The Spanning Tree Protocol (STP), is defined in the IEEE Standard 802.1D. As the name suggests, it creates a spanning tree within a mesh network of connected layer-2 bridges (typically Ethernet switches), and disables those links that are not part of the tree, leaving a single active path between any two network nodes.
The spanning tree that the bridges compute using the Spanning Tree Protocol can be determined using the following rules. The example network at the right, below, will be used to illustrate the rules.
Elect a root bridge. The root bridge of the spanning tree is the bridge with the smallest bridge ID. Each bridge has a unique identifier (ID) and a configurable priority number; the bridge ID contains both numbers. To compare two bridge IDs, the priority is compared first. If two bridges have equal priority, then the MAC addresses are compared. For example, if switches A (MAC=0000.0000.1111) and B (MAC=0000.0000.2222) both have a priority of 10, then switch A will be elected as the root bridge. If the network administrator would like switch B to become the root bridge, he must set its priority to be less than 10. (The default priority of Cisco switches is 32768 ).
Determine the least cost paths to the root bridge. The computed spanning tree has the property that messages from any connected device to the root bridge traverse a least cost path, i.e., a path from the device to the root that has minimum cost among all paths from the device to the root. The cost of traversing a single network segment is configurable; the cost of traversing a path is the sum of the costs of the segments on the path. Different technologies have different default costs for network segments. Also, an administrator can configure the cost of traversing a particular network segment.
The property that messages always traverse least-cost paths to the root is guaranteed by the following two rules.
Least cost path from each bridge. After the root bridge has been chosen, each bridge determines the cost of each possible path from itself to the root. From these, it picks the one with the smallest cost (the least-cost path). The port connecting to that path becomes the root port of the bridge.
Least cost path from each network segment. The bridges on a network segment collectively determine which bridge has the least-cost path from the network segment to the root. The port connecting this bridge to the network segment is then the designated port for the segment.
Disable all other root paths. Any active port that is not a root port or a designated port is a blocked port.
Modifications in case of ties. The above rules over-simplify the situation slightly, because it is possible that there are ties, for example, two or more ports on a single bridge are attached to least-cost paths to the root or two or more bridges on the same network segment have equal least-cost paths to the root. To break such ties:
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 e, there would be two paths of length 2 to the root, one path going through bridge 24 and the other through bridge 92. 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 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 figure 4, there are two least cost paths from network segment d to the root, one going through bridge 24 and the other through bridge 92. The lower bridge ID is 24, so the tie breaker dictates that the designated port is the port through which network segment is connected to bridge 24.
The final tie-breaker. In some cases, there may still be a tie, as when two bridges are connected by multiple cables. In this case, multiple ports on a single bridge are candidates for root port or designated port. In this case, the port with the lowest port priority is used.
|Data rate||STP Cost|
There are three types of BPDUs:
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.
When a device is first attached to a switch port, it will not immediately start to forward data. It will instead go through a number of states while it processes BPDUs and determines the topology of the network. When a host is attached such as a computer, printer or server the port will always go into the forwarding state, albeit after a delay of about 30 seconds while it goes through the listening and learning states (see below). The time spent in the listening and learning states is determined by a value known as the forward delay (default 15 seconds and set by the root bridge). However, if instead another switch is connected, the port may remain in blocking mode if it is determined that it would cause a loop in the network. Topology Change Notification (TCN) BPDUs are used to inform other switches of port changes. TCNs are injected into the network by a non-root switch and propagated to the root. Upon receipt of the TCN, the root switch will set a Topology Change flag in its normal BPDUs. This flag is propagated to all other switches to instruct them to rapidly age out their forwarding table entries.
STP switch port states:
To prevent the delay when connecting hosts to a switch and during some topology changes, Rapid STP was developed and standardized by IEEE 802.1w, which allows a switch port to rapidly transition into the forwarding state during these situations.
Although the purpose of a standard is to promote interworking of equipment from different vendors, 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.
Also, the original Perlman-inspired Spanning Tree Protocol, called DEC STP, is not a standard and differs from the IEEE version in message format as well as timer settings. 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.
RSTP bridge port roles:
RSTP is a refinement of STP and therefore shares most of its basic operation characteristics. However there are some notable differences as summarized below:
If there is only one Virtual LAN (VLAN) in the network, single (traditional) STP works appropriately. If the network contains more than one VLAN, the logical network configured by single STP would work, but it is possible to make better use of the redundant links available by using an alternate spanning tree for different (groups of) VLANs.
MSTP allows formation of MST regions that can run multiple MST instances (MSTI). Multiple regions and other STP bridges are interconnected using one single common spanning tree (CST).
MSTP was inspired by Cisco Systems' Multiple Instances Spanning Tree Protocol (MISTP), and is an evolution of the Spanning Tree Protocol and the Rapid Spanning Tree Protocol. It was introduced in IEEE 802.1s as amendment to 802.1Q, 1998 edition. Standard IEEE 802.1Q-2003 now includes MSTP.
Unlike some proprietary per-VLAN spanning tree implementations, MSTP includes all of its spanning tree information in a single BPDU format. Not only does this reduce the number of BPDUs required on a LAN to communicate spanning tree information for each VLAN, but it also ensures backward compatibility with RSTP (and in effect, classic STP too). MSTP does this by encoding additional region information after the standard RSTP BPDU as well as a number of MSTI messages (from 0 to 64 instances, although in practice many bridges support less). Each of these MSTI configuration messages conveys the spanning tree information for each instance. Each instance can be assigned a number of configured VLANs and frames (packets) assigned to these VLANs operate in this spanning tree instance whenever they are inside the MST region. In order to avoid conveying their entire VLAN to spanning tree mapping in each BPDU, bridges encode an MD5 digest of their VLAN to instance table in the MSTP BPDU. This digest is then used by other MSTP bridges, along with other administratively configured values, to determine if the neighboring bridge is in the same MST region as itself.
MSTP is fully compatible with RSTP bridges, in that an MSTP BPDU can be interpreted by an RSTP bridge as an RSTP BPDU. This not only allows compatibility with RSTP bridges without configuration changes, but also causes any RSTP bridges outside of an MSTP region to see the region as a single RSTP bridge, regardless of the number of MSTP bridges inside the region itself. In order to further facilitate this view of an MST region as a single RSTP bridge, the MSTP protocol uses a variable known as remaining hops as a time to live counter instead of the message age timer used by RSTP. The message age time is only incremented once when spanning tree information enters an MST region, and therefore RSTP bridges will see a region as only one "hop" in the spanning tree. Ports at the edge of an MST region connected to either an RSTP or STP bridge or an endpoint are known as boundary ports. As in RSTP, these ports can be configured as edge ports to facilitate rapid changes to the forwarding state when connected to endpoints.