The
Spanning Tree Protocol (
STP) is a network protocol that ensures a loop-free
topologyNetwork topology is the layout pattern of interconnections of the various elements of a computer or biological network....
for any
bridgedBridging is a forwarding technique used in packet-switched computer networks. Unlike routing, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding and examination of source addresses in received packet headers to locate unknown...
EthernetEthernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
local area networkA local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
. The basic function of STP is to prevent bridge loops and ensuing
broadcast radiationBroadcast radiation is the accumulation of broadcast and multicast traffic on a computer network. Extreme amounts of broadcast traffic constitute a broadcast storm. A broadcast storm can consume sufficient network resources so as to render the network unable to transport normal traffic.-Causes:Most...
. Spanning tree also allows a
network designNetwork planning and design is an iterative process, encompassingtopological design, network-synthesis, and network-realization, and is aimed at ensuring that a new network or service meets the needs of the subscriber and operator....
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 these backup links.
Spanning Tree Protocol (STP) is standardized as
IEEE 802.1D802.1D is the IEEE MAC Bridges standard which includes Bridging, Spanning Tree and others. It is standardized by the IEEE 802.1 working group. It includes details specific to linking many of the other 802 projects including the widely deployed 802.3 , 802.11 and 802.16 standards.VLANs are not...
. As the name suggests, it creates a
spanning treeIn the mathematical field of graph theory, a spanning tree T of a connected, undirected graph G is a tree composed of all the vertices and some of the edges of G. Informally, a spanning tree of G is a selection of edges of G that form a tree spanning every vertex...
within a mesh network of connected layer-2 bridges (typically
EthernetEthernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
switchesA network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
), and disables those links that are not part of the spanning tree, leaving a single active path between any two network nodes.
STP is based on an
algorithmIn mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
invented by
Radia PerlmanRadia Joy Perlman is a software designer and network engineer sometimes referred to as the "Mother of the Internet." She is most famous for her invention of the spanning-tree protocol , which is fundamental to the operation of network bridges, while working for Digital Equipment Corporation...
while working for
Digital Equipment CorporationDigital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
.
Protocol operation
The collection of bridges in a local area network (LAN) can be depicted as a
graphIn mathematics, a graph is an abstract representation of a set of objects where some pairs of the objects are connected by links. The interconnected objects are represented by mathematical abstractions called vertices, and the links that connect some pairs of vertices are called edges...
whose nodes are bridges and LAN segments (or cables), and whose edges are the interfaces connecting the bridges to the segments. To break loops in the LAN while maintaining access to all LAN segments, the bridges collectively compute a spanning tree. The spanning tree is not necessarily a minimum cost spanning tree. A network administrator can reduce the cost of a spanning tree, if necessary, by altering some of the configuration parameters in such a way as to affect the choice of the root of the spanning tree. 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.
Select a root bridge. The
root bridge of the spanning tree is the bridge with the smallest (lowest) 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=0200.0000.1111) and B (MAC=0200.0000.2222) both have a priority of 10, then switch A will be selected as the root bridge. If the network administrators would like switch B to become the root bridge, they must set its priority to be less than 10.
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 path is the sum of the costs of the segments on the path. Different technologies have different default costs for network segments. 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 one with the smallest cost (a least-cost path). The port connecting to that path becomes the
root port (RP) 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 (DP) 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 (BP).
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 d, 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 d is connected to bridge 24. If bridge IDs were equal, then the bridge with the lowest MAC address would have the designated port. In either case, the loser sets the port as being blocked.
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. In this case, the path which passes through the port on the neighbor bridge that has the lowest port priority is used.
In summary, the sequence of events to determine the best received BPDU (which is your best path to the root) is
- lowest root bridge id
- lowest root path cost
- lowest sender bridge id
- lowest sender port number
Data rate and STP path cost
The table below shows the default cost of an interface for a given data rate.>
| Data rate |
STP Cost (802.1D-1998) |
RSTP Cost (802.1W-2001) |
| 4 Mbit/s |
250 |
5,000,000 |
| 10 Mbit/s |
100 |
2,000,000 |
| 16 Mbit/s |
62 |
1,250,000 |
| 100 Mbit/s |
19 |
200,000 |
| 1 Gbit/s |
4 |
20,000 |
| 2 Gbit/s |
3 |
10,000 |
| 10 Gbit/s |
2 |
2,000 |
Bridge Protocol Data Units (BPDUs)
The above rules describe one way of determining what spanning tree will be computed by the algorithm, but the rules as written require knowledge of the entire network. The bridges have to determine the root bridge and compute the port roles (root, designated, or blocked) with only the information that they have. To ensure that each bridge has enough information, the bridges use special data frames called
Bridge Protocol Data Units (BPDUs) to exchange information about bridge IDs and root path costs.
A bridge sends a BPDU frame using the unique MAC address of the port itself as a source address, and a destination address of the STP
multicast addressA multicast address is a logical identifier for a group of hosts in a computer network, that are available to process datagrams or frames intended to be multicast for a designated network service...
01:80:C2:00:00:00.
There are three types of BPDUs:
- Configuration BPDU (CBPDU), used for Spanning Tree computation
- Topology Change Notification (TCN) BPDU, used to announce changes in the network topology
- Topology Change Notification Acknowledgment (TCA)
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
serverIn the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...
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
switchA network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
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:
- 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.
- 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)
- 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.
BPDU fields
The bridge ID, or BID, is a field inside a BPDU packet. It is eight
byteThe byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
s in length. The first two bytes are the Bridge Priority, an unsigned integer of 0-65,535. The last six
byteThe byte is a unit of digital information in computing and telecommunications that most commonly consists of eight bits. Historically, a byte was the number of bits used to encode a single character of text in a computer and for this reason it is the basic addressable element in many computer...
s are a
MAC addressA Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
supplied by the
switchIn electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
. In the event that
MAC AddressA Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
Reduction is used, the first two bytes are used differently. The first four
bitA bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s are a configurable priority, and the last twelve bits carry either the VLAN ID or MSTP instance number.
Evolutions and extensions
The first spanning tree protocol was invented in 1985 at the Digital Equipment Corporation by
Radia PerlmanRadia Joy Perlman is a software designer and network engineer sometimes referred to as the "Mother of the Internet." She is most famous for her invention of the spanning-tree protocol , which is fundamental to the operation of network bridges, while working for Digital Equipment Corporation...
. In 1990, the IEEE published the first standard for the protocol as 802.1D, based on the algorithm designed by Perlman. Subsequent
versions were published in 1998 and 2004, incorporating various extensions.
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 StatementA Protocol Implementation Conformance Statement or most commonly PICS is a structured document which asserts which specific requirements are met by a given implementation of a protocol standard. It is often completed as a record of formal protocol conformance test results, and some automated test...
", 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.
Rapid Spanning Tree Protocol (RSTP)
In 2001, the IEEE introduced Rapid Spanning Tree Protocol (RSTP) as 802.1w. RSTP provides significantly faster spanning tree convergence after a topology change, introducing new convergence behaviors and bridge port roles to do this. RSTP was designed to be backwards-compatible with standard STP.
While STP can take 30 to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3 × Hello times (default: 3 times 2 seconds) or within a few milliseconds of a physical link failure. The so-called Hello time is an important and configurable time interval that is used by RSTP for several purposes; its default value is 2 seconds.
Standard IEEE 802.1D-2004 now incorporates RSTP and obsoletes the original STP standard.
RSTP Operation
RSTP adds new bridge port roles in order to speed convergence following a link failure.
RSTP bridge port roles:
- Root - A forwarding port that is the best port from Nonroot-bridge to Rootbridge
- 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
Additional RSTP Operation Details:
- 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 that 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" its spanning tree information to its designated ports. If another RSTP bridge receives this information and determines this is the superior root information, it 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 that 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.
- RSTP will revert to legacy STP on an interface if a legacy version of an STP BPDU is detected on that port.
Per-VLAN Spanning Tree (PVST)
In
EthernetEthernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
switchedA network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...
environments where multiple
Virtual LANA virtual local area network, virtual LAN or VLAN, is a group of hosts with a common set of requirements that communicate as if they were attached to the same broadcast domain, regardless of their physical location...
s 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 protocolIn telecommunications, a proprietary protocol is a communications protocol owned by a single organization or individual.-Enforcement:Proprietors may enforce restrictions through patents and by keeping the protocol specification a trade secret...
s and they cannot be used on most 3rd party switches.
Some equipment from Force10 Networks, Extreme Networks, and Blade Network Technologies support PVST+. Extreme Networks does so with two limitations (lack of support on ports where the VLAN is untagged/native and also on the VLAN with ID 1).
PVST works only with
ISLCisco Inter-Switch Link is a Cisco Systems proprietary protocol that maintains VLAN information in Ethernet frames as traffic flows between switches and routers, or switches and switches....
(Cisco's proprietary protocol for VLAN encapsulation) due to its embedded Spanning tree ID. Due to high penetration of the
IEEE 802.1QIEEE 802.1Q is the networking standard that supports Virtual LANs on an Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying procedures to be used by bridges and switches in handling such frames...
VLAN trunking standard and PVST's dependence on ISL, Cisco defined a different PVST+ standard for 802.1Q encapsulation. PVST+ can tunnel across an MSTP Region.
VLAN Spanning Tree Protocol (VSTP)
In
Juniper NetworksJuniper Networks is an information technology and computer networking products multinational company, founded in 1996. It is head quartered in Sunnyvale, California, USA. The company designs and sells high-performance Internet Protocol network products and services...
environment, if compatibility to Cisco's proprietary PVST protocol is required, VLAN Spanning Tree Protocol (VSTP) can be configured. VSTP maintains a separate spanning-tree instance for each VLAN configured in the switch. The VSTP protocol is only supported by the EX Series from Juniper Networks. There are two restrictions to the compatibility of VSTP:
- VSTP supports only 253 different spanning-tree topologies. If there are more than 253 VLANs, it is recommended to configure RSTP in addition to VSTP, and VLANs beyond 253 will be handled by the RSTP protocol.
- MVRP does not support VSTP. If this protocol is in use, VLAN membership for trunk interfaces must be statically configuredhttp://www.juniper.net/techpubs/en_US/junos10.0/topics/concept/bridging-mvrp-ex-series.html.
By default, VSTP uses the RSTP protocol as its core spanning-tree protocol, but usage of STP can be forced if the network includes old bridges
https://www.juniper.net/techpubs/en_US/junos9.4/topics/concept/spanning-trees-ex-series-vstp-understanding.html.
For more information about configuring the VSTP protocol on Juniper Networks switches, go to the official documentation.
Understanding VSTP
Multiple Spanning Tree Protocol (MSTP)
The
Multiple Spanning Tree Protocol (MSTP), originally defined in IEEE 802.1s and later merged into
IEEE 802.1QIEEE 802.1Q is the networking standard that supports Virtual LANs on an Ethernet network. The standard defines a system of VLAN tagging for Ethernet frames and the accompanying procedures to be used by bridges and switches in handling such frames...
-2005, defines an extension to RSTP to further develop the usefulness of virtual LANs (VLANs). This "Per-VLAN" Multiple Spanning Tree Protocol configures a separate
Spanning TreeIn the mathematical field of graph theory, a spanning tree T of a connected, undirected graph G is a tree composed of all the vertices and some of the edges of G. Informally, a spanning tree of G is a selection of edges of G that form a tree spanning every vertex...
for each VLAN group and blocks all but one of the possible alternate paths within each Spanning Tree.
If there is only one
Virtual LAN (VLAN)A virtual local area network, virtual LAN or VLAN, is a group of hosts with a common set of requirements that communicate as if they were attached to the same broadcast domain, regardless of their physical location...
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 alternate paths available by using an alternate spanning tree for different VLANs or 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 is similar to
Cisco SystemsCisco Systems, Inc. is an American multinational corporation headquartered in San Jose, California, United States, that designs and sells consumer electronics, networking, voice, and communications technology and services. Cisco has more than 70,000 employees and annual revenue of US$...
'
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 an amendment to 802.1Q, 1998 edition. Standard IEEE 802.1Q-2005 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 fewer). 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.
Rapid Per-VLAN Spanning Tree (R-PVST)
Cisco's proprietary protocol that combines the functionalities of RSTP and PVST.
It is based on a per-VLAN instance that creates a spanning tree for each VLAN.
See also
- Bridging (networking)
Bridging is a forwarding technique used in packet-switched computer networks. Unlike routing, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding and examination of source addresses in received packet headers to locate unknown...
- Distributed minimum spanning tree
The distributed minimum spanning tree problem involves the construction of a minimum spanning tree by a distributed algorithm, in a network where nodes communicate by message passing...
- EtherChannel
EtherChannel is a port link aggregation technology or port-channel architecture used primarily on Cisco switches. It allows grouping of several physical Ethernet links to create one logical Ethernet link for the purpose of providing fault-tolerance and high-speed links between switches, routers and...
- Ethernet Automatic Protection Switching
- Flex Links
Flex links is a network switch feature which enables redundancy and load balancing at the layer 2 level. The feature serves as an alternative to Spanning Tree Protocol . A pair of layer 2 interfaces, such as switch ports or port channels has one interface configured as a backup to the other...
- IEEE 802.1D
- Minimum spanning tree
Given a connected, undirected graph, a spanning tree of that graph is a subgraph that is a tree and connects all the vertices together. A single graph can have many different spanning trees...
- Request for Comments
In computer network engineering, a Request for Comments is a memorandum published by the Internet Engineering Task Force describing methods, behaviors, research, or innovations applicable to the working of the Internet and Internet-connected systems.Through the Internet Society, engineers and...
- Shortest path bridging
- Unidirectional Link Detection
- Virtual LAN
A virtual local area network, virtual LAN or VLAN, is a group of hosts with a common set of requirements that communicate as if they were attached to the same broadcast domain, regardless of their physical location...
External links
- Cisco home page for the Spanning-Tree protocol family (discusses CST, MISTP, PVST, PVST+, RSTP, STP)
- STP article in the Wireshark wiki Includes a sample PCAP-file of captured STP traffic.
- IEEE Standards
- RFCs
- RFC 2674-1999, proposed standard, Definitions of Managed Objects for Bridges with Traffic Classes, Multicast Filtering and Virtual LAN Extensions
- RFC 1525-1993, - SBRIDGEMIB, proposed standard, Definitions of Managed Objects for Source Routing Bridges
- RFC 1493-1993 - BRIDGEMIB, draft standard, Definitions of Managed Objects for Bridges
- Spanning Tree Direct vs Indirect Link Failures - CCIE Study