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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
  • 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