module (spanning tree)

8/12/2019 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

100

100 100

1000

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Root1 2

3 4


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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

19

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1 2

3 4R

R

R

Root


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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

19

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1 2

3 4R

R

R

D

D

D D D

D

Root


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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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3 4R

R

R

D

D

D D D

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Root


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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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3 4R

R

R

DD

D D D

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Root


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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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Break


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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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1 2

3 4R

RD

DD D

D

R

Root


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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

module (spanning tree)

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