. If the primary uplink went down, up to 50 seconds could elapse before the redundant uplink could be used.
The UplinkFast feature on Catalyst switches enables leaf-node switches or switches at the ends of the spanning-tree branches to have a functioning root port while keeping one or more redundant or potential root ports in Blocking mode. When the primary root port uplink fails, another blocked uplink immediately can be brought up for use.
When UplinkFast is enabled, it is enabled for the entire switch and all VLANs. UplinkFast works by keeping track of possible paths to the root bridge. Therefore, the command is not allowed on the root bridge switch. UplinkFast also makes some modifications to the local switch to ensure that it does not become the root bridge and that the switch is not used as a transit switch to get to the root bridge. In other words, the goal is to keep UplinkFast limited to leaf-node switches that are farthest from the root.
После включения UplinkFast на коммутаторе:
- приоритет увеличивается до 49152
- стоимость портов устанавливается равной 3000
- отслеживаются альтернативные корневые порты, на которых были получены сообщения hello от корневого коммутатора.
Если основной RP выходит из строя, то коммутатор сразу переключается на запасной и переводит его в состояние forward.
Кроме того, UplinkFast позволяет коммутаторам обновить записи в таблицах коммутации, без использования TCN. Вместо TCN коммутатор находит MAC-адреса всех локальных устройств и отправляет один multicast фрейм с каждым MAC-адресом в поле отправитель. Удаляются также остальные записи в таблицы коммутации самого коммутатора.
BackboneFast
Позволяет быстрее найти альтернативный путь, после изменения топологии. Для того чтобы функция работала, необходимо включить её на всех коммутаторах в сети.
sw(config)# spanning-tree backbonefast
Where PortFast and UplinkFast enable immediate transitions, BackboneFast can reduce the maximum convergence delay only from 50 to 30
seconds.
Permissible combinations on a switch port:
Loop guard and UDLD
Root guard and UDLD
Not permissible on a switch port:
Root guard and Loop guard
Root guard and BPDU guard
