Transcript Switching Basics and Intermediate Routing CCNA 3 Chapter 7 www.ciscopress.com

Switching Basics and Intermediate
Routing CCNA 3
Chapter 7
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Spanning Tree Protocol
Introduction
• 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:
–
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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 Protocol
STP Background
• Spanning Tree Protocol (STP) was originally
developed by Digital Equipment Corporation
– The IEEE 802 committee revised the DEC spanningtree 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 Protocol
STP Background
STP Intelligently Blocks Selected Ports to Logically
Solve Problems That Physical Loops Cause
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Spanning Tree Protocol
Spanning 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 Protocol
Spanning 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 Protocol
Spanning Tree Operation
Various Spanning-Tree Parameters Include
Designated Ports, Nondesignated Ports, and
Root Ports
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Spanning Tree Protocol
Spanning 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 Protocol
Spanning 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 Protocol
Spanning Tree Operation
• As a result, for every switched network,
these elements exist:
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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 Protocol
Selecting 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 spanningtree 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 Protocol
Selecting the Root Bridge
Bridge ID Determines the Root Bridge
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Spanning Tree Protocol
Selecting 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 Protocol
Selecting the Root Bridge
Root Bridge Selection Relies on BPDUs
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Spanning Tree Protocol
Spanning Tree Port States
• With STP, ports transition through four states at powerup:
– 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 Protocol
Spanning Tree Port States
STP Flow Chart
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Spanning Tree Protocol
Spanning 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 Protocol
Spanning 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 Protocol
Spanning 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 Protocol
Spanning 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 Protocol
Spanning Tree Port States
Nondesignated Ports Are Blocking and
Others Are Forwarding
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Spanning Tree Protocol
Spanning Tree Port States
Spanning-Tree Operation with Three Switches
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Spanning Tree Protocol
Spanning-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 Protocol
Spanning-Tree Recalculation
STP Has Converged
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Spanning Tree Protocol
Spanning-Tree Recalculation
Port 1/2 Fails, Resulting in STP Recalculation
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Spanning Tree Protocol
Spanning-Tree Recalculation
STP Reconverges
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Spanning Tree Protocol
Rapid 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 Protocol
Rapid Spanning-Tree Protocol
RSTP Defines Five Port Roles (Backup Not Shown)
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Spanning Tree Protocol
Rapid 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
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Spanning Tree Protocol
Rapid 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 Protocol
Rapid 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 Protocol
Rapid 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 Protocol
Rapid Spanning-Tree Protocol
RSTP Incorporates the Concepts of Edge Ports
and Point-to-Point Links
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Spanning Tree Protocol
Rapid 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 Protocol
Rapid 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 pointto-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 Protocol
Summary
• 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 Protocol
Summary
• 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 Protocol
Summary
• 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 Protocol
Summary
• 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 Protocol
Summary
• 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 Protocol
Summary
• 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:
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•
•
•
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 Protocol
Summary
• 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 bridge’s 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
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