ccna exp3 - chapter05 - stp

8/3/2019 CCNA Exp3 - Chapter05 - STP

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Please purchase a personal license.

CCNA Ex loration 4.0


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Objectives

Explain the role of redundancy in a convergednetwork.

Summarize how STP works to eliminate Layer 2loops in a converged network.

Explain how the STP algorithm uses three steps to

H c vi n m ng Bch Khoa - Website: www.bkacad.com 2

converge on a loop-free topology. Implement rapid PVST+ in a LAN to prevent loops

between redundant switches.


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Redundancy

Redundancy in a hierarchical network Layer 2 redundancy improves the availability of the network by

implementing alternate network paths by adding equipment and

cabling.



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Redundancy

Examine a redundant design In a hierarchical design, redundancy is achieved at the distribution and

core layers through additional hardware and alternate paths throughthe additional hardware.



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Redundancy

Examine a redundant design



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Redundancy




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Redundancy




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Redundancy




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Types of Traffic


Types of traffic (Layer 2 perspective)1. Known Unicast : Destination addresses are in Switch Tables2. Unknown Unicast : Destination addresses are not in Switch Tables3. Multicast : Traffic sent to a group of addresses4. Broadcast : Traffic forwarded out all interfaces except incoming

interface.

Unknown Unicast


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Issues with Redundancy

A redundant switched topology (STP disabled) may cause:1. Layer 2 Loops

2. Broadcast Storms3. Duplicate Unicast Frames4. MAC address table instability



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Layer 2 Loops When multiple paths exist between two devices on the network and

STP has been disabled on those switches, a Layer 2 loop can occur.



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Broadcast Storms 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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Duplicate Unicast Frames Unicast frames sent onto a looped network can result in duplicate

frames arriving at the destination device.



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Incorrectly learn the MAC address



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Real-world Redundancy Issues

Loops in the Wiring Closet If the network cables are not properly labeled when they are terminated

in the patch panel in the wiring closet. Network loops that are a result

of accidental duplicate connections in the wiring closets are a commonoccurrence.



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Real-world Redundancy Issues

Loops in the Cubicles



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Prevent loop, storm bordcast?


Enable Spanning Tree Protocol (STP) 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. A port is considered blocked when networktraffic is prevented from entering or leaving that port.

Block redundant link and auto unblock redundant link when primary linkdown.


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Spanning-Tree Protocol (STP)


STP is a Layer 2 link-management protocol that is used to maintain aloop-free network.

The Spanning-Tree Protocol requires network devices to exchange messagesto detect bridging loops, is called a Bridge Protocol Data Unit (BPDU).

BPDUs continue to be received on blocked ports.


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Spanning-Tree Protocol


STP executes an algorithm called Spanning Tree Algorithm (STA). STA chooses a reference point, called a root bridge, and thendetermines the available paths to that reference point. If more than two paths exists, STA picks the best path and blocks

the rest STP calculations make extensive use of 2 key concepts in creating a

loop-free topology:1. Bridge ID2. Path Cost


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STP Algorithm


1. Root Bridge The lowest BID2. Root Ports - Switch ports closest to the root bridge.3. Designated ports - All non-root ports that are still permitted to forward traffic

on the network.4. Non-designated ports - All ports configured to be in a blocking state to

prevent loops.


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The Root Bridge

Every spanning-tree instance (switched LAN or broadcast domain) hasa switch designated as the root bridge. The root bridge serves as areference point for all spanning-tree calculations to determine which

redundant paths to block.


Lowest Bridge ID valueis the root !


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Extra: The Root Bridge


The root bridge maintains the stability of the forwarding paths between allswitches for a single STP instance.

A spanning tree instance is when all switches exchanging BPDUs andparticipating in spanning tree negotiation are associated with a single root. If this is done for all VLANs, it is called a Common Spanning Tree (CST)

instance. There is also a Per VLAN Spanning Tree (PVST) implementation that

provides one instance, and therefore one root bridge, for each VLAN.


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Bridge ID (BID)


For each Network, the switch with the highest switch priority (thelowest numerical priority value ) is elected as the root switch.

The BID is made up of a priority value , an extended system ID , andthe MAC address of the switch.


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Bridge ID (BID)



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Bridge ID (BID)


Priority-based decision The default value for the priority of all Cisco switches is 32768. The priority range is between 1 and 65536; therefore, 1 is the highest

priority.


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Bridge ID (BID)

MAC Address-based decision



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Configure and Verify the BID



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Configure and Verify the BID



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Best Paths to the Root Bridge

When the root bridge has been designated for the spanning-treeinstance, the STA starts the process of determining the best paths tothe root bridge from all destinations in the broadcast domain.



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Each bridge advertises the spanning tree path cost in the BPDU. Thisspanning tree path cost is the cumulative cost of all the links from the rootbridge to the switch sending the BPDU.

In Figure, switch Y receives a BPDU from the root bridge (switch X) on itsswitch port on the Fast Ethernet segment, and another BPDU on its switch porton the Ethernet segment. The root path cost in both cases is 0. The local path cost on the Fast Ethernet switch port is 19 , while the local

path cost on the Ethernet switch port is 100 .


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Bridge Protocol Data Unit (BPDU)


The BPDUs are transmitted in one direction from the root switch , andeach switch sends configuration BPDUs to communicate and tocompute the STP topology.


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BPDU Field Format

802.3 Header Destination: 01:80:C2:00:00:00 Mcast 802.1d Bridge group Source: 00:D0:C0:F5:18:D1 LLC Length: 38

802.2 Logical Link Control (LLC) Header Dest. SAP: 0x42 802.1 Bridge Spanning Tree Source SAP: 0x42 802.1 Bridge Spanning Tree Command: 0x03 Unnumbered Information

802.1 - Bridge Spanning Tree


ro oco en er:

Protocol Version ID: 0 Message Type: 0x00 Configuration Message Flags: 00000000 Root Priority/ID: 0x8000/ 00:D0:C0:F5:18:C0 Cost Of Path To Root: 0x00000000 (0) Bridge Priority/ID: 0x8000/ 00:D0:C0:F5:18:C0 Port Priority/ID: 0x80/ 0x1D Message Age: 0/256 seconds (exactly 0 seconds) Maximum Age: 5120/256 seconds (exactly 20 seconds) Hello Time: 512/256 seconds (exactly 2 seconds) Forward Delay: 3840/256 seconds (exactly 15 seconds)


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Extra: BPDU Field Format


802.1d uses 2 types of BPDUs: A configuration BPDU , used for initial STP configuraion. Type field=0x00 A topology change notification (TCN) BPDU used for topology changes.

Type field=0x80


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Extra: BPDU Field Format



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The BPDU Process



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The BPDU Process

When the network first starts, all bridges are announcing a chaotic mixof BPDUs.



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The BPDU Process



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The BPDU Process



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The BPDU Process

The BPDU Process



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The BPDU Process



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The BPDU Process



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The BPDU Process


h


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The BPDU Process


Th BPDU P


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The BPDU Process


P R l


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Port Roles


P t R l


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Port Roles

1. Root Port The root port exists on non-root bridges and is the switch port with the

best path to the root bridge.2. Designated Port

The designated port exists on root and non-root bridges. For root bridges , all switch ports are designated ports. For non-root bridges , a designated port is the switch port that receives and

forwards frames toward the root bridge as needed.


Only one designated port is allowed per segment .

3. Non-designated Port The non-designated port is a switch port that is blocked , so it is not

forwarding data frames and not populating the MAC address table withsource addresses.

A non-designated port is not a root port or a designated port.4. Disabled Port The disabled port is a switch port that is administratively shut down . A

disabled port does not function in the spanning-tree process.

C fig P t P i it


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Configure Port Priority

The port priority values range from 0 - 240, in increments of 16. Thedefault port priority value is 128.




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If all ports have the same priority, the port with the lowest port numberforwards frames.

(config-if)# spanning-tree port-priority {number}

(config-if)# spanning-tree port-priority {number}

Port Role Decisions


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Port Role Decisions


Port Role Decisions


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Port Role Decisions


Port Role Decisions


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Port Role Decisions


Port Role Decisions


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Port Role Decisions


Port Role Decisions


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Port Role Decisions


Port Role Decisions


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Port Role Decisions


Port Role Decisions


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Port Role Decisions


Port Roles - Summary


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Port Roles Summary


STP Port States and BPDU Timers


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Port States1. Blocking - The port is a non-designated port and does not participate

in frame forwarding. The port receives BPDU frames to determine thelocation and root ID of the root bridge switch and what port roles eachswitch port should assume in the final active STP topology.

2. Listening - STP has determined that the port can participate in frameforwarding according to the BPDU frames that the switch has receivedthus far. At this point, the switch port is not only receiving BPDU


adjacent switches that the switch port is preparing to participate in theactive topology.

3. Learning - The port prepares to participate in frame forwarding andbegins to populate the MAC address table. The port is still sendingand receiving BPDUs .

4. Forwarding - The port is considered part of the active topology andforwards frames and also sends and receives BPDU frames.

5. Disabled - The Layer 2 port does not participate in spanning tree anddoes not forward frames. The disabled state is set when the switchport is administratively disabled.



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If a bridge thinks it is the Root Bridge immediately after booting or in the absence of BPDUs for a certain period of time, the port transitions into the Listening state.



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The BPDU timers not be adjusted directly because the values have beenoptimized for the seven-switch diameter.

Adjusting the spanning-tree diameter value on the root bridge to a lower valueautomatically adjusts the forward delay and maximum age timers proportionallyfor the new diameter.

Extra: BPDU Timers


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Hello timer: Determines how often root bridge sendsconfiguration BPDUs . The default is 2 seconds.

Maximum A e Max A e : Tells the brid e how lon to


keep ports in the blocking state before listening. Thedefault is 20 seconds. Forward Delay (Fwd Delay): Determines how long to stay

in the listening state before going to the learning state, andhow long to stay in the learning state before forwarding.The default is 15 seconds.

Extra: STP Timers


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Cisco PortFast Technology PortFast is a Cisco technology . When a switch port configured with PortFast is

configured as an access port, that port transitions from blocking to forwardingstate immediately, bypassing the typical STP listening and learning states.


STP Convergence


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The STP algorithm uses three simple steps to converge on a loop-freetopology.

Switches go through three steps for their initial convergence:STP ConvergenceSTP Convergence

Step 1 Elect one Root BridgeStep 2 Elect Root PortsStep 3 Elect Designated Ports


All STP decisions are based on a the following predeterminedsequence:FourFour- -Step decision SequenceStep decision SequenceStep 1 - Lowest BIDStep 2 - Lowest Path Cost to Root BridgeStep 3 - Lowest Sender BIDStep 4 - Lowest Port ID

STP Convergence


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STP ConvergenceSTP ConvergenceStep 1 Elect one Root BridgeStep 2 Elect Root PortsStep 3 Elect Designated Ports


Step 1. Elect one Root Bridge


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Verify Root Bridge Election


STP Convergence


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STP ConvergenceSTP ConvergenceStep 1 Elect one Root Bridge

Step 2 Elect Root PortsStep 3 Elect Designated Ports


Step 2. Elect Root Ports


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Verify the Root Port


STP Convergence


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STP ConvergenceSTP ConvergenceStep 1 Elect one Root Bridge

Step 2 Elect Root PortsStep 3 Elect Designated Ports


Step 3. Electing Designated Ports and Non-Designated Ports


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Verify DP and Non-DP


STP Topology Change


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Topology change notification (TCN) Topology change acknowledgement (TCA) Topology change (TC)


designated bridge

In legacy STP, TCNs weregenerated for any active port that was not configured for PortFast.

STP Topology Change


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When the Root Bridge receives the (upstream) topology-changemessage (TCN BPDU), it sends out Configuration BPDUs to indicatethat a topology change is occurring (using the low-order bit in the Flagfield).

The Root Bridge sets the topology change in the configuration for aperiod of time equal to the sum of the Forward Delay and Max Ageparameters ( 20s+15s= 35s )


A bridge receiving a (downstream) topology change configurationmessage from the Root Bridge will use the Forward Delay timer (15seconds) to age out entries in the address table . This allows the device to age out entries faster than the normal 5-

minute default so that stations no longer available are aged out

faster. The bridge continues this process until it no longer receivestopology change configuration messages from the Root Bridge.

STP (802.1D) Enhancements


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UplinkFast is an access-layer STP solution that providesfast failover when the root port or root switch fails.

BackboneFast is a distribution and access-layer STPsolution that provides fast convergence in the network forindirect link failures.

-


port to enter the spanning tree forwarding stateimmediately, bypassing the listening and learning states.

Extra: PortFast


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(config-if)# spanning-tree portfast

(config)# spanning-tree portfast default Enable portfast by default on all access ports


When a device is connected to a port, the port normally enters the spanning tree Listening state.When the Forward Delay timer expires, the port enters the Learning state. When the Forward Delaytimer expires a second time, the port is transitioned to the Forwarding or Blocking state.

When PortFast is enabled on a switch or trunk port, the port is immediately transitioned to theForwarding state. As soon as the switch detects the link, the port is transitioned to the Forwardingstate ( less than 2 seconds after the cable is plugged in).

Extra: PortFast


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Extra: UplinkFast


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STP UplinkFast accelerates the choice of a new Root Port when a link or switch fails orwhen STP reconfigures itself. The Root Port transitions to the Forwarding stateimmediately without going through the Listening and Learning states , as it would withthe usual STP process.

UplinkFast also limits the burst of multicast traffic by reducing the max-update-rate . ForIOS the default for this parameter is 150 packets per second.

This change takes approximately 1 to 5 seconds

(config)# spanning-tree uplinkfast

Di bl U li kF

Extra: UplinkFast


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Disable UplinkFast


Enable UplinkFast

CAM T bl U d

Extra: UplinkFast


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CAM Table Update


Switch A begins to flood dummy packetswith the different MAC addresses

that it has in its CAM table as a source.

Extra: BackboneFast


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BackboneFast is a Catalyst feature that is initiated when a Root Port or Blocked port on a switchreceives inferior BPDUs from its Designated Bridge. An inferior BPDU identifies one switch as boththe Root Bridge and the Designated Bridge . When a switch receives an inferior BPDU, it means that alink to which the switch is not directly connected (an indirect link) has failed. That is, the DesignatedBridge has lost its connection to the Root Bridge. Under STP rules, the switch ignores inferior BPDUsfor the configured Max Age (the default is 20 seconds).

This switchover takes approximately 30 seconds , twice the Forward Delay time if the default ForwardDelay time of 15 seconds is set. This saves up to 20 seconds.

(config)# spanning-tree backbonefast

Extra: BackboneFast


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Other Example


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PVST+ RSTP d R id PVST+


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PVST+, RSTP and Rapid PVST+


Cisco and STP Variants


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PVST+

Cisco developed PVST+ so that a network can run an STP instance for


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peach VLAN in the network. And Creating different STP root switchesper VLAN creates a more redundant network.

With PVST+, more than one trunk can block for a VLAN and loadsharing can be implemented.


PVST+

PVST+ Bridge ID


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The following provides more details on the PVST+ fields: Bridge priority - A 4-bit field carries the bridge priority Extended system ID - A 12-bit field carrying the VID for PVST+.

MAC address - A 6-byte field with the MAC address of a single switch.


PVST+


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Default Switch Configuration

The table shows the default spanning-tree configuration for a Cisco


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Catalyst 2960 series switch. Notice that the default spanning-treemode is PVST+.


Configure PVST+


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Configure PVST+


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RSTP

What is RSTP?S ( 802 1 ) i l i f h 802 1 d d S


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RSTP (IEEE 802.1w) is an evolution of the 802.1D standard. RSTP canachieve much faster convergence in a properly configured network, sometimesin as little as a few hundred milliseconds.


If a port is configured to be an alternate or a backup port it can immediately change to a forwarding state without waiting for thenetwork to converge.

RSTP


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The immediate consideration with STP is convergence time. Depending on the type offailure, it takes anywhere from 30 to 50 seconds to converge the network. RSTP helps with convergence issues that plague legacy STP.

RSTP has additional features similar to UplinkFast and BackboneFast that offer betterrecovery at Layer 2.

RSTP is proactive and therefore negates the need for the 802.1D delay timers . RSTP (802.1w) supersedes 802.1D, while still retaining backward compatibility. Much of

the 802.1D terminology remains, and most parameters are unchanged. In addition,802.1w is capable of reverting back to 802.1D to interoperate with legacy switches on aper-port basis.

Because the RSTP and Cisco-proprietary enhancements are functionally similar,features such as UplinkFast and BackboneFast are not compatible with RSTP .

RTSP BPDU

RSTP (802.1w) uses type 2, version 2 BPDUs, so an RSTP bridge can communicate 802.1D on anyshared link or with any switch running 802.1D.


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Because BPDUs are used as a keepalive mechanism, 3 consecutively missed BPDUs indicatelost connectivity between a bridge and its neighboring root or designated bridge.


Extra: Examining the RSTP BPDU


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RSTP sends BPDUs and populates the flag byte in a slightly different manner than802.1D:


(2 seconds by default), even if it does not receive any BPDUs from the root bridge .

Protocol information can be immediately aged on a port if hellos are not received forthree consecutive hello times or if the max age timer expires . Because BPDUs are now used as a keepalive mechanism, three consecutively

missed BPDUs indicate lost connectivity between a bridge and its neighboring rootor designated bridge. This fast aging of the information allows quick failuredetection .

Unlike in legacy STP, each switch generates its own BPDUs regardless if it hearsBPDUs from the root. In legacy STP, BPDUs were only generated by the root and propagated throughout thespanning tree domain. As a result, when a switch did not receive a configuration BPDU,it did not know where the failure occurred.

In RSTP mode, the switch needs to worry only about its immediate neighbors.

Edge Ports

Ports not participating in spanning tree are called edge ports.

The edge port concept is already well known to Cisco spanning tree users, as it basicallyd t th P tF t f t


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corresponds to the PortFast feature. All ports directly connected to end stations cannot create bridging loops in the network.

Therefore, the edge port directly transitions to the forwarding state, and skips thelistening and learning stages .


Unlike PortFast, an RSTP edge port that receives a BPDU loses its edge port statusimmediately and becomes a normal spanning-tree port.

The edge port immediately becomes a non-edge port if a BPDU is heard on the port.

Extra: Explaining Edge Ports


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Unlike PortFast, an edge port that receives a BPDU loses its edge portstatus immediately and becomes a normal spanning tree port. When aswitch with an edge port receives a BPDU, it generates a TCN.

Link Types

Non-edge ports are categorized into 2 link types:1. point-to-point2 shared


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2. shared. The link type is automatically derived from the duplex mode of a port. A port that operates in

fullduplex is assumed to be pointtopoint , while a halfduplex port is considered as a shared portby default.

RSTP can only achieve rapid transition to the forwarding state on edge ports and on pointtopointlinks .

Non-edge ports participate in the spanning tree algorithm and only non-edge ports generate to olo chan es TCs on the network when transitionin to forwardin state. TCs are not


generated for any other RSTP states.

Extra: Describing RSTP Link Types


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Root ports do not use the link type parameter . Root ports are able to make a rapidtransition to the forwarding state as soon as the port is in sync. In addition, alternate and backup ports do not use the link type parameter in most cases . Designated ports make the most use of the link type parameter . Rapid transition to the

forwarding state for the designated port occurs only if the link type parameter indicates apoint-to-point link.

RSTP Port States

RSTP provides rapid convergence following a failure or during re-establishment of aswitch, switch port, or link. A RSTP t l g h g t iti i th i t it h t t th


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An RSTP topology change causes a transition in the appropriate switch ports to theforwarding state through either explicit handshakes or a proposal and agreementprocess and synchronization.

With RSTP, the role of a port is separated from the state of a port . For example, adesignated port could be in the discarding state temporarily, even though its final state isto be forwarding.


RSTP Port Roles


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The role is now a variable assigned to a given port. The root port and designated port roles remain. The blocking port role is now split into the backup and alternate port roles. The Spanning Tree Algorithm (STA) determines the role of a port based on

Bridge Protocol Data Units (BPDUs). To keep things simple, the thing to remember about a BPDU is that there is

always a way of comparing any two of them and deciding whether one is moreuseful than the other.

This is based on the value stored in the BPDU and occasionally on the port onwhich they are received.

RSTP Port Roles


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Alternate Port


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An alternate port provides


an a ternate pat to t e rootbridge and could thereforereplace the root port should itfail.

Backup Port


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A backup port provides


segment and cannot guarantee an

alternate connectivity to the rootbridge. It was therefore excludedfrom the uplink group.

RSTP Proposal or Agreement Process


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In IEEE 802.1D STP, when a port has been selected by spanning tree tobecome a designated port, it must wait two times the forward delay beforetransitioning the port to the forwarding state.

RSTP significantly speeds up the recalculation process after a topologychange, because it converges on a link-by-link basis and does not rely ontimers expiring before ports can transition. Rapid transition to the forwarding state can only be achieved on edge ports

and point-to-point links.


A port is in-sync if it meets eitherof the following criteria:


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It is in a Blocking state(which means discarding, ina stable topology).

It is an edge port.




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Switch A has a path to the root via switch B and switch C. A new link is then createdbetween the root and switch A, and both ports are in designated blocking state untilthey receive a BPDU from their counterpart. When a designated port is in a discardingor learning state (and only in this case), it sets the proposal bit on the BPDUs it sendsout. This is what happens for port P0 of the root bridge.

Switch A sees that the proposal BPDU has a superior path cost. It blocks all non-edgedesignated ports other than the one over which the proposal-agreement process are

occurring. This operation is called sync and prevents switches below A from causing aloop during the proposal-agreement process. Edge ports do not have to be blockedand remain unchanged during sync.

Bridge A sends an agreement that allows the root bridge to put root port P0 inforwarding state. Port P1 becomes the root port for A.


Animation 5.4.6.3


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Configuring rapid PVST+


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Design STP for Trouble Avoidance

Know Where the Root Is You now know that the primary function of the STA is to break loops that redundant links

create in bridge networks.


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g Generally, choose a powerful bridge in the middle of the network . If you put the root

bridge in the center of the network with a direct connection to the servers and routers,

you reduce the average distance from the clients to the servers and routers.



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VTP Pruning Only switch D1 receives unnecessary broadcast and multicast traffic for VLAN

20, but it is also blocking one of its ports for VLAN 30.


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, g p The are three redundant paths between core switch C1 and core switch C2.

This redundancy results in more blocked ports and a higher likelihood of aloop.



Manual Pruning VTP pruning can help, but this feature is not necessary in the core of the

network.


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network. Only one port is blocked per VLAN.



What is Layer 3 Switch? Layer 3 switching means routing approximately at the speed of

switching A router performs two main functions:


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switching. A router performs two main functions: It builds a forwarding table. The router generally exchanges

information with peers by way of routing protocols. It receives packets and forwards them to the correct interface


High-end Cisco Layer 3 switches are now able to perform this second

function, at the same speed as the Layer 2 switching function. In thefigure: There is no speed penalty with the routing hop and an additional

segment between C1 and C2.

Core switch C1 and core switch C2 are Layer 3 switches. VLAN 20and VLAN 30 are no longer bridged between C1 and C2, so thereis no possibility for a loop.



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Use Layer 3 Switching The design ensures a convergence that is even faster than convergence with STP.

STP no longer blocks any single port, so there is no potential for a bridging loop. Leaving the VLAN by Layer 3 switching is as fast as bridging inside the VLAN.



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Troubleshoot STP Operation


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Switch or Link Failure

(Animation 5.4.9.1) For some reason port F0/3 on switch S2 fails to receive BPDUs for the Default

max_age time of 20 seconds.


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(Animation 5.4.9.1) For some reason port F0/3 on switch S2 fails to receive BPDUs for the Default

max_age time of 20 seconds.


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Problem During normal operation, bridge B is designated on the link BC.

Bridge B sends BPDUs down to C, which is blocking the port. The


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g g pport is blocked while C sees BPDUs from B on that link.

Now, consider what happens if the link BC fails in the direction ofC. C stops receiving traffic from B, however, B still receives trafficfrom C.



Solution: Uses Unidirectional Link Detection feature on switch.


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PortFast Configuration Error

Problem You typically enable PortFast only for a port or interface that connects

to a host. When the link comes up on this port, the bridge skips the first


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stages of the STA and directly transitions to the forwarding mode.


PortFast Configuration Error

Solution To prevent this situation, most Catalyst switches that run Cisco IOS softwarehave a feature called BPDU guard . BPDU guard disables a PortFast-configured port or interface if the port or interface receives a BPDU.


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g p p


Network Diameter Issues

The conservative default values for the STP timers impose a maximum network diameterof seven . When a BPDU propagates from the root bridge toward the leaves of the tree, the age

field increments each time the BPDU goes though a switch . Eventually, the switch


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discards the BPDU when the age field goes beyond maximum age.


Extra: STP Vulnerabilities


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Activity


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Activity


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Lab - Basic Spanning Tree Protocol


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Challenge Lab Spanning Tree Protocol


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Troubleshooting Spanning Tree Protocol


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Summary


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ccna exp3 - chapter05 - stp

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