STP Operations

Steps to a Loop-Free Topology

Now you know how loops are created and the basics of using spanning tree protocol to prevent them. This topic will take you, step by step, through the operation of STP. Using the STA, STP builds a loop-free topology in a four-step process:

1. Elect the root bridge.
2. Elect the root ports.
3. Elect designated ports.
4. Elect alternate (blocked) ports.

During STA and STP functions, switches use Bridge Protocol Data Units (BPDUs) to share information about themselves and their connections. BPDUs are used to elect the root bridge, root ports, designated ports, and alternate ports. Each BPDU contains a bridge ID (BID) that identifies which switch sent the BPDU. The BID is involved in making many of the STA decisions including root bridge and port roles. As shown in the figure, the BID contains a priority value, the MAC address of the switch, and an extended system ID. The lowest BID value is determined by the combination of these three fields.

The BID includes the Bridge Priority, the Extended System ID, and the MAC Address of the switch.

Bridge Priority

The default priority value for all Cisco switches is the decimal value 32768. The range is 0 to 61440 in increments of 4096. A lower bridge priority is preferable. A bridge priority of 0 takes precedence over all other bridge priorities.

Extended System ID

The extended system ID value is a decimal value added to the bridge priority value in the BID to identify the VLAN for this BPDU.

Early implementations of IEEE 802.1D were designed for networks that did not use VLANs. There was a single common spanning tree across all switches. For this reason, in older switches, the extended system ID was not included in the BPDUs. As VLANs became common for network infrastructure segmentation, 802.1D was enhanced to include support for VLANs, which required that the 12-bit VLAN ID be included in the BPDU frame. VLAN information is included in the BPDU frame through the use of the extended system ID.

The extended system ID allows later implementations of STP, such as Rapid STP (RSTP) to have different root bridges for different sets of VLANs. This can allow for redundant, non-forwarding links in a STP topology for one set of VLANs to be used by a different set of VLANs using a different root bridge.

MAC address

When two switches are configured with the same priority and have the same extended system ID, the switch having the MAC address with the lowest value, expressed in hexadecimal, will have the lower BID.

(1) Elect the Root Bridge

The STA designates a single switch as the root bridge and uses it as the reference point for all path calculations. Switches exchange BPDUs to build the loop-free topology beginning with selecting the root bridge.

An election process determines which switch becomes the root bridge. All switches in the broadcast domain participate in the election process. After a switch boots, it begins to send out BPDU frames every two seconds. These BPDU frames contain the BID of the sending switch and the BID of the root bridge, known as the Root ID.

The switch with the lowest BID will become the root bridge. At first, all switches declare themselves as the root bridge with their own BID set as the Root ID. Eventually, the switches learn through the exchange of BPDUs which switch has the lowest BID and will agree on one root bridge.

In the figure, S1 is elected the root bridge because it has the lowest BID.

Impact of Default BIDs

Because the default BID is 32768, it is possible for two or more switches to have the same priority. In this scenario, where the priorities are the same, the switch with the lowest MAC address will become the root bridge. To ensure that the root bridge decision best meets network requirements, it is recommended that the administrator configure the desired root bridge switch with a lower priority.

In the figure, all switches are configured with the same priority of 32769. Here the MAC address becomes the deciding factor as to which switch becomes the root bridge. The switch with the lowest hexadecimal MAC address value is the preferred root bridge. In this example, S2 has the lowest value for its MAC address and is elected as the root bridge for that spanning tree instance.

Determine the Root Path Cost

When the root bridge has been elected for a given spanning tree instance, the STA starts the process of determining the best paths to the root bridge from all destinations in the broadcast domain. The path information, known as the internal root path cost, is determined by the sum of all the individual port costs along the path from the switch to the root bridge.

When a switch receives the BPDU, it adds the ingress port cost of the segment to determine its internal root path cost.

The default port costs are defined by the speed at which the port operates. The table shows the default port costs suggested by IEEE. Cisco switches by default use the values as defined by the IEEE 802.1D standard, also known as the short path cost, for both STP and RSTP. However, the IEEE standard suggests using the values defined in the IEEE-802.1w, also known as long path cost, when using 10 Gbps links and faster.

Link Speed	STP Cost: IEEE 802.1D-1998	RSTP Cost: IEEE 802.1w-2004
10 Gbps	2	2,000
1 Gbps	4	20,000
100 Mbps	19	200,000
10 Mbps	100	2,000,000

Although switch ports have a default port cost associated with them, the port cost is configurable. The ability to configure individual port costs gives the administrator the flexibility to manually control the spanning tree paths to the root bridge.

(2) Elect the Root Ports

After the root bridge has been determined, the STA algorithm is used to select the root port. Every non-root switch will select one root port. The root port is the port closest to the root bridge in terms of overall cost (best path) to the root bridge. This overall cost is known as the internal root path cost.

The internal root path cost is equal to the sum of all the port costs along the path to the root bridge, as shown in the figure. Paths with the lowest cost become preferred, and all other redundant paths are blocked. In the example, the internal root path cost from S2 to the root bridge S1 over path 1 is 19 (based on the IEEE-specified individual port cost) while the internal root path cost over path 2 is 38. Because path 1 has a lower overall path cost to the root bridge, it is the preferred path and F0/1 becomes the root port on S2.

(3) Elect Designated Ports

The loop prevention part of spanning tree becomes evident during these next two steps. After each switch selects a root port, the switches will then select designated ports.

Every segment between two switches will have one designated port. The designated port is a port on the segment (with two switches) that has the internal root path cost to the root bridge. In other words, the designated port has the best path to receive traffic leading to the root bridge.

What is not a root port or a designated port becomes an alternate or blocked port. The end result is a single path from every switch to the root bridge.

Click each button for an explanation of how STA elects the designated ports.

(4) Elect Alternate (Blocked) Ports

If a port is not a root port or a designated port, then it becomes an alternate (or backup) port. Alternate ports and backup ports are in discarding or blocking state to prevent loops. In the figure, the STA has configured port F0/2 on S3 in the alternate role. Port F0/2 on S3 is in the blocking state and will not forward Ethernet frames. All other inter-switch ports are in forwarding state. This is the loop-prevention part of STP.

The Fa0/2 interface of S3 is not a root port or a designated port, so it becomes an alternate or blocked port.

Elect a Root Port from Multiple Equal-Cost Paths

Root port and designated ports are based on the lowest path cost to the root bridge. But what happens if the switch has multiple equal-cost paths to the root bridge? How does a switch designate a root port?

When a switch has multiple equal-cost paths to the root bridge, the switch will determine a port using the following criteria:

1. Lowest sender BID
2. Lowest sender port priority
3. Lowest sender port ID

Click each criteria for an example and explanation.

STP Timers and Port States

STP convergence requires three timers, as follows:

• Hello Timer -The hello time is the interval between BPDUs. The default is 2 seconds but can be modified to between 1 and 10 seconds.
• Forward Delay Timer -The forward delay is the time that is spent in the listening and learning state. The default is 15 seconds but can be modified to between 4 and 30 seconds.
• Max Age Timer -The max age is the maximum length of time that a switch waits before attempting to change the STP topology. The default is 20 seconds but be modified to between 6 and 40 seconds.

Note: The default times can be changed on the root bridge, which dictates the value of these timers for the STP domain.

STP facilitates the logical loop-free path throughout the broadcast domain. The spanning tree is determined through the information learned by the exchange of the BPDU frames between the interconnected switches. If a switch port transitions directly from the blocking state to the forwarding state without information about the full topology during the transition, the port can temporarily create a data loop. For this reason, STP has five ports states, four of which are operational port states as shown in the figure. The disabled state is considered non-operational.

The details of each port state are shown in the table.

Port State	Description
Blocking	The port is an alternate port and does not participate in frame forwarding. The port receives BPDU frames to determine the location and root ID of the root bridge. BPDU frames also determine which port roles each switch port should assume in the final active STP topology. With a Max Age timer of 20 seconds, a switch port that has not received an expected BPDU from a neighbor switch will go into the blocking state.
Listening	After the blocking state, a port will move to the listening state. The port receives BPDUs to determine the path to the root. The switch port also transmits its own BPDU frames and informs adjacent switches that the switch port is preparing to participate in the active topology.
Learning	A switch port transitions to the learning state after the listening state. During the learning state, the switch port receives and processes BPDUs and prepares to participate in frame forwarding. It also begins to populate the MAC address table. However, in the learning state, user frames are not forwarded to the destination.
Forwarding	In the forwarding state, a switch port is considered part of the active topology. The switch port forwards user traffic and sends and receives BPDU frames.
Disabled	A switch port in the disabled state does not participate in spanning tree and does not forward frames. The disabled state is set when the switch port is administratively disabled.

Operational Details of Each Port State

The table summarizes the operational details of each port state.

Port State	BPDU	MAC Address Table	Forwarding Data Frames
Blocking	Receive only	No update	No
Listening	Receive and send	No update	No
Learning	Receive and send	Updating table	No
Forwarding	Receive and send	Updating table	Yes
Disabled	None sent or received	No update	No

Per-VLAN Spanning Tree

Up until now, we have discussed STP in an environment where there is only one VLAN. However, STP can be configured to operate in an environment with multiple VLANs.

In Per-VLAN Spanning Tree (PVST) versions of STP, there is a root bridge elected for each spanning tree instance. This makes it possible to have different root bridges for different sets of VLANs. STP operates a separate instance of STP for each individual VLAN. If all ports on all switches are members of VLAN 1, then there is only one spanning tree instance.

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