Module 5 Spanning Tree Overview and Configuration

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Module 5Spanning Tree Overview and Configuration
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Module Objectives

• Describe Spanning Tree in an Overview
• Define Building Blocks of Spanning Tree
• Describe How Spanning Tree Works
• Bridge Communication Reconfiguration
• Ports, VLANs, and Spanning Tree Domains
• CLI Commands for STP Configuration
• Lab Exercises
• Summary

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Spanning Tree

• Developed by DEC
• Adopted by IEEE 802.1 Network Management
  Committee
• 802.1d - Bridging, Switching Spanning Tree
  specification
• Allows for redundant bridging paths
• Detect and reroute any broken network paths
• Prevent network loops from occurring

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Spanning Tree Algorithm

• Bridged networks must allow for redundancy
• Only one path should be enabled to any
  destination on the network

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Switch Ports in Active ST Topology

• Certain switch ports are allowed to forward
  packets
• Other ports will participate in spanning tree but
  do not forward packets
• Switch ports participating in the active topology
  are called forwarding ports
• Switch port that do not participate are placed in
  blocking mode
• These ports still participate in the spanning
  tree protocol

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Reconfiguration In Spanning Tree

• Occurs when there is a change in the active
  topology

Link Lost
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Building Blocks of Spanning Tree

• Designated Bridge
• Designated Port
• Root Bridge
• Root port
• BPDU - Bridge Protocol Data Unit

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How Spanning Tree WorksRoot Bridge

• One bridge- the one with the highest priority
  (lowest bridge ID) will become the root bridge
• All ports on the root bridge are placed in the
  forwarding state
• This bridge is the root of the spanning tree

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How Spanning Tree WorksRoot Port

• Other bridges select a port with the lowest path
  cost to the root bridge
• This port becomes the root port and is placed in
  the forwarding state
• There is only one root port per spanning tree per
  bridge, regardless of the number of ports in that
  tree.
• EXCEPTION
• ExtremeWare allows for multiple trees per device

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How Spanning Tree WorksDesignated Bridge

• Selected as the bridge that provides a LAN with
  the lowest path cost to the root for that
  individual cable segment.

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How Spanning Tree WorksDesignated Ports

• These ports are placed in the forwarding state
• They are the designated best path to the root
  bridge
• They either have workstations or branch hubs with
  a single connection to the network attached

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How Spanning Tree WorksOther Bridge or Ports

• Those that are not selected are placed in the
  blocking state.
• The blocking state prevents data packets from
  being forwarded on that port
• These ports are still participating in spanning
  tree for future updates

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How Spanning Tree WorksStable Active Topology

• The root bridge transmits configuration messages
  out all of its active ports
• Designated bridges should receive these messages
  on their root port
• Designated bridges should propagate the
  information out all of its designated ports

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Root Bridge Determination

• Each bridge, when initialized will transmit
  topology change messages indicating that it is
  the root bridge.
• All bridges will receive this information and
  each will save the best information.
• The best information is based on three items in
  the topology message
• the Root ID field,
• the Transmitting bridge ID, and
• the cost field.

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Root Bridge Determination (cont)

• A bridge receiving a topology message from a root
  bridge will perform these tests
• If root ID of received message is of higher
  priority than previously saved message (including
  its own),
• it will discard the previously saved message and
  keep the new one.
• If the root IDs are equal, check the cost.
• If the cost is lower, discard previously saved
  message and keep the new one.
• If the root IDs and costs are equal, compare the
  transmitter ID.
• If the new transmitter ID is lower, the
  previously saved message will be discarded and
  the new one kept.

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Building the Spanning Tree Domain
1,0,1
3,0,3
Bridge 1
Bridge 3
1,0,1
3,0,3
4,0,4
Bridge 4
4,0,4
6,0,6
7,0,7
Bridge 6
Bridge 7
6,0,6
7,0,7
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Building the Spanning Tree Domain
Cable segment 4
1,1,3 Root port
Bridge 1
Bridge 3
1,0,1
Designated bridge
Standby Bridge
Root bridge
X
1,0,1
Blocking
1,1,3
Cable segment 3
Root port
Form (Root ID, Cost, Transmitting bridge ID)
1,1,4
Designated bridge
Bridge 4
Designated port
1,1,4
Cable segment 2
Root port
1,2,6
Bridge 6
1,2,7
Root port
Bridge 7
Designated bridge
Standby bridge
Designated port
1,2,6
1,2,7
X
Blocking
Cable segment 1
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Spanning Tree Bridge Communications

• BPDU - Bridge Protocol Data Unit
• Configuration BPDU
• Topology Change BPDU

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Spanning Tree Link Failure

• The spanning tree topology is kept active through
  the transmission of BPDUs
• BPDU - Bridge Protocol Data Unit
• The root bridge will transmit a Configuration
  BPDU on all its active ports every two seconds
• Any bridge that changes the active topology will
  send a Topology Change BPDU message

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Spanning Tree Link Failure

• BPDUs are key to the Max aging timer
• BPDUs reset the Max age timer on each
  participating bridge receiving the BPDU
• The timers max age is 20 seconds
• If 20 seconds expire, the bridge assumes spanning
  tree has failed and the bridge will go into
  convergence state

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Spanning Tree Link Failure (cont)

• Reconfiguration of the active topology will occur
  if a bridge receives a topology change packet
  containing better information than what it has
  previously stored.

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Spanning Tree Link Failure

• Every bridge adjacent to the Root receives the
  BPDU on its root port
• Upon receipt of packet, each switch will
  retransmit its version of the BPDU with
• The root ID will remain the same
• Path Cost incremented by cost associated with
  port the message is forwarded on
• Transmitting bridge ID will be changed
• Message Age field will be set to 0 (zero)

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Extreme Switches and Spanning Tree

• Multiple Domains
• STPD - Spanning Tree Protocol Domain
• S0 - default STPD
• VLANs
• Things to remember

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Multiple Domains and Spanning Tree

• The switch can be partitioned into multiple
  virtual bridges
• Each spanning tree instance in the switch is
  called an STPD
• STPD - Spanning Tree Protocol Domain or just
  Spanning Tree Domain
• Each STPD has its own Root Bridge and active path

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STPD s0 and VLANs

• Default switch configuration contains one STPD
  called s0
• By default, spanning tree is disabled on s0
• Once the STPD is created, one or more VLANs can
  be assigned to it
• Spanning Trees have VLANs as members
• VLANs are assigned to STPDs
• All VLANs are automatically made members of s0
• You cannot delete a VLAN from s0, however, you
  can add it to another STPD

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Ports, VLANs, and STPD

• A VLAN cannot be a member of more than one STPD
• A physical port can belong to only one STPD
• If a port is a member of multiple VLANs, then
  those VLANs must belong to the same STPD

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Spanning Tree Configuration Recommendations

• Draw the physical network topology
• If there is a loop anywhere, some links will get
  blocked
• You must remove all VLANs associated with the STP
  before deleting the STPD
• If you delete an STPD, it will delete its member
  VLANs!!!
• STPD are blind to VLAN membership
• STP and load-sharing will work together
• STP and the redundant physical port/link will
  work well together.

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VLANs and STP - Things To Remember

• Each VLAN forms an independent broadcast domain
• STP blocks paths to create a loop free
  environment
• No data can be transmitted or received on an STP
  blocked port
• Within any given STP Domain (STPD), all VLANs
  belonging to it use the same spanning tree

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STP Configuration CLI Commands

• create/delete stpd
• enable/disable stpd
• enable/disable stpd port
• config stpd add vlan
• config stpd priority
• config stpd port cost
• config stpd port priority
• config stpd hellotime
• config stpd forwarddelay
• config stpd maxage
• unconfig stpd
• show stpd
• show stpd port

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CLI Command - create/delete stpd

• create stpd ltstpd_namegt
• delete stpd ltstpd_namegt
• Creates an STPD. When created, an STPD has the
  following default parameters
• Bridge priority 32,768
• Hello time 2 seconds
• Forward delay 15 seconds
• Remember If you do not remove member VLANs from
  the STPD, those VLANs and their configurations
  will be deleted when the STPD is deleted.

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CLI Command - enable/disable stpd
enable stpd ltstpd_namegt disable stpd ltstpd_namegt
Enables / disables the STP protocol for one or
all STPDs. The default setting is disabled.
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CLI Command - enable/disable stpd port
enable stpd ltstpd_namegt port ltportlistgt disable
stpd ltstpd_namegt port ltportlistgt Enables /
disables the STP protocol on one or more ports.
If STPD is enabled for a port, BPDUs will be
generated on that port if STP is enabled for the
associated STPD. The default setting is enabled.
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CLI Command - config stpd add vlan
config stpd ltstpd_namegt add vlan ltnamegt Adds a
VLAN to the STPD. A VLAN cannot be a member of
more than one spanning tree domain.
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CLI Command - config stpd priority
config stpd ltstpd_namegt priority
ltvaluegt Specifies the bridge priority in the
STPD. By changing the priority, the bridge can be
configured to be more or less likely elected the
Root Bridge. The range is 0 through 65,535. The
default setting is 32,768. A setting of 0
indicates the highest priority.
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CLI Command - config stpd port cost

• config stpd ltstpd_namegt port cost ltvaluegt
  ltportlistgt
• Specifies the path cost of the port in this
  STPD.
• The range is 1 through 65,535. The switch
• automatically assigns a default path cost based
  on the speed of the port, as follows
• For a 10Mbps port, the default cost is 100.
• For a 100Mbps port, the default cost is 19.
• For a 1000Mbps port, the default cost is 4.

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CLI Command - config stpd port priority
config stpd ltstpd_namegt port priority ltvaluegt
ltportlistgt Specifies the priority of the port in
this STPD. By changing the priority of the port,
you can make it more or less likely to become the
Root Port. The range is 0 through 255. The
default setting is 128. A setting of 0 indicates
the lowest priority.
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CLI Command - config stpd hellotime
config stpd ltstpd_namegt hellotime
ltvaluegt Specifies the time delay (in seconds)
between the transmission of BPDUs from this STPD
when it is the Root Bridge. The hellotime
default setting is 2 seconds
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CLI Command - config stpd forwarddelay
config stpd ltstpd_namegt forwarddelay
ltvaluegt Specifies the time (in seconds) that the
ports in this STPD spend in the listening and
learning states when the switch is the Root
Bridge. The range is 4 through 30. The default
setting is 15 seconds.
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CLI Command - config stpd maxage
config stpd ltstpd_namegt maxage
ltvaluegt Specifies the maximum age of a BPDU in
this STPD. The range is 6 through 40. The
default setting is 20 seconds. Note that the time
must be greater than, or equal to 2 X (Hello Time
1) and less than, or equal to 2 X (Forward
Delay 1).
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CLI Command - unconfig stpd
unconfig stpd ltstpd_namegt Restores default STP
values to a particular STPD or to all STPDs.
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CLI Command - show stpd
show stpd ltstpd_namegt Displays STP information
for one or all STP domains.
Stpd s0 Stp ENABLED
Number of Ports 3 Ports 16,17,22 Vlans
Default red blue Bridge Priority 32768 BridgeID
800000e02b03eb00 Designated
root 800000e02b031800 RootPathCost
4 MaxAge 20s HelloTime 2s
ForwardDelay 15s CfgBrMaxAge 20s
CfgBrHelloTime 2s CfgBrForwardDelay
15s Topology Change Time 35s
Hold time 1s Topology Change Detected FALSE
Topology Change TRUE Number of
Topology Changes 0 Time Since Last Topology
Change 9s

If this matches, then this is the ROOT Bridge
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CLI Command - show stpd port
show stpd ltstpd_namegt port ltportlistgt Displays
the STP state of a port.
Summit246 show stpd s0 port 1 Stpd s0
Port 1 PortId 8001 Stp ENABLED Path
Cost 100 Port State FORWARDING
Topology Change Ack FALSE Port Priority
128 Designated Root 0000000000000000
Designated Cost 0 Designated Bridge
0000000000000000 Designated Port Id
0 Press ltSPACEgt to continue or ltQgt to quit
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CLI Command - enable ignore-stp vlan
enable ignore-stp vlan ltnamegt Enables a VLAN
from using STP port information. When enabled,
all virtual ports associated with the VLAN are in
STP forwarding mode. The default setting is
disabled.
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Lab Exercise

• Lab 9 - Spanning Tree Configuration - I
• Lab 10 - Spanning Tree Configuration - II

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Lab 9 - Spanning Tree - I
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Lab 10 - Spanning Tree - II
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Summary

• Describe Spanning Tree in an Overview
• Define related Spanning Tree Terminology
• Describe How Spanning Tree Works
• Bridge Communications Reconfiguration
• SPTD, VLAN and Ports with Spanning Tree
• CLI Commands for STP Configuration
• Lab Exercises