Objectives
Implement Spanning
Tree Protocols
LAN Switching and Wireless – Chapter 5
ƒ
Explain the role of redundancy in a converged
network
ƒ
Summarize how STP works to eliminate Layer 2 loops
in a converged network
ƒ
Explain how the STP algorithm uses three steps to
converge on a loop-free topology
ƒ
Implement rapid per VLAN spanning tree (rapid
PVST+) in a LAN to prevent loops between redundant
switches.
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ITE I Chapter 6
© 2006 Cisco Systems, Inc. All rights reserved.
Cisco Public
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The Role of Redundancy in a Switched
Network
Layer 2 Loops
ƒ Need to eliminate single points of failure in the LAN, as
far as possible
ƒ Redundant links between switches results in Layer 2
loops
ƒ Redundant links connecting switches in the three layers
provides more than one path between hosts
ƒ Ethernet frames do not have a time-to-live (TTL) like IP
packets
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Broadcast Storms
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More problems with Layer 2 loops
ƒ A switch will flood a frame out of all ports except the one
it arrived on, if
ƒ Unicast frames sent onto a looped network can result in
duplicate frames arriving at the destination device.
- the frame is a broadcast
- some network protocols cannot handle duplicate
frames
- the destination MAC address is not in the switch table
ƒ In a loop, broadcasts will
ƒ Loops can cause MAC address tables to become
unstable
- loop endlessly
- increase in number exponentially
ƒ Loops result in high CPU load on all switches caught in
the loop.
- bring down the LAN (usually within seconds)
ƒ Network loops that are a result of accidental duplicate
connections in the wiring closets are a common
occurrence.
ƒ A broadcast storm occurs when there are so many
broadcast frames caught in a Layer 2 loop that all
available bandwidth is consumed.
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Spanning Tree Protocol (STP)
The Spanning Tree Topology
ƒ STP prevents Layer 2 loops.
ƒ Nodes are connected as a tree if
ƒ STP runs on all Layer 2 switches and bridges by default
straight out of the box
- each node has one and only
one parent node
ƒ 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
- except for the root node
Root node
ƒ Change a general structure into a
tree structure by removing some
of the links
ƒ Switches and bridges running STP cooperate to
produce a logical loop-free layer 2 topology
ƒ A spanning tree means all nodes
remain connected
ƒ If a link goes down, the STP algorithm will automatically
be run again to determine a new spanning tree
topology. Hence, redundancy is maintained.
ƒ A tree structure will not contain
any loops
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STP in action
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BPDUs
ƒ STP puts port F0/2 on S3 into blocking mode, thus
removing a loop
ƒ Switches running STP need to cooperate with each
other
ƒ Now there is only one path between any source and
destination device
ƒ They use Bridge Protocol Data Units (BPDUs) to
exchange messages
ƒ An Ethernet frame encapsulates the BPDU
ƒ Uses an Ethernet multicast address for the spanningtree group
ƒ Each BPDU contains a BID number that identifies the
switch that sent the BPDU
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The BPDU Fields
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Bridge ID (BID)
ƒ Each switch has a unique Bridge ID number (BID) made
up of:
Bridge priority – 16-bit customizable, default 32768
MAC address – guarantees uniqueness
ƒ The BID identifies the switch and is used in elections.
ƒ The admin can determine the outcome of an election by
setting the value of the Bridge Priority field
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2
STP Convergence Steps
ƒ When bootup has completed, each switch determines
the logical spanning tree topology by running the
Spanning Tree Algorithm (STA)
ƒ The STP Algorithm uses three steps to converge on a
loop-free topology:
ƒ Step 1: Elect a Root Bridge
ƒ Step 2: Elect the Root Ports
ƒ Step 3: Elect the Designated and Non-Designated
ports
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Step 1: Elect a Root Bridge
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Port Roles
ƒ Each switch port connecting to another switch (i.e. that
receives BPDUs) is assigned one of the following port roles
by the STA:
ƒ The first step is to elect a Root Bridge. The switch with
the lowest BID wins.
ƒ All switches continuously transmit BPDUs out of all ports
ƒ Root port
ƒ Each BPDU contains the BID of the sender and the BID
of the current Root Bridge
This is the port closest to the Root Bridge
This port will forward frames
ƒ Each switch initially assumes that it is the Root Bridge
ƒ Designated port
ƒ When a BPDU is received with a lower Root Bridge BID, it
replaces the current one in future BPDU transmissions.
If two or more switches connect to the same segment,
only one will be the designated port
ƒ After no more than 20 seconds, a single Root Bridge will
have been identified by all switches.
This port will forward frames
ƒ Non-designated port
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The port is in blocking mode; it will not forward user
frames
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Step 2: Elect Root Ports
ƒ All of the Root Bridge ports become Designated Ports
ƒ A Root Port exists on each non-root bridge. It is the
switch port with the best path to the Root Bridge
ƒ Choosing the Root Port:
- The Root Bridge continues to send BPDUs which
are relayed through the network by the other
switches
- As each BPDU is sent out of a port, the cost field is
updated, in accordance with the port bandwidth
- As a switch receives BPDUs from the Root Bridge,
the one with the lowest cost identifies the Root Port
- Each switch will have one and only one Root Port
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Step 3: Elect Designated and Non-Designated
Ports
ƒ The remaining ports on a switch which connect to other
switches will be either Designated Ports or Nondesignated Ports
ƒ If two switches connect to the same segment, the port
on the switch with the lowest BID becomes a
Designated Port. The port on the other switch
becomes a Non-designated Port.
ƒ Designated Ports will forward user frames.
ƒ Non-designated Ports will be blocked
ƒ Result is each segment will have only one switch
forwarding frames onto it
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Port States
ƒ While the STA is running, each switch port will be in one of
these port states:
ƒ Blocking - A non-designated port; does not participate in
frame forwarding. Receives BPDU frames only
ƒ Listening - STP has determined that the port can
participate in frame forwarding. Receives BPDU frames
and also transmits its own BPDU frames
ƒ Learning - The port prepares to participate in frame
forwarding and begins to populate the MAC address table.
ƒ Forwarding - The port is part of the active topology and
forwards frames, sends and receives BPDU frames.
ƒ Disabled - The switch port is administratively disabled.
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Port States
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BPDU Timers
ƒ
The spanning tree is determined immediately after a
switch has finished booting up.
ƒ Network diameter is the number of devices that a packet
has to cross before it reaches its destination.
ƒ
All ports are initially put in Blocking mode
ƒ Default convergence times are based on a seven-switch
diameter network
(LEDs on switch port will be amber)
ƒ
When the STA has completed, each port will be in either
Blocking mode or Forwarding mode
ƒ
A port that becomes part of the final spanning tree
topology will transition between modes in this order:
Hello time – Default 2 secs.
Forward delay – time spent in listening and learning
state. Default 15 secs each.
Maximum age – Default 20 secs.
1. Blocking
2. Listening
ƒ Optimize timers by reconfiguring the network diameter, not
the BPDU timers.
3. Learning
ƒ On root bridge only:
4. Forwarding
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ƒ spanning-tree vlan vlan-id root primary diameter
value
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4
Summary Spanning Tree Protocol (STP)
STP Variants
ƒ Standard IEEE 802.1D STP
ƒ STP’s lengthy convergence time (50 seconds) facilitated
the development of:
ƒ Only one spanning tree instance in a network (i.e.
broadcast domain).
ƒ RSTP
IEEE standard (IEEE 802.1w)
convergence time is slightly over 6 seconds
ƒ BID is 2-byte bridge priority + MAC address.
ƒ Only one Root Bridge elected in the network.
ƒ Each non-root switch has one Root Port – shortest path
to the Root Bridge
ƒ Each segment is connected by no more than one
Designated Port
ƒ All other ports on a switch are non-designated ports
and are in blocking mode.
ƒ Convergence time is 50 seconds
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PVST+ Feature: PortFast
ƒ Rapid PVST+
Cisco proprietary technology
This is the preferred STP on a Cisco switched
network
Adds VLAN support to RSTP
Separate Root Bridge for each instance means better
redundancy.
Can load balance VLANs on trunks
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PVST+: Bridge ID (BID)
ƒ PortFast is a Cisco proprietary technology.
ƒ When an access switch port is configured with PortFast
it transitions from blocking to forwarding state
immediately.
ƒ Use only on access ports connected to a single
workstation,etc. to allow those devices to connect to the
network immediately.
ƒ The Bridge ID number (BID) is made up of three fields:
Bridge priority – 4-bit customizable
Extended System ID – 12-bit VLAN ID number
MAC address – guarantees uniqueness
ƒ If a port configured with PortFast receives a BPDU
frame, spanning tree can put the port into the blocking
state using a feature called BPDU guard.
ƒ The first two fields are displayed as a single number.
ƒ PortFast technology can be used to support DHCP.
ƒ Therefore changing the Bridge Priority changes the
number in steps of 4096
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ƒ Default is 32769 – i.e. 32768 + VLAN 1
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RSTP (IEEE 802.1w)
ƒ RSTP - speeds the recalculation of the spanning tree
when the Layer 2 network topology changes.
ƒ RSTP supersedes STP (802.1D) while retaining
backward compatibility.
ƒ RSTP keeps the same BPDU format with version set to
2.
ƒ Edge port - corresponds to the Cisco PortFast feature
ƒ Non-edge ports are categorized into two link types,
point-to-point and shared.
ƒ Possible RSTP port states: discarding, learning, and
forwarding
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