Spanning Tree Protocol (STP)

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Spanning Tree Protocol (STP)

• W.lilakiatsakun

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Redundancy (1)
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Redundancy (2)
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Redundancy (3)
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Examine Redundancy (1)
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Examine Redundancy (2)
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Examine Redundancy (3)
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Examine Redundancy (4)
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Examine Redundancy (5)
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Issues with Redundancy- layer2 loop (1)

• LAYER 2 Loop
• Ethernet frames do not have a time to live (TTL)
  like IP packets traversing routers. As a result,
  if they are not terminated properly on a switched
  network, they continue to bounce from switch to
  switch endlessly or until a link is disrupted and
  breaks the loop.
• Broadcast frames are forwarded out all switch
  ports, except the originating port.
• This ensures that all devices in the broadcast
  domain are able to receive the frame.
• If there is more than one path for the frame to
  be forwarded out, it can result in an endless
  loop.

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Issues with Redundancy - layer2 loop (2)
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Issues with Redundancy - layer2 loop (3)
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Issues with Redundancy - layer2 loop (4)
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Issues with Redundancy - layer2 loop (5)
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Issues with Redundancy broadcast storm (1)

• Broadcast storm
• A broadcast storm occurs when there are so many
  broadcast frames caught in a Layer 2 loop that
  all available bandwidth is consumed.
• Consequently, no bandwidth is available bandwidth
  for legitimate traffic, and the network becomes
  unavailable for data communication.

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Issues with Redundancy broadcast storm (2)
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Issues with Redundancy Duplicate Unicast frame
(1)

• Duplicate Unicast Frames
• Broadcast frames are not the only type of frames
  that are affected by loops.
• Unicast frames sent onto a looped network can
  result in duplicate frames arriving at the
  destination device.

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Issues with Redundancy Duplicate Unicast frame
(2)
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Issues with Redundancy Duplicate Unicast frame
(3)
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Issues with Redundancy Duplicate Unicast frame
(4)
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Real world Redundancy issues - Loops in the
Wiring Closet(1)

• Loops in the Wiring Closet
• Network cables between access layer switches,
  located in the wiring closets, disappear into the
  walls, floors, and ceilings where they are run
  back to the distribution layer switches on the
  network.
• If the network cables are not properly labeled
  when they are terminated in the patch panel in
  the wiring closet, it is difficult to determine
  where the destination is for the patch panel port
  on the network.
• Network loops that are a result of accidental
  duplicate connections in the wiring closets are a
  common occurrence.

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Real world Redundancy issues - Loops in the
Wiring Closet(2)
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Real world Redundancy issues - Loops in the
Wiring Closet(3)
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Real world Redundancy issues - Loops in the
Cubicles (1)

• Loops in the Cubicles
• Wiring closets are typically secured to prevent
  unauthorized access, so often the network
  administrator is the only one who has full
  control over how and what devices are connected
  to the network.
• Unlike the wiring closet, the administrator is
  not in control of how personal hubs and switches
  are being used or connected, so the end user can
  accidentally interconnect the switches or hubs.

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Real world Redundancy issues - Loops in the
Cubicles (2)
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STP Topology (1)

• STP ensures that there is only one logical path
  between all destinations on the network by
  intentionally blocking redundant paths that could
  cause a loop.
• Blocking the redundant paths is critical to
  preventing loops on the network.
• The physical paths still exist to provide
  redundancy, but these paths are disabled to
  prevent the loops from occurring.
• If the path is ever needed to compensate for a
  network cable or switch failure, STP recalculates
  the paths and unblocks the necessary ports to
  allow the redundant path to become active.

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STP Topology (2)
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STP Topology (3)
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STP Algorithm (1)

• STP uses the Spanning Tree Algorithm (STA) to
  determine which switch ports on a network need to
  be configured for blocking to prevent loops from
  occurring.
• The STA designates a single switch as the root
  bridge and uses it as the reference point for all
  path calculations.
• All switches participating in STP exchange BPDU
  frames to determine which switch has the lowest
  bridge ID (BID) on the network.
• The switch with the lowest BID automatically
  becomes the root bridge for the STA calculations.

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STP Algorithm (2)
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STP Algorithm (2)

• After the root bridge is selected, the STA
  calculates the shortest path to the root bridge.
• Each switch uses the STA to determine which ports
  to block.
• The STA considers both path and port costs when
  determining which path to leave unblocked.
• The path costs are calculated using port cost
  values associated with port speeds for each
  switch port along a given path.
• The sum of the port cost values determines the
  overall path cost to the root bridge.
• If there is more than one path to choose from,
  STA chooses the path with the lowest path cost.

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STP Algorithm (3)

• Root ports - Switch ports closest to the root
  bridge.

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STP Algorithm (4)

• Designated ports - All non-root ports that are
  still permitted to forward traffic on the
  network.

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STP Algorithm (5)

• Non-designated ports - All ports configured to be
  in a blocking state to prevent loops.

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Selecting The root bridge (1)

• The Root Bridge
• Every spanning-tree instance (switched LAN or
  broadcast domain) has a switch designated as the
  root bridge.
• The root bridge serves as a reference point for
  all spanning-tree calculations to determine which
  redundant paths to block.

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Selecting The root bridge (2)

• After a switch boots, it sends out BPDU frames
  containing the switch BID and the root ID every 2
  seconds.
• By default, the root ID matches the local BID for
  all switches on the network.
• The root ID identifies the root bridge on the
  network.
• Initially, each switch identifies itself as the
  root bridge after bootup.

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Selecting The root bridge (3)

• If the root ID from the BPDU received is lower
  than the root ID on the receiving switch, the
  receiving switch updates its root ID identifying
  the adjacent switch as the root bridge.
• Eventually, the switch with the lowest BID ends
  up being identified as the root bridge for the
  spanning-tree instance.

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Selecting The root bridge (4)
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Best Path to the root bridge (1)

• The path information is determined by summing up
  the individual port costs along the path from the
  destination to the root bridge.
• The default port costs are defined by the speed
  at which the port operates.
• 10-Gb/s Ethernet ports have a port cost of 2,
• 1-Gb/s Ethernet ports have a port cost of 4,
• 100-Mb/s Fast Ethernet ports have a port cost of
  19,
• 10-Mb/s Ethernet ports have a port cost of 100.

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Best Path to the root bridge (2)

• Default port cost

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Best Path to the root bridge (3)
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Best Path to the root bridge (4)
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Best Path to the root bridge (5)
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STP - BPDU (1)
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STP - BPDU (2)
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STP - BPDU (3)
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STP - BPDU (4)
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BPDU Process (1)

• Each switch in the broadcast domain initially
  assumes that it is the root bridge for the
  spanning-tree instance, so the BPDU frames sent
  contain the BID of the local switch as the root
  ID.
• By default, BPDU frames are sent every 2 seconds
  after a switch is booted that is, the default
  value of the hello timer specified in the BPDU
  frame is 2 seconds.
• Each switch maintains local information about its
  own BID, the root ID, and the path cost to the
  root.

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BPDU Process (2)
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BPDU Process (3)

• When adjacent switches receive a BPDU frame, they
  compare the root ID from the BPDU frame with the
  local root ID.
• If the root ID in the BPDU is lower than the
  local root ID, the switch updates the local root
  ID and the ID in its BPDU messages.
• Also, the path cost is updated to indicate how
  far away the root bridge is.
• If the root ID in the BPDU is higher than the
  local root ID, the switch discard the BPDU frame

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BPDU Process (4)
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BPDU Process (5)
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BPDU Process (6)

• After a root ID has been updated to identify a
  new root bridge, all subsequent BPDU frames sent
  from that switch contain the new root ID and
  updated path cost.
• As the BPDU frames pass between other adjacent
  switches, the path cost is continually updated to
  indicate the total path cost to the root bridge.
• Each switch in the spanning tree uses its path
  costs to identify the best possible path to the
  root bridge.

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BPDU Process (7)
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BPDU Process (8)
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BPDU Process (8)
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BPDU Process (9)
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BPDU Process (10)
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BPDU Process (11)
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Bridge ID field (1)

• The bridge ID (BID) is used to determine the root
  bridge on a network.
• The BID field of a BPDU frame contains three
  separate fields bridge priority, extended system
  ID, and MAC address.
• Each field is used during the root bridge
  election.

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Bridge ID field (2)
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Bridge ID field (3)

• Bridge Priority
• The bridge priority is a customizable value that
  you can use to influence which switch becomes the
  root bridge.
• The switch with the lowest priority, which means
  lowest BID, becomes the root bridge (the lower
  the priority value, the higher the priority).
• The default value for the priority of all Cisco
  switches is 32768.
• The priority range is between 1 and 65536
  therefore, 1 is the highest priority.

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Bridge ID field (4)

• Extended System ID
• The early implementation of STP was designed for
  networks that did not use VLANs.
• There was a single common spanning tree across
  all switches.
• When VLANs started became common for network
  infrastructure segmentation, STP was enhanced to
  include support for VLANs.
• As a result, the extended system ID field
  contains the ID of the VLAN with which the BPDU
  is associated.

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Bridge ID field (5)

• When the extended system ID is used, it changes
  the number of bits available for the bridge
  priority value, so the increment for the bridge
  priority value changes from 1 to 4096.
• Therefore, bridge priority values can only be
  multiples of 4096.
• The extended system ID value is added to the
  bridge priority value in the BID to identify the
  priority and VLAN of the BPDU frame.

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Bridge ID field (6)

• MAC Address
• When two switches are configured with the same
  priority and have the same extended system ID,
  the switch with the MAC address with the lowest
  hexadecimal value has the lower BID.
• Initially, all switches are configured with the
  same default priority value. The MAC address is
  then the deciding factor on which switch is going
  to become the root bridge. This results in an
  unpredictable choice for the root bridge.

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Bridge ID field (7)

• It is recommended to configure the desired root
  bridge switch with a lower priority to ensure
  that it is elected root bridge.
• This also ensures that the addition of new
  switches to the network does not trigger a new
  spanning-tree election, which could disrupt
  network communication while a new root bridge is
  being selected.

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Bridge ID field (8)
Priority Based Decision
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Bridge ID field (9)
MAC Based Decision
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Configure BID (1)

• Method 1 - To ensure that the switch has the
  lowest bridge priority value, use the
  spanning-tree vlan vlan-id root primary command
  in global configuration mode.
• The priority for the switch is set to the
  predefined value of 24576 or to the next 4096
  increment value below the lowest bridge priority
  detected on the network.

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Configure BID (2)
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Configure BID (3)

• If an alternate root bridge is desired, use the
  spanning-tree vlan vlan-id root secondary
• global configuration mode command.
• This command sets the priority for the switch to
  the predefined value of 28672.
• This ensures that this switch becomes the root
  bridge if the primary root bridge fails and a new
  root bridge election occurs and assuming that the
  rest of the switches in the network have the
  default 32768 priority value defined.

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Configure BID (4)
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Port Roles (1)

• Root Port
• The root port exists on non-root bridges and is
  the switch port with the best path to the root
  bridge.
• Root ports forward traffic toward the root
  bridge.
• The source MAC address of frames received on the
  root port are capable of populating the MAC
  table.
• Only one root port is allowed per bridge.
• In the example, switch S1 is the root bridge and
  switches S2 and S3 have root ports defined on the
  trunk links connecting back to S1.

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Port Roles (2)
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Port Roles (3)

• Designated Port
• The designated port exists on root and non-root
  bridges.
• For root bridges, all switch ports are designated
  ports.
• For non-root bridges, a designated port is the
  switch port that receives and forwards frames
  toward the root bridge as needed.
• Only one designated port is allowed per segment.
• If multiple switches exist on the same segment,
  an election process determines the designated
  switch, and the corresponding switch port begins
  forwarding frames for the segment.
• Designated ports are capable of populating the
  MAC table.

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Port Roles (4)

• Non-designated Port
• The non-designated port is a switch port that is
  blocked, so it is not forwarding data frames and
  not populating the MAC address table with source
  addresses.
• A non-designated port is not a root port or a
  designated port.
• For some variants of STP, the non-designated port
  is called an alternate port.
• In the example, switch S3 has the only
  non-designated ports in the topology. The
  non-designated ports prevent the loop from
  occurring.

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Port Roles (5)
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Port Roles (6)

• Disabled Port
• The disabled port is a switch port that is
  administratively shut down.
• A disabled port does not function in the
  spanning-tree process.

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Port Roles (7)

• When determining the root port on a switch, the
  switch compares the path costs on all switch
  ports participating in the spanning tree.
• The switch port with the lowest overall path cost
  to the root is automatically assigned the root
  port role because it is closest to the root
  bridge.
• In a network topology, all switches that are
  using spanning tree, except for the root bridge,
  have a single root port defined.

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Port Roles (8)

• When there are two switch ports that have the
  same path cost to the root bridge and both are
  the lowest path costs on the switch, the switch
  needs to determine which switch port is the root
  port.
• The switch uses the customizable port priority
  value, or the lowest port ID if both port
  priority values are the same.

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Port Roles (9)
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Configure Port Priority (1)

• You can configure the port priority value using
  the spanning-tree port-priority value interface
  configuration mode command
• The port priority values range from 0 - 240, in
  increments of 16.
• The default port priority value is 128.
• As with bridge priority, lower port priority
  values give the port higher priority.

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Configure Port Priority (2)
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Port Role Decision (1)
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Port Role Decision (2)
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Port Role Decision (3)
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Port Role Decision (4)
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Port Role Decision (5)
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Port Role Decision (6)
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Port Role Decision (7)
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Port States (1)

• STP introduces five port states
• Blocking
• The port is a non-designated port and does not
  participate in frame forwarding.
• The port receives BPDU frames to determine the
  location and root ID of the root bridge switch
  and what port roles each switch port should
  assume in the final active STP topology.

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Port States (2)

• Listening
• STP has determined that the port can participate
  in frame forwarding according to the BPDU frames
  that the switch has received thus far.
• At this point, the switch port is not only
  receiving BPDU frames, it is also transmitting
  its own BPDU frames and informing adjacent
  switches that the switch port is preparing to
  participate in the active topology.

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Port States (3)

• Learning
• The port prepares to participate in frame
  forwarding and begins to populate the MAC address
  table.
• Forwarding
• The port is considered part of the active
  topology and forwards frames and also sends and
  receives BPDU frames.
• Disabled
• The Layer 2 port does not participate in spanning
  tree and does not forward frames.
• The disabled state is set when the switch port is
  administratively disabled

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Port States (4)
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BPDU Timers (1)

• The amount of time that a port stays in the
  various port states depends on the BPDU timers.
• Only the switch in the role of root bridge may
  send information through the tree to adjust the
  timers.
• The following timers determine STP performance
  and state changes
• Hello time
• Forward delay
• Maximum age

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BPDU Timers (2)
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BPDU Timers (3)

• When STP is enabled, every switch port in the
  network goes through the blocking state and the
  transitory states of listening and learning at
  power up.
• The ports then stabilize to the forwarding or
  blocking state, as seen in the example.
• During a topology change, a port temporarily
  implements the listening and learning states for
  a specified period called the "forward delay
  interval."

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BPDU Timers (4)
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BPDU Timers (5)

• These values allow adequate time for convergence
  in a network with a switch diameter of seven.
• To review, switch diameter is the number of
  switches a frame has to traverse to travel from
  the two farthest points on the broadcast domain.
• A seven-switch diameter is the largest diameter
  that STP permits because of convergence times.
• Convergence in relation to spanning tree is the
  time it takes to recalculate the spanning tree if
  a switch or a link fails.

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BPDU Timers (6)

• It is recommended that the BPDU timers not be
  adjusted directly because the values have been
  optimized for the seven-switch diameter.
• Adjusting the spanning-tree diameter value on the
  root bridge to a lower value automatically
  adjusts the forward delay and maximum age timers
  proportionally for the new diameter.
• Typically, you do not adjust the BPDU timers nor
  reconfigure the network diameter.

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BPDU Timers (7)
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Cisco PortFast Technology(1)

• When a switch port configured with PortFast is
  configured as an access port, that port
  transitions from blocking to forwarding state
  immediately, bypassing the typical STP listening
  and learning states.
• You can use PortFast on access ports, which are
  connected to a single workstation or to a server,
  to allow those devices to connect to the network
  immediately rather than waiting for spanning tree
  to converge.

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Cisco PortFast Technology(2)
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Cisco PortFast Technology(3)
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Cisco PortFast Technology(4)
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STP convergence (1)

• Convergence is the time it takes for the network
  to determine which switch is going to assume the
  role of the root bridge, go through all the
  different port states, and set all switch ports
  to their final spanning-tree port roles where all
  potential loops are eliminated.
• The convergence process takes time to complete
  because of the different timers used to
  coordinate the process.

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STP convergence (2)

• To understand the convergence process more
  thoroughly, it has been broken down into three
  distinct steps
• Step 1. Elect a root bridge
• Step 2. Elect root ports
• Step 3. Elect designated and non-designated ports

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STP convergence Elect a root bridge (1)

• A root bridge election is triggered after a
  switch has finished booting up, or when a path
  failure has been detected on a network.
• Initially, all switch ports are configured for
  the blocking state, which by default lasts 20
  seconds.
• This is done to prevent a loop from occurring
  before STP has had time to calculate the best
  root paths and configure all switch ports to
  their specific roles.

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STP convergence Elect a root bridge (2)

• While the switch ports are in a blocking state,
  they are still able to send and receive BPDU
  frames so that the spanning-tree root election
  can proceed.
• Spanning tree supports a maximum network diameter
  of seven switch hops from end to end.
• This allows the entire root bridge election
  process to occur within 14 seconds, which is less
  than the time the switch ports spend in the
  blocking state.

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STP convergence Elect a root bridge (3)
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STP convergence Elect a root bridge (4)
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STP convergence Elect a root port (1)

• Every switch in a spanning-tree topology, except
  for the root bridge, has a single root port
  defined.
• The root port is the switch port with the lowest
  path cost to the root bridge.
• If switch ports have equivalent path costs to the
  root, it uses the configurable port priority
  value.
• They use the port ID to break a tie.
• When a switch chooses one equal path cost port as
  a root port over another, the losing port is
  configured as the non-designated to avoid a loop.

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STP convergence Elect a root port (2)
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STP convergence Elect a root port (3)
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STP convergence Elect a root port (4)
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STP convergence Electing Designated Ports and
Non-Designated Ports (1)

• Each segment in a switched network can have only
  one designated port.
• When two non-root port switch ports are connected
  on the same LAN segment, a competition for port
  roles occurs.
• The two switches exchange BPDU frames to sort out
  which switch port is designated and which one is
  non-designated.

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STP convergence Electing Designated Ports and
Non-Designated Ports (2)

• Generally, when a switch port is configured as a
  designated port, it is based on the BID.
• However, keep in mind that the first priority is
  the lowest path cost to the root bridge and that
  only if the port costs are equal, is the BID of
  the sender.

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STP convergence Electing Designated Ports and
Non-Designated Ports (3)

• When two switches exchange their BPDU frames,
  they examine the sending BID of the received BPDU
  frame to see if it is lower than its own.
• The switch with the lower BID wins the
  competition and its port is configured in the
  designated role

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STP convergence Electing Designated Ports and
Non-Designated Ports (4)
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STP convergence Electing Designated Ports and
Non-Designated Ports (5)
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STP Topology change (1)

• A switch considers it has detected a topology
  change either
• when a port that was forwarding is going down
  (blocking for instance) or
• when a port transitions to forwarding and the
  switch has a designated port.
• When a change is detected,
• the switch notifies the root bridge of the
  spanning tree.
• The root bridge then broadcasts the information
  into the whole network.

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STP Topology change (2)

• STP Topology Change Notification Process
• When a switch needs to signal a topology change,
  it starts to send TCNs (Topology Change
  Notification) on its root port to the root
  bridge.
• The TCN is a very simple BPDU that contains no
  information and is sent out at the hello time
  interval.
• The receiving switch is called the designated
  bridge and it acknowledges the TCN by immediately
  sending back a normal BPDU with the Topology
  Change Acknowledgement (TCA) bit set.
• This exchange continues until the root bridge
  responds.

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STP Topology change (3)
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STP Topology change (4)

• Broadcast Notification
• Once the root bridge is aware that there has been
  a topology change event in the network, it starts
  to send out its configuration BPDUs with the
  topology change (TC) bit set.
• These BPDUs are relayed by every switch in the
  network with this bit set.
• As a result, all switches become aware of the
  topology change and can reduce their aging time
  to forward delay.
• Switches receive topology change BPDUs on both
  forwarding and blocking ports.

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STP Topology change (5)
The TC bit is set by the root for a period of max
age forward delay seconds, which is 201535
seconds by default.
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Cisco and STP Variants
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PVST (Per VLAN Spanning Tree) (1)

• Cisco developed PVST so that a network can run
  an STP instance for each VLAN in the network.
• With PVST, more than one trunk can block for a
  VLAN and load sharing can be implemented.
• However, implementing PVST means that all
  switches in the network are engaged in converging
  the network, and the switch ports have to
  accommodate the additional bandwidth used for
  each PVST instance to send its own BPDUs.

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PVST (Per VLAN Spanning Tree) (2)
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PVST (Per VLAN Spanning Tree) (3)

• In a Cisco PVST environment, you can tune the
  spanning-tree parameters so that half of the
  VLANs forward on each uplink trunk.
• This is accomplished by configuring one switch to
  be elected the root bridge for half of the total
  number of VLANs in the network, and a second
  switch to be elected the root bridge for the
  other half of the VLANs.
• In the figure, switch S3 is the root bridge for
  VLAN 20, and switch S1 is the root bridge for
  VLAN 10.
• As a result, port F0/3 on switch S2 is the
  forwarding port for VLAN 20, and F0/2 on switch
  S2 is the forwarding port for VLAN 10.

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PVST (Per VLAN Spanning Tree) (4)
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PVST (Per VLAN Spanning Tree) (5)
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Configure PVST(1)
The goal is to configure - S3 as the root bridge
for VLAN 20 and S1 as the root bridge for VLAN
10. - Port F0/3 on S2 is the forwarding port for
VLAN 20 and the blocking port for VLAN 10. -
Port F0/2 on S2 is the forwarding port for VLAN
10 and the blocking port for VLAN 20.
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Configure PVST(2)

• Step 1. Select the switches you want for the
  primary and secondary root bridges for each VLAN.
• Step 2. Configure the switch to be a primary
  bridge for one VLAN, for example switch S3 is a
  primary bridge for VLAN 20.
• Step 3. Configure the switch to be a secondary
  bridge for the other VLAN, for example, switch S3
  is a secondary bridge for VLAN 10.
• Optionally, set the spanning-tree priority to be
  low enough on each switch so that it is selected
  as the primary bridge.

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Configure PVST(3)
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Configure PVST(4)
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Configure PVST(5)
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RSTP (Rapid Spanning Tree Protocol) (1)

• RSTP (IEEE 802.1w) is an evolution of the 802.1D
  (Bridge - STP) standard.
• The 802.1w STP terminology remains primarily the
  same as the IEEE 802.1D STP terminology.
• Most parameters have been left unchanged, so
  users familiar with STP can rapidly configure the
  new protocol.

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RSTP (Rapid Spanning Tree Protocol) (2)
Discard State (No blocking State)
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RSTP (Rapid Spanning Tree Protocol) (3)

• RSTP speeds the recalculation of the spanning
  tree when the Layer 2 network topology changes.
• RSTP can achieve much faster convergence in a
  properly configured network, sometimes in as
  little as a few hundred milliseconds.
• RSTP redefines the type of ports and their
  state.
• If a port is configured to be an alternate or a
  backup port it can immediately change to a
  forwarding state without waiting for the network
  to converge.

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RSTP (Rapid Spanning Tree Protocol) (5)

• RSTP (802.1w) supersedes STP (802.1D) while
  retaining backward compatibility.
• Much of the STP terminology remains, and most
  parameters are unchanged.
• In addition, 802.1w is capable of reverting back
  to 802.1D to interoperate with legacy switches on
  a per-port basis.
• For example, the RSTP spanning-tree algorithm
  elects a root bridge in exactly the same way as
  802.1D.
• RSTP keeps the same BPDU format as IEEE 802.1D,
  except that the version field is set to 2 to
  indicate RSTP, and the flags field uses all 8
  bits.
• RSTP is able to actively confirm that a port can
  safely transition to the forwarding state without
  having to rely on any timer configuration.

141
RSTP (Rapid Spanning Tree Protocol) (4)

• RSTP Characteristics
• RSTP is the preferred protocol for preventing
  Layer 2 loops in a switched network environment.
• Many of the differences were informed by
  Cisco-proprietary enhancements to 802.1D.
• These enhancements, such as BPDUs carrying and
  sending information about port roles only to
  neighboring switches, require no additional
  configuration and generally perform better than
  the earlier Cisco-proprietary versions.
• They are now transparent and integrated in the
  protocol's operation.
• Cisco-proprietary enhancements to 802.1D, such as
  UplinkFast and BackboneFast, are not compatible
  with RSTP.

142
RSTP BPDU (1)

• RSTP (802.1w) uses type 2, version 2 BPDUs, so an
  RSTP bridge can communicate 802.1D on any shared
  link or with any switch running 802.1D.
• RSTP sends BPDUs and populates the flag byte in a
  slightly different manner than in 802.1D
• Protocol information can be immediately aged on a
  port if hellos are not received for three
  consecutive hello times, 6 seconds by default, or
  if the max age timer expires.
• Because BPDUs are used as a keepalive mechanism,
  three consecutively missed BPDUs indicate lost
  connectivity between a bridge and its neighboring
  root or designated bridge.
• The fast aging of the information allows failures
  to be detected quickly.

143
RSTP BPDU (2)
144
RSTP BPDU (3)

• RSTP uses the flag byte of version 2 BPDU as
  shown in the figure
• Bits 0 and 7 are used for topology change
  notification and acknowledgment as they are in
  802.1D.
• Bits 1 and 6 are used for the Proposal Agreement
  process (used for rapid convergence).
• Bits 2-5 encode the role and state of the port
  originating the BPDU.
• Bits 4 and 5 are used to encode the port role
  using a 2-bit code.

145
RSTP Edge Port (1)

• An RSTP edge port is a switch port that is never
  intended to be connected to another switch
  device.
• It immediately transitions to the forwarding
  state when enabled.

Edge Port
146
RSTP Edge Port (2)

• Cisco uses Port Fast function as RSTP Edge
  Port except, an RSTP edge port that receives a
  BPDU loses its edge port status immediately and
  becomes a normal spanning-tree port.
• Neither edge ports nor PortFast-enabled ports
  generate topology changes when the port
  transitions to a disabled or enabled status.
• The Cisco RSTP implementation maintains the
  PortFast keyword using the spanning-tree portfast
  command for edge port configuration.

147
RSTP Link Types (1)

• Non-edge ports are categorized into two link
  types, point-to-point and shared.
• The link type is automatically determined, but
  can be overwritten with an explicit port
  configuration.
• Edge ports, the equivalent of PortFast-enabled
  ports, and point-to-point links are candidates
  for rapid transition to a forwarding state.

148
RSTP Link Types (2)
149
RSTP Link Types (3)
150
RSTP Port States (1)
151
RSTP Port States (2)
152
RSTP Port Roles (1)
153
RSTP Port Roles (2)
154
Configuring RSTP (1)

• Rapid PVST is a Cisco implementation of RSTP.

155
Configuring RSTP (2)
156
Configuring RSTP (3)
157
Trouble Avoidance for STP design (1)

• Know where the root is
• Do not leave it up to the STP to decide which
  bridge is root.
• For each VLAN, you can usually identify which
  switch can best serve as root.
• Generally, choose a powerful bridge in the middle
  of the network.

158
Trouble Avoidance for STP design (2)
If switch S2 is the root, the link from S1 to S3
is blocked on S1 or S3. In this case, hosts that
connect to switch S2 can access the server and
the router in two hops. Hosts that connect to
bridge S3 can access the server and the router in
three hops. The average distance is two and
one-half hops.
If switch S1 is the root, the router and the
server are reachable in two hops for both hosts
that connect on S2 and S3. The average distance
is now two hops.
159
Trouble Avoidance for STP design (3)

• Minimize the Number of Blocked Ports
• The only critical action that STP takes is the
  blocking of ports.
• A single blocking port that mistakenly
  transitions to forwarding can negatively impact a
  large part of the network.
• A good way to limit the risk inherent in the use
  of STP is to reduce the number of blocked ports
  as much as possible.

160
Trouble Avoidance for STP design (4)
161
Trouble Avoidance for STP design (5)
162
Trouble Avoidance for STP design (6)

• Use Layer 3 Switching
• Layer 3 switching means routing approximately at
  the speed of switching. A router performs two
  main functions
• It builds a forwarding table. The router
  generally exchanges information with peers by way
  of routing protocols.
• It receives packets and forwards them to the
  correct interface based on the destination
  address.

163
Trouble Avoidance for STP design (7)
164
Trouble Avoidance for STP design (8)

• Redundancy is still present, with a reliance on
  Layer 3 routing protocols.
• The design ensures a convergence that is even
  faster than convergence with STP.
• STP no longer blocks any single port, so there is
  no potential for a bridging loop.
• Leaving the VLAN by Layer 3 switching is as fast
  as bridging inside the VLAN.

165
Trouble Avoidance for STP design (9)