Switching Basics and Intermediate Routing CCNA 3 Chapter 7

1
Switching Basics and Intermediate Routing CCNA
3Chapter 7
2
Spanning Tree ProtocolIntroduction

• Redundancy is desirable in a network
• Helps minimize network downtime
• Downside increased likelihood of Layer 2 or
  Layer 3 loops
• Spanning Tree Protocol (STP) was invented to
  address issues caused by physical redundancy in a
  switched topology
• Two major solutions
• IEEE 802.1d original standard, five states
• IEEE 802.1w enhancements, becoming the standard

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Redundant Topologies Introduction

• Redundancy is critical in a network
• Allows a network to be fault tolerant
• A network without redundancy can suffer downtime
  from the failure of a single link, port, or
  device
• Goal is to balance the cost of redundancy with
  the need for network availability
• Switched networks have some drawbacks
• Broadcast storms
• Multiple frame transmissions
• MAC address database instability

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Redundant Topologies Introduction

• Switched networks have benefits
• Smaller collision domains
• Microsegmentation
• Full duplex operation
• Better network performance
• Redundancy protects against lost connectivity
  because of a failed individual component
• Can result in physical topologies with loops
• Physical layer loops can cause serious problems
  in switched networks

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Redundant Topologies Redundancy

• If the network is down, productivity and customer
  satisfaction decline
• Companies require continuous network
  availability, or uptime
• 100 uptime is nearly impossible
• Five nines uptime (99.999) is the goal of many
  organizations
• Means one hour of downtime for every 4000 days
  (5.25 minutes of downtime a year)

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Redundant Topologies Redundancy

• Network reliability is achieved through reliable
  equipment and network designs that are tolerant
  to failures and faults
• Networks should reconverge rapidly to bypass the
  fault
• Goal of redundant topologies is to eliminate
  outages caused by a single point of failure

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Redundant Topologies Redundant Switched
Topologies

• Problems that can occur with redundant links and
  devices in switched or bridged networks
• Broadcast storms without a loop-avoidance
  process in place, each switch or bridge
  broadcasts endlessly
• Multiple frame transmission multiple copies of
  unicast frames can be delivered to destination
  stations can cause unrecoverable errors
• MAC address instability results from copies of
  the same frame being received on different ports
  of the switch data forwarding can be impaired

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Redundant Topologies Redundant Switched
Topologies

• A Redundant Switched Topology Can Be a Source of
  Layer 2 Problems

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Redundant Topologies Redundant Switched
Topologies

• Layer 2 LAN protocols, such as Ethernet, lack a
  mechanism to recognize and eliminate endlessly
  looping frames
• Some Layer 3 protocols utilize a Time to Live
  (TTL) mechanism that limits how many times a
  packet can be retransmitted by a Layer 3
  networking device
• Layer 2 devices lack such a capability, so a
  loop-avoidance mechanism is required

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Redundant Topologies Broadcast Storms

• Broadcasts and multicasts can cause problems in a
  switched network
• Without specialized switch configurations,
  switches treat multicasts the same as broadcasts
• Broadcast and multicast frames are flooded out
  all ports except the one on which the frame was
  received
• Broadcast storms are not as prevalent due to the
  move to Layer 3 switching

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Redundant Topologies Broadcast Storms

• Broadcast Storm

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Redundant Topologies Broadcast Storms

• How a broadcast storm can occur in the previous
  slide
• Host X sends a broadcast frame, such as an ARP
  Switch A receives the frame
• Switch A examines the Destination Address field
  in the frame and determines the frame must be
  flooded to segment 2
• When the copy of the frame arrives at Switch B,
  the process repeats and a copy of the frame is
  transmitted to the Ethernet, segment 1 near
  Switch B
• Because the original copy of the frame also
  arrives at Switch B via the top Ethernet, the
  frames travel around the loop in both directions,
  even after the destination has received a copy of
  the frame

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Redundant Topologies Broadcast Storms

• A broadcast storm can disrupt normal traffic flow
• Every device on the switched or bridged network
  must process the frames because they are
  broadcasts
• Takes CPU cycles
• A loop-avoidance mechanism (spanning tree)
  eliminates this problem by preventing one of the
  four interfaces from transmitting frames during
  normal operation, thus breaking the loop

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Redundant Topologies Multiple Frame Transmissions

• Multiple copies of the same frame can arrive at
  the intended host
• Can cause problems with the receiving protocol as
  most protocols do not cope with or recognize
  duplicate transmissions
• Protocols that use a sequence numbering mechanism
  assume that many transmissions have failed and
  that the protocol is recycling numbers
• Other protocols attempt to hand the duplicate
  transmission to the appropriate upper-layer
  protocol, with unpredictable results

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Redundant Topologies Multiple Frame Transmissions

• Multiple Frame Transmissions Can Occur in a
  Redundant Switched Network

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Redundant Topologies Multiple Frame Transmissions

• How multiple copies of frames can arrive at the
  intended host in previous slide
• Host X sends a unicast frame to Router Y one
  copy is received over Ethernet segment 1 at the
  same time Switch A receives a copy of the frame
• Switch A examines the Destination Address field
  in the frame, finds no entry in its table, and
  floods the frame
• Switch B receives the frame and forwards it to
  segment 1 if the table has no entry for Router Y
• Router Y receives a second copy of the frame

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Redundant Topologies MAC Database Instability

• MAC database instability results when multiple
  copies of a frame arrive on different ports of a
  switch
• Depending on the internal architecture of the
  switch, it might or might not cope well with
  rapid changes in its MAC database
• STP eliminates this problem by preventing one of
  the interfaces from transmitting frames during
  normal operation

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Redundant Topologies MAC Database Instability

• MAC Database Instability Can Also Occur in
  Redundant Switched Networks

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Spanning Tree ProtocolSTP Background

• Spanning Tree Protocol (STP) was originally
  developed by Digital Equipment Corporation
• The IEEE 802 committee revised the DEC
  spanning-tree algorithm in the IEEE 802.1d
  specification
• IEEE 802.1d is used by Cisco switches
• STP is enabled by default on Catalyst switches
• Purpose of STP is to maintain a loop-free network
  topology
• STP continually probes the network so in can
  detect the addition or failure of a link

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Spanning Tree ProtocolSTP Background

• STP Intelligently Blocks Selected Ports to
  Logically Solve Problems That Physical Loops Cause

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Spanning Tree ProtocolSpanning Tree Operation

• Convergence in STP is a state in which all switch
  and bridge ports have transitioned into a
  forwarding or blocking state
• Necessary for normal network operations
• Amount of time for convergence is a key issue
  fast convergence time is desirable
• 30 to 50 seconds with IEEE 802.1d
• STP uses two key concepts when converging a
  loop-free logical topology
• Bridge ID
• Path cost

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Spanning Tree ProtocolSpanning Tree Operation

• Spanning-tree path cost based on cumulative link
  costs
• Link costs are based on the speed of the link
• Spanning-Tree Path Costs for the Revised and
  Previous IEEE Specification

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Spanning Tree ProtocolSpanning Tree Operation

• Various Spanning-Tree Parameters Include
  Designated Ports, Nondesignated Ports, and Root
  Ports

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Spanning Tree ProtocolSpanning Tree Operation

• STP performs three steps when it initially
  converges on a logically loop-free topology
• Elects one root bridge on the root bridge, all
  ports are designated ports that are normally in
  the forwarding state that can send and receive
  traffic
• Selects the root port on the nonroot bridge STP
  establishes one root port on the nonroot bridge
  (any bridge that is not the root bridge)
• Root ports are normally in the forwarding state

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Spanning Tree ProtocolSpanning Tree Operation

• STP performs three steps when it initially
  converges on a logically loop-free topology
  (continued)
• Selects the designated port on each segment only
  one designated port is selected on each segment
• The designated port has the lowest-cost path to
  the root bridge
• Designated ports are normally in the forwarding
  state
• Nondesignated ports are normally in the blocking
  state to logically break the loop topology

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Spanning Tree ProtocolSpanning Tree Operation

• As a result, for every switched network, these
  elements exist
• One root bridge per network
• One root port per nonroot bridge
• One designated port per segment
• Unused, or nondesignated ports
• Root ports and designated ports are used for
  forwarding data traffic
• Nondesignated ports discard all data traffic and
  are called blocking or discarding ports

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Spanning Tree ProtocolSelecting the Root Bridge

• The root bridge is the bridge with the lowest
  bridge ID
• The bridge ID (BID) includes the priority and MAC
  address of the bridge
• Switches and bridges that run the spanning-tree
  algorithm exchange configuration messages every 2
  seconds by default
• They use a multicast frame called the bridge
  protocol data unit (BPDU)

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Spanning Tree ProtocolSelecting the Root Bridge

• Bridge ID Determines the Root Bridge

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Spanning Tree ProtocolSelecting the Root Bridge

• Each bridge must have a unique BID assigned
• The default in IEEE 802.1d is 32,768
• Binary 1000 0000 0000 0000 hex 0x8000
• Is the midrange value
• The root bridge is the bridge with the lowest
  BID it is a combination of bridge priority and
  MAC address values
• Setting the switch priority smaller makes the BID
  smaller

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Spanning Tree ProtocolSelecting the Root Bridge

• Root Bridge Selection Relies on BPDUs

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Spanning Tree ProtocolSpanning Tree Port States

• With STP, ports transition through four states at
  power-up
• Blocking
• Listening
• Learning
• Forwarding
• Ports then stabilize to forwarding or blocking
  states
• Forwarding ports provide the lowest cost path to
  the root bridge
• During a topology change, ports temporarily go
  through listening and learning states

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Spanning Tree ProtocolSpanning Tree Port States

• STP Flow Chart

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Spanning Tree ProtocolSpanning Tree Port States

• Initially, all bridge ports start in the blocking
  state, listening for BPDUs
• When a bridge first boots up, it thinks it is the
  root bridge, so it transitions to the listening
  state
• An absence of BPDUs for a certain period of time
  is called the max_age
• Default setting of 20 seconds
• If a port is in the blocking state and does not
  receive a BPDU within the max_age, it transitions
  from the blocking state to the listening state
• When in the listening state, it can determine the
  active topology

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Spanning Tree ProtocolSpanning Tree Port States

• During the listening state, no user data is
  passed through the switch port
• The bridge selects the root bridge
• The bridge selects the root ports on the nonroot
  bridges
• The bridge selects designated ports on each
  segment
• The time it takes for a port to transition from
  listening to learning or learning to forwarding
  is called the forward delay has a default value
  of 15 seconds

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Spanning Tree ProtocolSpanning Tree Port States

• The learning state reduces the amount of flooding
  required when data forwarding begins
• If a port is still a designated or root port at
  the end of the learning state, the port
  transitions to the forwarding state
• It can send and receive user data
• Ports that are not designated or root ports
  transition back to the blocking state

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Spanning Tree ProtocolSpanning Tree Port States

• A port normally transitions from the learning
  state to the forwarding state in 30 to 50 seconds
• If a Cisco switch port is connected only to
  end-user stations (not to another switch or
  bridge), a feature called PortFast can be enabled
• Automatically transitions from blocking to
  forwarding

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Spanning Tree ProtocolSpanning Tree Port States

• Nondesignated Ports Are Blocking and Others Are
  Forwarding

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Spanning Tree ProtocolSpanning Tree Port States

• Spanning-Tree Operation with Three Switches

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Spanning Tree ProtocolSpanning-Tree Recalculation

• When a network topology changes, switches must
  recompute STP
• Disrupts user traffic
• A switched network has converged when all switch
  and bridge ports are in either forwarding or
  blocking states
• Forwarding ports send and receive data traffic
  and BPDUs
• Blocking ports receive only BPDUs

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Spanning Tree ProtocolSpanning-Tree Recalculation

• STP Has Converged

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Spanning Tree ProtocolSpanning-Tree Recalculation

• Port 1/2 Fails, Resulting in STP Recalculation

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Spanning Tree ProtocolSpanning-Tree Recalculation

• STP Reconverges

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Spanning Tree ProtocolRapid Spanning-Tree
Protocol

• Rapid Spanning Tree Protocol (RSTP) significantly
  reduces the time to reconverge the active
  topology when physical or configuration changes
  occur
• Defines additional port RSTP port roles
• Alternate
• Backup
• Defines port states as discarding, learning, or
  forwarding

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Spanning Tree ProtocolRapid Spanning-Tree
Protocol

• RSTP Defines Five Port Roles (Backup Not Shown)

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Spanning Tree ProtocolRapid Spanning-Tree
Protocol

• RSTP provides rapid connectivity following the
  failure of a switch, a switch port, or a LAN
• A new root port and the designated port on the
  other side of the bridge transition to forwarding
  through an explicit handshake
• RSTP allows switch port configuration so that the
  ports can transition to forwarding directly when
  the switch reinitializes

46
Spanning Tree ProtocolRapid Spanning-Tree
Protocol

• RSTP (IEEE 802.1w) supercedes STP while remaining
  compatible with STP
• RSTP port roles
• Root a forwarding port elected for the spanning
  tree topology
• Designated a forwarding port elected on every
  LAN segment
• Alternate an alternate path to the root bridge
• Backup a backup path that provides a redundant
  but less desirable path
• Disabled a port with no role in spanning tree

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Spanning Tree ProtocolRapid Spanning-Tree
Protocol

• RSTP has a different set of port states
• The RSTP port state controls the forwarding and
  learning processes and provides the values of
  discarding, learning and forwarding
• RSTP Port States

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Spanning Tree ProtocolRapid Spanning-Tree
Protocol

• In a stable topology, RSTP ensures that every
  root port and designated port transitions to
  forwarding
• All alternate and backup ports are always in the
  discarding state
• STP waits passively for topology changes to
  occur RSTP actively confirms a port can
  transition safely without relying on a timer
  configuration, uses edge ports and point-to-point
  links
• Results in faster convergence

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Spanning Tree ProtocolRapid Spanning-Tree
Protocol

• RSTP Incorporates the Concepts of Edge Ports and
  Point-to-Point Links

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Spanning Tree ProtocolRapid Spanning-Tree
Protocol

• With edge ports, no ports directly connected to
  end stations can create bridging loops
• Edge ports go directly to forwarding, skipping
  listening and learning states
• RSTP can achieve rapid transition to forwarding
  only on edge ports, new root ports and
  point-to-point links
• Edge ports immediately transitions to
  forwarding, same as a PortFast port
• Root ports if RSTP elects a new root port, it
  blocks the old one and transitions the new one to
  forwarding
• Point-to-point links if one port connects to
  another through a p-to-p link and it becomes a
  designated port, a rapid transition is negotiated
  with the other port

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Spanning Tree ProtocolRapid Spanning-Tree
Protocol

• The link-type variable is automatically derived
  from the duplex mode of the port
• A port operating in full-duplex mode is
  point-to-point
• A port operating in half-duplex mode is
  considered shared by default
• The automatic link-type setting can be overridden
  with an explicit configuration

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Spanning Tree ProtocolSummary

• Redundancy is the duplication of components that
  allows continued functionality despite the
  failure of an individual component
• In a network, this means having a backup method
  to connect all devices
• Network downtime is decreased because single
  points of failure are reduced or eliminated

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Spanning Tree ProtocolSummary

• A redundant switched topology might cause
• Broadcast storms
• Caused by multiple hosts sending and receiving
  broadcast messages
• Network appears to be down or extremely slow
• Multiple frame transmission
• A router receives multiple copies of a frame from
  multiple switches because of an unknown MAC
  address
• MAC address table instability
• If a switch incorrectly learns the MAC address of
  a device on a port, it can cause a loop situation

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Spanning Tree ProtocolSummary

• Switches operate at OSI Layer 2
• Decisions are made at this level
• No TTL value is decremented
• Physical network topologies need switching or
  bridging loops to provide reliability, but a
  switched network cannot have loops
• Solution allow physical loops but create a
  loop-free logical topology

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Spanning Tree ProtocolSummary

• The loop-free topology is called a spanning tree
• Star or extended star that spans the network
• All devices are reachable
• The algorithm that creates the loop-free logical
  topology is the spanning-tree algorithm
• STP establishes a root node, called the root
  bridge

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Spanning Tree ProtocolSummary

• STP constructs a topology that has one node for
  every device on the network
• Results in a tree that originates from the root
  bridge
• Redundant links that are not part of the shortest
  path tree are blocked
• A loop-free logical topology is possible because
  certain paths are blocked
• Data frames received on blocked links are dropped

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Spanning Tree ProtocolSummary

• Switches send messages called bridge protocol
  data units (BPDUs)
• Allow a loop-free logical topology to be formed
• Blocked ports continue to receive BPDUs
• BPDUs contain information that allows switches
  to
• Select a single switch that will act as the root
• Calculate the shortest path to the root switch
• Designate one of the switches as the designated
  switch
• Choose one of its ports as the root port, for
  each nonroot switch
• Select the ports (designated ports) that are part
  of the spanning tree

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Spanning Tree ProtocolSummary

• The IEEE 802.1w standard defines RSTP
• Clarifies port states and roles
• Defines a set of link types
• Allows switches in a converged network to
  generate BPDUs rather than use the root bridges
  BPDUs
• The STP blocking state of a port is renamed as
  the discarding state
• The role of a discarding port is that of an
  alternate port
• The discarding port can become the designated
  port if the designated port of the segment fails