Jump to content

Spanning Tree Protocol

From Wikipedia, the free encyclopedia

This is an old revision of this page, as edited by Boscobiscotti (talk | contribs) at 20:13, 2 July 2007 (→External links: add link protocol). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

Based on an algorithm invented by Radia Perlman while working for Digital Equipment Corporation, Spanning Tree Protocol Is an OSI layer-2 protocol which ensures a loop free topology for any bridged LAN.

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 the links which are not part of that tree, leaving a single active path between any two network nodes.

Multiple active paths between network nodes causes a bridge loop. Bridge loops create several problems. First, the MAC address table (also known as "switch forwarding table") used by the switch or bridge can fail, since the same MAC addresses (and hence the same network hosts) are seen on multiple ports. Second, a broadcast storm occurs. This is caused by broadcast packets being forwarded in an endless loop between switches. A broadcast storm can consume all available CPU resources and bandwidth.

Spanning tree allows a network design to include spare (redundant) links to provide automatic backup paths if an active link fails, without the danger of bridge loops, or the need for manual enabling/disabling of backup links.

There are two different standards of STP: IEEE and DEC. IEEE is the most common type. Problems can be caused in networks where both standards of STP are implemented. It is likely that both standards will elect their own root bridge and cause loops in the topology. The way in which both standards handle Bridge Protocol Data Units (see below) also differs.

Protocol operation

  • Elect a root bridge.
  • Find paths to root bridge.
  • Determine least cost (measured as sum of all traversed port costs) path to root bridge.
  • Disable all other root paths.

Electing a root bridge

Each switch has a unique identifier (ID) and a configurable priority number; both of these numbers make up the Bridge Identification or BID. The BID is used to elect a root bridge based upon the lowest priority number; if this is a tie then the numerically lowest ID wins. Because it is next to impossible that two IDs will be the same (they are uniquely assigned), one switch should always be successfully elected as the root bridge with the switch that has the next lowest priority/cost acting as a secondary (backup) root bridge. Other switches in the network that use spanning tree will use the same algorithmic process to calculate the shortest path to the root bridge and so produce a loop free tree topology where multiple paths to the root bridge exist. The priority number is normally left at its default value but can be reconfigured to a lower number if the network administrator wishes a particular switch to be elected; otherwise the whole process is fully automated. The Bridge ID is the concatenation of bridge priority number and MAC address of the switch. The bridge priority number varies from 0 to 65535. The Switch with least administrative cost is selected as a Root Bridge.

Bridge Protocol Data Units (BPDUs)

BIDs and other Spanning Tree Protocol information are carried in special data frames called bridge protocol data units (BPDUs). BPDUs are exchanged regularly (every 2 seconds by default) and enable switches to keep track of network changes and activate or disable 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:

  • Listening - The switch processes BPDUs and awaits possible new information that would cause it to return to the blocking state.
  • Learning - While the port does not yet forward frames (packets) it does learn source addresses from frames received and adds them to the filtering database (switching database)
  • Blocking - A port that would cause a switching loop, no user data is sent or received but it may go into forwarding mode if the other links in use were to fail and the spanning tree algorithm determines the port may transition to the forwarding state. BPDU data is still received in blocking state.
  • Forwarding - A port receiving and sending data, normal operation. STP still monitors incoming BPDUs that would indicate it should return to the blocking state to prevent a loop.
  • Disabled - Not strictly part of STP, a network administrator can manually disable a port

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.

Evolutions and extensions

Per-VLAN Spanning Tree (PVST)

In Ethernet switched environments where multiple Virtual LANs exist, spanning tree can be deployed per Virtual LAN. Cisco's name for this is per VLAN spanning tree (PVST and PVST+ which is the default protocol used by Cisco switches). Both PVST and PVST+ protocols are Cisco proprietary protocols and they cannot be used on 3rd party switches. PVST only works with ISL (Cisco's proprietary protocol for VLAN encapsulation) due to its embedded Spanning tree ID. Due to high penetration of the IEEE 802.1Q VLAN trunking standard and PVST's incompatibility with 802.1Q, Cisco redefined its PVST standard and called it PVST+. PVST+ can tunnel across a MST Region.

Rapid Spanning Tree Protocol (RSTP)

In 1998, the IEEE introduced an evolution of the Spanning Tree Protocol: Rapid Spanning Tree Protocol (RSTP) or 802.1w. In the 2004 edition of 802.1D, STP is superseded by the RSTP.

RSTP is an evolution of the Spanning Tree Protocol; it was introduced in the extension IEEE 802.1w, and provides for faster spanning tree convergence after a topology change. Standard IEEE 802.1D-2004 now incorporates RSTP and obsoletes STP.

RSTP bridge port roles:

  • Root - A forwarding port that has been elected for the spanning-tree topology
  • Designated - A forwarding port for every LAN segment
  • Alternate - An alternate path to the root bridge. This path is different than using the root port.
  • Backup - A backup/redundant path to a segment where another bridge port already connects.
  • Disabled - Not strictly part of STP, a network administrator can manually disable a port

RSTP is a refinement of STP and therefore shares most of its basic operation characteristics. However there are some notable differences as summarized below:

  • Detection of root switch failure is done in 3 hello times, which is 6 seconds if default hello times have not been changed.
  • Ports may be configured as edge ports if they are attached to a LAN which has no other bridges attached. These edge ports transition directly to the forwarding state. RSTP still continues to monitor the port for BPDUs in case a bridge is connected. RSTP can also be configured to automatically detect edge ports. As soon as the bridge detects a BPDU coming to an edge port, the port becomes a non-edge port.
  • Unlike in STP, RSTP will respond to BPDUs sent from the direction of the root bridge. An RSTP bridge will "propose" to its designated ports its spanning tree information. If another RSTP bridge receives this information, determines this is the superior root information, and sets all its other ports to discarding. The bridge may send an "agreement" to the first bridge confirming its superior spanning tree information. The first bridge, upon receiving this agreement, knows it can rapidly transition that port to the forwarding state bypassing the traditional listening/learning state transition. This essentially creates a cascading effect away from the root bridge where each designated bridge proposes to its neighbors to determine if it can make a rapid transition. This is one of the major elements which allows RSTP to achieve faster convergence times than STP.
  • As discussed in the port role details above, RSTP maintains backup details regarding the discarding status of ports. This avoids timeouts if the current forwarding ports were to fail or BPDUs were not received on the root port in a certain interval.

Multiple Spanning Tree Protocol (MSTP)

The Multiple Spanning Tree Protocol (MSTP), originally defined in IEEE 802.1s and later merged into IEEE 802.1Q-2003, defines an extension to the RSTP protocol to further develop the usefulness of virtual LANs (VLANs). This "Per-VLAN" Multiple Spanning Tree Protocol configures a separate Spanning Tree for each VLAN group and blocks the links that are redundant within each Spanning Tree.

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 which 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,[citation needed] 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 classical 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 a 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.

Poetical Description

Radia Perlman, the inventor of the algorithm, summarized it in a poem titled "Algorhyme"
(adapted from "Trees", by Joyce Kilmer):

I think that I shall never see
A graph more lovely than a tree.
A tree whose crucial property
Is loop-free connectivity.
A tree which must be sure to span
So packets can reach every LAN.
First the Root must be selected
By ID it is elected.
Least cost paths from Root are traced
In the tree these paths are placed.
A mesh is made by folks like me
Then bridges find a spanning tree.


You can listen to author of STP playing on piano and her daughter Dawn Perlner (voice) performing at MIT's Lincoln Laboratory, which can be downloaded from NetworkWorld Podcasts Section[1]

References

  • Perlman, Radia (2000). Interconnections, Second Edition. USA: Addison-Wesley. ISBN 0-201-63448-1. {{cite book}}: Cite has empty unknown parameter: |coauthors= (help)

See also

Retrieved from "https://en.wikipedia.org/enwiki/w/index.php?title=Spanning_Tree_Protocol&oldid=142080771"