module (spanning tree)
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Spanning Tree
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Student Objectives
Upon completion of this module, the successful studentwill be able to:Define the spanning tree protocol.
Explain how spanning tree works.
Identify the building blocks of STP.Describe the relationship between ports, VLANs, andthe Spanning Tree Domain.
Configure STP on Extreme Networks switches.
Describe Extreme Networks STP enhancements.Note: Depending on the needs of the students, the instructor may choose
to reduce or eliminate the protocol overview portion of this module.
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Spanning Tree
802.1D - Bridging, Switching andSpanning Tree specification
Developed by DEC
Adopted by the IEEE and is definedin the IEEE 802.1D specification
Allows forredundantbridging
paths
Preventsnetwork
loops fromoccurring
Detects andreroutes any
brokennetwork
paths
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Spanning Tree Algorithm
Ports in the forwarding state are allowed to forwardpackets.
Ports in blocking state participate in spanning tree butdo not forward data packets.
Redundant paths are pruned from the tree
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Reconfiguration after a LinkFailure
Occurs when there is a change in the active topology Link failure Bridge failure Administrative configuration change
Link Lost
LinkRestored
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Spanning Tree Port StatesPorts participating in STP can be in any one of five states:
Blocking Initial state or determined by STP
No forwarding, no MAC address learning
Listening Temporary state preparing for forwarding
Learning
MAC addresses are learned and entered in FDBForwarding
Active port for frame transmission
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How Spanning Tree BPDUs areUsed
A stable active topology is maintained by the root bridge
The root bridge transmits Configuration BPDUs out of all of itsactive ports to determine the least cost path.
When a designated bridge detects a topology change, it sendsout a Topology Change BPDU through it's root port to the root
bridge.Root
Bridge
Root Port
Root Port
Root Port
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Spanning Tree Protocol BuildingBlockSpanning Tree uses the following building blocks:
Bridge ID
Bridge Protocol Data Unit
Root Bridge
Root Port
Designated Bridge
Designated Port
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Selecting the Root BridgeThe Root Bridge controls the STP topology
Root Bridge = Lowest Bridge ID
Bridge ID = Bridge Priority and MAC Address Priority = 0 65,535, Default = 32,768 (80:00 Hex) e.g. Bridge ID = 80:00: 00:01:30:12:34:56
Every bridge transmits CBPDUs claiming to be theroot bridge.
Each bridge saves and transmits the CBPDU
information of the bridge with the lowest Bridge ID If the Bridge IDs are equal, save the information withthe lower MAC address.
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Selecting the Root BridgeContinued
In this example, Switch 1 becomes the Root Bridge
100 1000
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Selecting the Root Port
Each switch except the root bridge selects the RootPort as follows:
Port that provides lowest total cost to root bridge If cost is the same, whichever has lowest port number
Root ports are allowed to forward trafficOnly one root port per spanning tree per bridge
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Selecting the Root PortContinued
In 802.1D, port costs are : 10Mb = 100, 100Mb = 19, 1000Mb = 4, 10Gb = 2
In this example the following root ports are selected: Switch 1 = Root Bridge Switch 2 Root Port = 1 Switch 3 Root Port = 1 Switch 4 Root Port = 3
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Selecting the Designated Bridge
One designated bridge per directly connectedsegment (collision domain)
Provides the lowest path cost to root If the cost is the same, the lowest bridge ID is selected
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Selecting the Designated Port
Ports on a designated bridge that attach to thesegments for which that bridge is the designatedbridge
Designated port is allowed to forward traffic
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Ports in Forwarding State andBlocking State
All root and designated ports are allowed to forward
All other ports are blocked
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Active Topology
Now there is only a single active path
What if an active link fails?
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Detecting Topology Changes
STP detects changes to the active topology in thefollowing ways:
Failure of communications with Root Bridge
The root bridge periodically sends out a configuration BPDU(Type 0) every 2 seconds as a keep alive. If a non-root bridge
does not receive this BPDU within 20 seconds, it presumes tohave lost communications with the root bridge Local topology change / receive topology change BPDU
A topology change triggers a topology change BPDU (Type 80)which is sent out and repeated by all the root ports
If a change in active topology is detected, STP canrecalculate the necessary port states.
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A Change in Active TopologySwitch 4 is not able to exchange frames with the other switches
Switch 2 sends out a topology change BPDU
When the Root bridge receives this BPDU, it changes a flag inthe configuration BPDU triggering re-calculation
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Re-Calculating Port States
All root and designated ports are allowed to forward
All other ports are blocked
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New Active Topology
Switch 4 can now exchange frames with the otherswitches
If the connection between switches 2 and 4 isrestored, another topology change BPDU is releasedtriggering another re-calculation which restores theactive topology to it original state.
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Break
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Planning a Spanning TreeTopology
Diagram the network topology and identify loops.Determine the root bridge, designated bridges, anddesignated ports using default values.
Identify if there is a more desirable topology and whatneeds to be done to implement it.
Test all link and bridge failures on paper to identifyresulting topology.
Identify if there are more desirable topologies andwhat needs to be done to implement it.
Document everything.
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Configuring a Single SpanningTree in dot1w Mode
Configure the default STP domain for 802.1w mode.
configure stpd s0 mode dot1w
Add VLANs to the spanning tree domain.
configure stpd add vlan port
Optionally configure bridge priority to control election of the rootbridge.
configure stpd priority For RSTP and MSTP priorities are in increments of 4096
Optionally configure port cost to control election of designatedbridge and designated port.
configure stpd ports cost
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Configuring STP Continued
Configure the port priority to select the designatedport in case of a tie.configure stpd {} ports priority
Enable the STP protocol for the STPD.enable stpd {}
Note: The default STP domain (STPD) is
named s0.
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Configuring STP Parameters
Typically default values are sufficient
Parameter changes require advanced STPknowledge.
Parameters include: Hello time Forward delay Max Age
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Verifying STP Configuration
If the local BridgeID and the Designated root ID match,
then this switch is a root bridge.
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Verifying STP Ports
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Notes on Spanning TreeConfiguration
The 802.1D ports must be untagged.
A VLAN and port can belong to only one 802.1DSTPD.
A VLAN can only be a member of one 802.1DSTPD.
A physical port can belong to only one 802.1D STPD.
If a port is a member of multiple VLANs, then thoseVLANs must belong to the same 802.1D STPD
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Notes on Spanning TreeOperation
You should remove all VLANs associated with theSTP before deleting the STPD. Because, if youdelete an STPD, member VLANs are automaticallydeleted.
STP, load-sharing, and redundant physical ports/linkswork together.
Blocked ports only process BPDU packets.
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Spanning Tree EnhancementsExtreme Networks switches support the following modes:
802.1D Using IEEE standard 802.1D (1998)
802.1w Rapid Spanning Tree Easy to enable. Provides faster convergence. Compatible with 802.1D
Extreme Multiple Instance Spanning Tree Protocol (EMISTP) Default mode for user created STPD
Per VLAN Spanning Tree (PVST+) For compatibility with third party switches that run this version of
STP
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Spanning Tree EnhancementsContinuedExtreme Networks switches support:
Multiple Spanning Tree Protocol (MSTP) Logically divides a Layer 2 network into regions Each region can contain multiple instances of spanning trees
Uses rapid spanning tree as a converging algorithm and isfully interoperable with earlier versions of STP.
IEEE standard 802.1Q-2004 (previously 802.1s)
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Summary
You should now be able to:
Define the spanning tree protocol.
Explain how spanning tree works.
Identify the building blocks of STP.
Describe the relationship between ports, VLANs, andthe Spanning Tree Domain.
Configure STP on Extreme Networks switches.
Describe Extreme Networks STP enhancements.
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Spanning Tree Configuration Lab
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