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Spanning tree Protocol (STP) Variants. Rapid Spanning Tree Protocol (RSTP) -The reason behind the word «rapid» Multiple Spanning Tree Protocol (MSTP). Introduction. Spanning Tree Protocol (STP) developed in the late 80s Later standardized by IEEE (IEEE-802.1D, 1990)

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Spanning tree protocol stp variants

Spanning tree Protocol (STP) Variants

Rapid Spanning Tree Protocol (RSTP) -The reason behind the word «rapid»

Multiple Spanning Tree Protocol (MSTP)


  • Spanning Tree Protocol (STP) developed in the late 80s

    • Later standardized by IEEE (IEEE-802.1D, 1990)

  • Switches and Bridges do not age-out packets

    • Loops in the network -> frames may live forever -> congestion

  • STP prevents loops allowing redundant connections

  • But STP is too slow

    • After a failure -> recovery time 30-50 seconds

  • Rapid Spanning Tree Protocol is an improved and faster version

    • Preserves the basic concepts of STP

    • Also standardized (IEEE-802.1W)

    • In IEEE-802.1D from 2004 STP has been suppressed

  • Tree topology
    Tree Topology

    • Spanning Tree can be thought of a tree:

      • Root -> Root Bridge

      • Branches -> LANs and Designated Switches

      • Leaves -> End nodes

  • No disconnected parts

  • No loops

  • Only one path from leaf to leaf

  • Root and designated bridges
    Root and Designated Bridges

    • Both STP and RSTP use Root and Designated Bridges

    • Root bridge -> from which all branches spring

      • There is only one

      • Any switch could be the Root (Bridge ID)

  • Designated bridge -> traffic from the Root to any link

    • Only one Designated bridge per link

    • No loops

  • The Root bridge is the Designated bridge for all links connected to it

  • Port roles stp i
    Port Roles – STP (I)

    • Three types of ports in STP

    • Root port: closest to the Root bridge (path cost)

    • Designated port: connectivity in the direction away from the Root

      • Sends the best Bridge Protocol Data Unit (BPDU) on the segment it is connected

  • Blocking port: disables redundant links

    • Do not forward data

    • Prevents loops

  • Port roles rstp i
    Port Roles – RSTP (I)

    • Maintains Root and Designated ports

    • Splits Blocking port into two (do not forward data):

    • Alternate port

      • Provides redundant connection to the Root bridge

      • May become a new Root port

  • Backup port

    • Connected to the same LAN segment as a Designated port

    • Or two ports are connected together in a loopback

  • Edge ports

    • Connected directly to end stations -> cannot create loops

    • Do not follow regular states

  • Port states stp i
    Port States – STP (I)

    • 5 states

    • Disabled: not receiving or transmitting any data

    • Blocking: enabled and listen for BPDU messages

    • Listening: not forwarding data, but listening and sending BPDU messages

    • Learning: preparing to forward data -> building up forwarding table

    • Forwarding: forwards data

    • Duration of listening and learning states is 15 seconds by default (forwarding delay timer)

    Port states rstp
    Port States – RSTP

    • RSTP has only 3 port states

    • Forwarding: forwards data and learns MAC addresses

    • Learning: does not forward data, but learns MACs

    • Discarding: does not forward data and does not learn MACs


    • Bridge Protocol Data Units (BPDUs) to learn and exchange information

    • STP uses two BPDUs

      • Configuration BPDUs: from Root every hello time (typically 2 seconds)

        • Other bridges forward on Designated ports

      • Topology Change (TCN) BPDUs: from the bridge that detected a change to the Root

        • Root answers setting a Topology Change (TC) flag

        • A bridge receiving a BPDU with a TC flag -> switches aging time to short

  • RSTP uses one BPDU

    • All the bridges

    • Includes TC flag, role and state of the port and flags for handshake

  • Filtering database aging
    Filtering Database Aging

    • Database of MAC-to-port entries

    • STP

      • Bridge detecting a topology change do not flush its filtering database

      • Send a TCN BPDU to Root

      • The Root responds with the TC flag activated

      • Bridges wait the aging timer before removing entries from database

  • RSTP

    • Switches detecting a topology change send a BPDU with TC flag

    • Purges old entries

    • Every switch receiving the BPDU purges old entries

  • Keep alive bpdus
    «Keep-alive» BPDUs

    • STP bridges do not generate BPDUs (unless failures)

      • Receive them on Root port and forward them on Designated ports

      • If no BPDU is received in a “max age time” (default 20 seconds) the Root is declared dead

      • The bridge assumes to be the Root and starts from the beginning

  • RSTP bridges send BPDUs every “hello time”

    • If no BPDU is received in three “hello times” -> connection is lost

    • Immediately assumes it is the new Root or

    • Alternate ports can move to Forwarding state without delay

  • Rstp behavior
    RSTP Behavior

    • RSTP does not relies on timers:

    • Monitors MAC operational states and retires ports

    • Processes inferior BPDUs (STP discards them)

    • If a Root port fails, an Alternate port can be put into operation without delay

    • If bridges are connected via point-to-point links, handshake is used to transition a Designated port to Forwarding state

    Example ii stp case
    Example (II) – STP Case

    • 222 and 444 wait max age timer (default 20 seconds) before deciding connection to the Root is broken

    • 444 ages out information -> path to Root through port 02 -> advertises to 222 through port 01

    • 444’s port 02 is new Root port -> port 01 is Designated port

    • Both ports must move through listening and learning states -> other switches agree -> 30 seconds (15 each)

    • 222 makes port 03 a new root port -> transition through listening and learning

    • Total time: 20 + 15 + 15 = 50 seconds

    Example iii rstp case
    Example (III) –RSTP Case

    • 222 loses connection to Root -> decides it is the new Root

    • 444 recognizes BPDUs from 222 as inferior -> connection to Root through 222 is broken

    • 444’s Alternate port 02 is immediately placed in Forwarding state

    • 444’s port 01 is set as Designated port -> advertises new path to the Root to 222

    • 222 accepts and makes port 03 Root port

    • 444 performs a handshake (“sync operation) with 222 to transition port 01 to Forwarding state

    • No timers

    Multiple spanning tree protocol i
    Multiple Spanning Tree Protocol (I)

    • MSTP is based on RSTP and aims at

      • A more balanced load across the network

      • Failures only affect a region of the network

  • The network is divided in regions (MST regions):

    • Internal Spanning Tree (IST)

      • Spanning Tree within a region

      • Can communicate with other regions

    • Multiple Spanning Tree Instance (MSTIn)

      • Spanning Trees within a region

      • Cannot communicate with other regions

    • Multiple VLANs could be mapped to a Spanning Tree Instance

  • Multiple spanning tree protocol ii
    Multiple Spanning Tree Protocol (II)

    • MST regions are interconnected using a Common Spanning Tree (CST)

      • Using one Regional Root Bridge

  • The Common Internal Spanning Tree is comprised of:

    • The CST connecting all regions

    • The IST providing connectivity inside each region

    • MST regions are seen as “big bridges” (pseudobridge or superbridge) by CST

    • Allows separated management of the regions

    • No change in internal topologies is influenced or produced by outside region changes

  • References

    • W. Wojdak, “Rapid Spanning Tree Protocol: A New Solution from an old Technology”, CompactPCI Systems Magazine, Telecom Special Feature, March 2003

    • G. Prytz, “Redundancy in Industrial Ethernet Networks”, IEEE International Workshop on Factory Communication Systems, 2006

    • Cisco White Paper, “Understanding Spanning-Tree Protocol, Cisco Systems Inc., 1997

    • Cisco White Paper, “Understanding Rapid Spanning Tree Protocol”, Cisco Systems Inc., 2006

    • G. Ibanez, A. Garcia, A. Azcorra, “Alternative Multiple Spanning Tree Protocol (AMSTP) for Optical Ethernet Backones”, Proc. of LCN’04, November 2004