Spanning Tree Protocol (STP)
STP Topology
Redundancy increases the availability of the network topology by protecting the network from a single point of failure, such as a failed network cable or switch.
STP ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop. A port is considered blocked when network traffic is prevented from entering or leaving that port.
STP Algorithm
STP uses the Spanning Tree Algorithm (STA) to determine which switch ports on a network need to be configured for blocking to prevent loops from occurring.
All switches participating in STP exchange BPDU frames to determine which switch has the lowest bridge ID (BID) on the network. The switch with the lowest BID automatically becomes the root bridge for the STA calculations.
The BPDU is the message frame exchanged by switches for STP. Each BPDU contains a BID that identifies the switch that sent the BPDU. The BID contains a priority value, the MAC address of the sending switch, and an optional extended system ID. The lowest BID value is determined by the combination of these three fields.
Root ports – Switch ports closest to the root bridge. In the example, the root port on switch S2 is F0/1 configured for the trunk link between switch S2 and switch S1. The root port on switch S3 is F0/1, configured for the trunk link between switch S3 and switch S1.
Designated ports – All non-root ports that are still permitted to forward traffic on the network. In the example, switch ports F0/1 and F0/2 on switch S1 are designated ports. Switch S2 also has its port F0/2 configured as a designated port.
Non-designated ports – All ports configured to be in a blocking state to prevent loops. In the example, the STA configured port F0/2 on switch S3 in the non-designated role. Port F0/2 on switch S3 is in the blocking state.
BID
BID is made up of a priority value, an extended system ID, and the MAC address of the switch.
Best Paths to the Root Bridge
Path cost is the sum of all the port costs along the path to the root bridge.
The BPDU Fields
The BPDU frame contains 12 distinct fields that are used to convey path and priority information that STP uses to determine the root bridge and paths to the root bridge.
Root Port
The root port exists on non-root bridges and is the switch port with the best path to the root bridge. Root ports forward traffic toward the root bridge.
Designated Port
The designated port exists on root and non-root bridges. For root bridges, all switch ports are designated ports. For non-root bridges, a designated port is the switch port that receives and forwards frames toward the root bridge as needed. Only one designated port is allowed per segment.
Non-designated Port
The non-designated port is a switch port that is blocked, so it is not forwarding data frames and not populating the MAC address table with source addresses.
Port States
The spanning tree is determined immediately after a switch is finished booting up. If a switch port were to transition directly from the blocking to the forwarding state, the port could temporarily create a data loop if the switch was not aware of all topology information at the time.
Blocking – The port is a non-designated port and does not participate in frame forwarding. The port receives BPDU frames to determine the location and root ID of the root bridge switch and what port roles each switch port should assume in the final active STP topology.
Listening – STP has determined that the port can participate in frame forwarding according to the BPDU frames that the switch has received thus far. At this point, the switch port is not only receiving BPDU frames, it is also transmitting its own BPDU frames and informing adjacent switches that the switch port is preparing to participate in the active topology.
Learning – The port prepares to participate in frame forwarding and begins to populate the MAC address table.
Forwarding – The port is considered part of the active topology and forwards frames and also sends and receives BPDU frames.
Disabled – The Layer 2 port does not participate in spanning tree and does not forward frames. The disabled state is set when the switch port is administratively disabled.
BPDU Timers
• Hello time
• Forward delay
• Maximum age
STP Convergence Steps
Three Steps
Step 1: Elect a Root Bridge
Step 2: Elect the Root Ports
Step 3: Elect the Designated and Non-Designated ports
Step 1. Electing a Root Bridge
The root bridge is the basis for all spanning-tree path cost calculations and ultimately leads to the assignment of the different port roles used to prevent loops from occurring.
A root bridge election is triggered after a switch has finished booting up, or when a path failure has been detected on a network.
Step 2: Elect the Root Ports
The root port is the switch port with the lowest path cost to the root bridge. Normally path cost alone determines which switch port becomes the root port.
Step 3. Electing Designated Ports and Non-Designated Ports
Each segment in a switched network can have only one designated port. When two non-root port switch ports are connected on the same LAN segment, a competition for port roles occurs. The two switches exchange BPDU frames to sort out which switch port is designated and which one is non-designated.
Cisco Proprietary
Per-VLAN spanning tree protocol (PVST) – Maintains a spanning-tree instance for each VLAN configured in the network. It uses the Cisco proprietary ISL trunking protocol that allows a VLAN trunk to be forwarding for some VLANs while blocking for other VLANs.
Per-VLAN spanning tree protocol plus (PVST+) – Cisco developed PVST+ to provide support for IEEE 802.1Q trunking. PVST+ provides the same functionality as PVST, including the Cisco proprietary STP extensions. PVST+ is not supported on non-Cisco devices.
Rapid per-VLAN spanning tree protocol (rapid PVST+) – Based on the IEEE 802.1w standard and has a faster convergence than STP (standard 802.1D). Rapid PVST+ includes Cisco-proprietary extensions such as BackboneFast, UplinkFast, and PortFast.
IEEE Standards
Rapid spanning tree protocol (RSTP) – First introduced in 1982 as an evolution of STP (802.1D standard). It provides faster spanning-tree convergence after a topology change. RSTP implements the Cisco-proprietary STP extensions, BackboneFast, UplinkFast, and PortFast, into the public standard.
Multiple STP (MSTP) – Enables multiple VLANs to be mapped to the same spanning-tree instance, reducing the number of instances needed to support a large number of VLANs. MSTP was inspired by the Cisco-proprietary Multiple Instances STP (MISTP) and is an evolution of STP and RSTP
What is RSTP?
RSTP (IEEE 802.1w) is an evolution of the 802.1D standard. The 802.1w STP terminology remains primarily the same as the IEEE 802.1D STP terminology. Most parameters have been left unchanged, so users familiar with STP can rapidly configure the new protocol.
