JEX 11.a C4 SpanningTree

Junos Enterprise Switching

Chapter 4: Spanning Tree

© 2011 Juniper Networks, Inc. All rights reserved. | www.juniper.net | Worldwide Education Services

Chapter Objectives  After successfully completing this chapter, you will be

able to: •Explain when a spanning tree is required •Describe STP and RSTP operations •List some advantages of using RSTP over STP •Configure and monitor RSTP •Describe the BPDU, loop, and root protection features •Configure and monitor the BPDU, loop, and root protection features

© 2011 Juniper Networks, Inc. All r ights reserved.

Worldwide Education Services

www.junip er.net | 4-2

Agenda: Spanning Tree Spanning Tree Protocol  Rapid Spanning Tree Protocol  Configuring and Monitoring RSTP  Protection Features

•BPDU Protection •Loop Protection •Root Protection




Test Your Knowledge  What will Switch-1 and Switch-2 do if they receive a

broadcast frame or a frame destined to an unknown MAC address?

User A MAC: 00:26:88:02:74:86

Switch-1

Switch-2

User B MAC: 00:26:88:02:74:87

User C MAC: 00:26:88:02:74:88

User D MAC: 00:26:88:02:74:89

Both switches would flood the frames out all ports except the port on which the frames arrived




What If …?  What if a broadcast frame or a frame with an

unknown destination MAC address were sent into a Layer 2 network with redundant paths?

User A MAC: 00:26:88:02:74:86

Switch-1

Switch-2

User B MAC: 00:26:88:02:74:87

User C MAC: 00:26:88:02:74:88

User D MAC: 00:26:88:02:74:89

Switch-3

User E MAC: 00:26:88:02:74:90


User F MAC: 00:26:88:02:74:91



Spanning Tree Protocol  STP

•Defined in the IEEE 802.1D-1998 specification •Builds loop-free paths in redundant Layer 2 networks •Automatically rebuilds tree when topology changes Switch-1

Host B

Host A Switch-2


Switch-3



How Does it Work?  Steps for creating a spanning tree include:

1. Switches exchange bridge protocol data units (BPDUs) 2. Root bridge is elected 3. Port role and state are determined 4. Tree is fully converged Switch-1

Switch-2

Switch-1 (Root Bridge)

Switch-3


Switch-2


Switch-3


Terms and Concepts (1 of 2)  Key terms and concepts of STP:

• Bridge ID: Unique identifier for each switch • Root bridge: Switch with the lowest bridge ID • Root port: The port on each bridge closest to the root bridge • Root : A bridge’s calculated cost to get from itself to thepath root cost bridge • Equal to the received root path cost from configuration BPDUs plus the port cost of the root port on the bridge

• Port cost: Every interface on a bridge has an assigned port cost value • Used in the calculation of the root path cost for the local bridge • Configurable value (1–200000000) • The default value is 20000 for 1 Gigabit Ethernet © 2011 Juniper Networks, Inc. All r ights reserved.



Terms and Concepts (2 of 2)  Key terms and concepts of STP (contd.):

• Designated bridge: A switch representing the LAN segment • Port ID: A unique identifier for each port on each switch • Designated port: The designated bridge’s forwarding port on a LAN segment

• The port used by a designated bridge to send traffic from the direction of the root to the LAN or from the LAN toward the root

• Bridge protocol data unit: Packets used to exchange information between switches • Configuration BPDU • Topology change notification BPDU




Port States  Each individual port of each bridge can be in one of

four states: •Blocking • The port drops all data packets and listens to BPDUs • The port is not used in active topology

•Listening • The port drops all data packets and listens to BPDUs • The port is transitioning and will be used in active topology

•Learning • The port drops all data packets and listens to BPDUs • The port is transitioning and the switch is learning MAC addresses

•Forwarding • The port receives and forwards data packets and sends and receives BPDUs • The port has transitioned and the switch continues to learn MAC addresses © 2011 Juniper Networks, Inc. All r ights reserved.



BPDU—Ethernet Frame Format  Ethernet Frame : The bridge group address (01:80:C2:00:00:00)

: MAC of the outgoing port of the srcinating switch

DSAP and SSAP = 0x42 (Bridge Spanning Tree Protocol)

 BPDU types:

•Configuration BPDUs • Used to build the spanning-tree topology

•Topology change notification (TCN) BPDUs • Reports topology changes © 2011 Juniper Networks, Inc. All r ights reserved.



BPDU Format Octets Protocol ID

0x00 (Configuration BPDU) 0x80 (TCN BPDU)

1

BPDU Type Flags

1 1

Root ID

Used as the tiebreaker to determine the designated port, the root port, or both for a LAN (lower is better)

2

Protocol Version

Root Path Cost

A unique ID of the bridge that the transmitting bridge believes to be the root 8

4

The ID of the transmitting port

1

The priority of becoming the root bridge, the designated bridge, or both (lower is better)

2 Bridge ID

8

Port ID

2

6

The unique MAC address of the bridge itself

1


Message Age

2

Max Age

2

Hello Time

2

Forward Delay

2



Building a Spanning Tree (1 of 3)  Switches exchange configuration BPDUs: •They do not flood—instead each bridge uses information in the received BPDUs to generate its own  Root bridge is elected based on BPDU information: •Criterion for election is the bridge ID • The election process reviews priority first—lowest priority wins • If the priority values are the same, bridge addresses (MAC) are compared—the lowest identifier wins Switch-1 (Root Bridge)

Switches initially exchange configuration BPDUs, claiming themselves as the root bridge.

Switch-1 is elected as the root bridge based on the received configuration BPDU information.

Host B

Host A Switch-2 © 2011 Juniper Networks, Inc. All r ights reserved.

Switch-3 Worldwide Education Services


Building a Spanning Tree (2 of 3)  Least-cost path calculation to root bridge determines

port role; port role determines port state: All ports on root bridge assume designated port role and forwarding state Root ports on switches are placed in the forwarding state; root bridge has no root ports Designated ports on designated bridges are placed in the forwarding state All other ports are placed in the blocking state Switch-1 (Root Bridge)

Host A

Host B Switch-2




Building a Spanning Tree (3 of 3)  The tree is fully converged

•All traffic between Host A to Host B flows through the root bridge (Switch-1)


Host B

Host A Switch-2


Switch-3



Reconvergence Example (1 of 2)  Steps:

1. Switch G fails 2. Switch E’s port leaves forwarding state 3. Switch E sends TCNs out root port every 2 seconds until E’s root port receives TCN ACK (configuration BPDU) 4. Switch B sends TCN ACK 5. Switch B sends TCN out root port 6. Switch A sends TCN ACK

Port leaves forwarding state Switch fails




Reconvergence Example (2 of 2)  Steps (contd.):

7. The root bridge sets the topology change flag and sends an updated configuration BPDU 8. Switches B and C relay the topology change flag to downstream switches 9. All nonroot bridges change the MAC address forwarding table aging timer to equal the forwarding delay time (default: 15 seconds)

MAC Fwd Table Aging Time: 15 Sec








Agenda: Spanning Tree  Spanning Tree Protocol Rapid Spanning Tree Protocol  Configuring and Monitoring RSTP  Protection Features





STP Drawbacks  Slow convergence time

•STP uses timers to transition between port states • STP can take 30 to 50 seconds to respond to a topology change (20 seconds for a BPDU to age out, 15 seconds for the listening state, and 15 seconds for the learning state)

•Root bridge is responsible for communicating the current tree topology




Rapid Spanning Tree Protocol  RSTP was first defined in IEEE 802.1w and later

incorporated into IEEE 802.1D-2004  Convergence improvements:

•Point-to-point link designation •Edge port designation • A port that connects to a LAN with no other bridges attached • It is always in the forwarding state

•Allows for rapid recovery from failures • A new root port or designated port can transition to forwarding without waiting for the protocol timers to expire

•Direct and indirect link failure and recovery




RSTP Port Roles  RSTP introduces new port roles:


•Alternate port: D D D D

• Provides an alternate path to the root bridge (essentially a backup root port) • Blocks traffic while receiving superior BPDUs from a neighboring switch

R A

R A

D B

A A

Switch-2

Switch-3

•Backup port: • Provides a redundant path to a segment (on designated switches only) • Blocks traffic while a more preferred port functions as the designated port

 RSTP continues to use the root

and designated port roles © 2011 Juniper Networks, Inc. All r ights reserved.


R D A B


STP and RSTP Port States  RSTP uses fewer states than STP but has the same

functionality

Alternate Backup, and Disabled Ports

Blocking Discarding Listening Learning

Learning

Forwarding

Forwarding



Root and Designated Ports


Rapid Spanning Tree BPDUs  Rapid Spanning Tree BPDUs:

•Act as keepalives • RSTP-designated ports send Configuration BPDUs every hello time (default of 2 seconds)

•Provide faster failure detection • If a neighboring bridge receives no BPDU within 3 times the hello interval (3 x 2 = 6 seconds), connectivity to the neighbor is faulty Switch-1 (Root Bridge)

DD DD

Switch-2


R A

R A

D B

A A

Switch-3



RST BPDU Format Octets Protocol ID

2

Protocol Version BPDU Type Flags

1 1

Root ID

8

 RST BPDU fields that differ from STP:

1

•Protocol Version—0x02 (IEEE 802.1D-2004) •BPDU Type—0x02 (RST BPDU) •Flags

Root Path Cost

4

• Topology Change Acknowledgement Flag (Bit 8) • Agreement Flag (Bit 7)

Bridge ID

8

• Forwarding Flag (Bit 6) • Learning Flag (Bit 5)

Port ID

2

Message Age

2

Max Age

2

Hello Time

2

Forward Delay Version 1 Length

2 2


• Port Role (Bits 3 and 4) • Proposal Flag (Bit 2) • Topology Change Flag (Bit 1)

•Version 1 Length—0x0000 Worldwide Education Services


Transitioning to the Forwarding State  STP:

•Takes 30 seconds before the ports start forwarding traffic after port enablement • 2x forwarding delay (listening + learning)



RSTP:

•Uses a proposal-and-agreement handshake on point-topoint links instead of timers • Exceptions are alternate ports that immediately transition to root, and edge ports that immediately transition to the forwarding state • Nonedge-designated ports transition to the forwarding state once they receive explicit agreement




Topology Change Reconvergence  Topology changes occur only when nonedge ports

transition to the forwarding state: •Port transitions to the discarding state no longer trigger the STP TCN/TCN Acknowledgment sequence •The initiator sends RSTP TCNs (RST BPDU with TCN flag set) out of all designated ports as well as out of the root port •Because of the received RSTP TCN, switches flush the majority of MAC addresses in the bridge table • Switches do not flush MAC addresses learned from edge ports • Switches do not flush MAC addresses learned on port receiving TCN




Indirect Link Failure  When an indirect link failure occurs:

•Switch-2’s root port fails—it assumes it is the new root •Switch-3 receives inferior BPDUs from Switch-2—it moves the alternate port to the designated port role •Switch-2 receives superior BPDUs, knows it is not the root, and designates the port connecting to Switch-3 as the root port Note: The failure is from the perspective of Switch-3

Before


After


Inferior PDU

Switch-2

Superior PDU

Switch-3



Switch-3 www.junip er.net | 4-28

Direct Link Failure  When a direct link failure occurs:

•Alternate port transitions to forwarding state and assumes root port role following the failure of the old root port •Switch-3 signals upstream switches to flush their MAC tables by sending RSTP TCNs out new root port • Upstream switches only flush MAC entries that they learned on active ports that did not receive the RSTP TCNs (except edge ports) Before


Switch-2

Switch-3


After



Note: The failure is from the perspective of Switch-3


RSTP Interoperability with STP  STP and RSTP interoperability considerations:

•If a switch supports only the STP protocol, it discards any RSTP BPDUs it receives •If an RSTP-capable switch receives BPDUs, it reverts to STP mode on the receiving interface only and sends STP BPDUs

Switch-1

Switch-2

Switch-3

Protocol Version—0 (STP)

Protocol Version—0x02 (RSTP)

Protocol Version—0x02 (RSTP)




Agenda: Spanning Tree  Spanning Tree Protocol  Rapid Spanning Tree Protocol Configuring and Monitoring RSTP  Protection Features





Configuring RSTP [edit protocols rstp] user@switch# show bridge-priority 32k; max-age 20; hello-time 2; forward-delay 15; interface ge-0/0/10.0 { disable; } interface ge-0/0/13.0 { cost 20000; mode point-to-point; } interface ge-0/0/14.0 { priority 128; mode shared; } interface ge-0/0/2.0 { edge; }


Default RSTP settings

Excludes interface from participating in RSTP

Default cost value for interfaces operating at 1 Gbps Default interface mode for interfaces operating in full-duplex mode Default priority value (used to influence downstream device’s least -cost path calculation to root bridge—lower is better) Default interface mode for interfaces operating in half-duplex mode

Default value for interfaces that do not connect to STP-enabled devices



Monitoring STP and RSTP (1 of 2) user@switch> show spanning-tree ? Possible completions: bridge Show STP bridge parameters interface Show STP interface parameters mstp Show Multiple Spanning Tree Protocol information statistics Show STP statistics user@switch> show spanning-tree bridge STP bridge parameters Context ID Enabled protocol Root ID Root cost Root port Hello time Maximum age Forward delay Message age Number of topology changes Time since last topology change Local parameters Bridge ID Extended system ID Internal instance ID © 2011 Juniper Networks, Inc. All r ights reserved.

: : : : : : : : : : :

Root Bridge’s ID

0 RSTP Cumulative Cost to 4096.00:19:e2:55:36:00 Root Bridge 40000 ge-0/0/13.0 Root Port 2 seconds 20 seconds 15 seconds 2 Local Device’s Bridge ID 2 72 seconds

: 32768.00:19:e2:55:1d:40 : 0 : 0 Worldwide Education Services


Monitoring STP and RSTP (2 of 2) user@switch> show spanning-tree interface Spanning tree interface parameters for instance 0 Interface

Port ID

ge-0/0/10.0 ge-0/0/11.0 ge-0/0/12.0

128:523 128:524 128:525

Designated port ID 128:523 128:524 128:525

ge-0/0/13.0 ge-0/0/14.0 ge-0/0/15.0

128:526 128:527 128:528

128:526 128:527 128:528

Designated bridge ID 32768.0019e2507c00 32768.0019e2507c00 32768.0019e2507c00

Port Cost 20000 20000 20000

State

Role

BLK BLK BLK

ALT ALT ALT

32768.0019e2503fe0 32768.0019e2503fe0 32768.0019e2503fe0

20000 20000 20000

FWD BLK BLK

ROOT ALT ALT

user@switch> show spanning-tree statistics interface Interface ge-0/0/10.0 ge-0/0/11.0 ge-0/0/12.0 ge-0/0/13.0 ge-0/0/14.0 ge-0/0/15.0

BPDUs sent 7 7 7 7 7 7


BPDUs received 5 5 5 4 5 5

Next BPDU transmission 0 0 0 0 0 0



Test Your Knowledge (1 of 4)  Which switch will be elected the root bridge? {master:0}[edit protocols rstp] user@Switch-2# show bridge-priority 8k; interface ge-0/0/10.0 { cost 1; } interface all {

{master:0}[edit protocols rstp] user@Switch-1# show bridge-priority 4k; interface ge-0/0/8.0 { cost 1; } interface all {

priority 128; cost 200000;

Switch-1

ge-0/0/1.0

}

}

{master:0}[edit protocols rstp] user@Switch-3# show bridge-priority 32k; interface all { priority 16; cost 2000; }


priority 16; cost 20000;

Switch-2

.0 8 / 0 / -0 e g

Switch-3

.0 8 / 0 / -0 e g

ge-0/0/12.0

Switch-4




Test Your Knowledge (2 of 4)  What role and state will be assigned to the various

switch ports? {master:0}[edit protocols rstp] user@Switch-1# show bridge-priority 4k; interface ge-0/0/8.0 { cost 1; } interface all { priority 128; cost 200000; }



Switch-1

Switch-2 ge-0/0/1.0

.0 8 / 0 / -0 e g

Switch-3

.0 8 / 0 / -0 e g

ge-0/0/12.0

Switch-4

{master:0}[edit protocols rstp] user@Switch-2# show bridge-priority 8k; interface ge-0/0/10.0 { cost 1; } interface all { priority 16; cost 20000; }




Test Your Knowledge (3 of 4)  Assume ge-0/0/8 on Switch-1 has failed, what role

and state will be assigned to the remaining ports? {master:0}[edit protocols rstp] user@Switch-1# show bridge-priority 4k; interface ge-0/0/8.0 { cost 1; } interface all { priority 128; cost 200000; }



Switch-1

Switch-2 ge-0/0/1.0

.0 8 / 0 / -0 e g

Switch-3

.0 8 / 0 / -0 e g

ge-0/0/12.0

Switch-4

{master:0}[edit protocols rstp] user@Switch-2# show bridge-priority 8k; interface ge-0/0/10.0 { cost 1; } interface all { priority 16; cost 20000; }




Test Your Knowledge (4 of 4)  Based on the modified configurations, what role and

state will be assigned to Switch-4’s ports? {master:0}[edit protocols rstp] user@Switch-1# show bridge-priority 4k; interface all { priority 128; }

Switch-1

Switch-2 ge-0/0/1.0

cost 20000;

} .0 8 / 0 / -0 e g



{master:0}[edit protocols rstp] user@Switch-2# show bridge-priority 32k; interface all { priority 16;

Switch-3

cost 20000;

.0 8 / 0 / -0 e g

ge-0/0/12.0

Switch-4

{master:0}[edit protocols rstp] user@Switch-4# show bridge-priority 36k; interface ge-0/0/8.0 { priority 32; } interface ge-0/0/12.0 { priority 16; }



Agenda: Spanning Tree  Spanning Tree Protocol  Rapid Spanning Tree Protocol  Configuring and Monitoring RSTP Protection Features BPDU

Protection

•Loop Protection •Root Protection




What If…?  Given the topology below, what if User A connects a

personal (unauthorized) switch running the spanning tree protocol to Switch-2? Switch-1 (Root Bridge)

User A

Part of the spanning tree

Switch-1

User A Switch-2

Switch-2

Switch-3

Switch-3

BPDUs would be exchanged, a new STP calculation would occur, and the rogue switch would become part of the spanning tree, potentially leading to a network outage




BPDU Protection  BPDU protection prevents rogue switches from

connecting to the network and causing undesired Layer 2 topology changes and possible outages •If a BPDU is received on a protected interface, the interface is disabled and transitions to the blocking state Edge port is disabled if BPDU is received on protected interface


User A Switch-2



Switch-3


Configuring BPDU Protection  BPDU protection can be enabled on switches whether

or not the spanning tree protocol enabled: {master:0}[edit protocols rstp] user@Switch-2# show interface ge-0/0/6.0 { edge; }

Use bpdu-block-on-edge option when spanning tree protocol is enabled

bpdu-block-on-edge;

{master:0}[edit ethernet-switching-options] user@Switch-2# show bpdu-block { interface ge-0/0/6.0; }

User A

Switch-2

Use bpdu-block option when spanning tree protocol is not enabled


ge-0/0/6.0



Monitoring BPDU Protection Before BPDU is received on protected interface ge-0/0/6.0

{master:0} user@Switch-2> show spanning-tree interface ge-0/0/6.0

User A Switch-2

Spanning tree interface parameters for instance 0 Interface

ge-0/0/6.0

Port ID

128:519

Designated port ID 128:519

Designated bridge ID 32768.0019e2516580

Port State Cost 20000 FWD

Role

DESG

{master:0} user@Switch-2> show ethernet-switching interfaces ge-0/0/6.0 Interface State VLAN members Tag Tagging Blocking ge-0/0/6.0 up default untagged unblocked

Before BPDU violation

After BPDU is received on protected interface {master:0} user@Switch-2> show spanning-tree interface ge-0/0/6.0

After BPDU violation

{master:0} user@Switch-2> show ethernet-switching interfaces ge-0/0/6.0 Interface State VLAN members Tag Tagging Blocking ge-0/0/6.0 down default untagged Disabled by bpdu-control

{master:0} user@Switch-2> clear ethernet-switching bpdu-error interface ge-0/0/6.0



Re-enables interface www.junip er.net | 4-43

Agenda: Spanning Tree  Spanning Tree Protocol  Rapid Spanning Tree Protocol  Configuring and Monitoring RSTP Protection Features

•BPDU Protection Loop

Protection

•Root Protection




What If…?  Given the topology below, what if BPDUs sent by

Switch-2 were not received by Switch-3? Switch-1 (Root Bridge)


Layer 2 Loop

Switch-2

BPDUs not received due to a uni-directional link failure or a software configuration issue


Switch-3

Switch-2

Switch-3

Switch-3 waits until the max-age timer expires then transitions its alternate port to the designated port role and the forwarding state thus removing the blocked port and causing a Layer 2 loop Worldwide Education Services


Loop Protection  The loop protection feature provides additional

protection against Layer 2 loops by preventing nondesignated ports from becoming designated ports •Enable loop protection on all non-designated ports • Ports that detect the loss of BPDUs transition to the “loop inconsistent” role which maintains the blocking state • Port automatically transitions back to previous or new role when it receives a BPDU Switch-1 (Root Bridge)

Switch-2 © 2011 Juniper Networks, Inc. All r ights reserved.



Configuring Loop Protection  Configure loop protection on non-designated ports

(root and alternate ports): {master:0}[edit protocols rstp] user@Switch-3# show interface ge-0/0/10.0 { bpdu-timeout-action { block;


} } interface ge-0/0/12.0 { bpdu-timeout-action { block; } }

Use the block or alarm action in conjunction with the loop protection feature


ge-0/0/12.0 Switch-2



Monitoring Loop Protection When BPDUs are received on protected interface: {master:0} user@Switch-3> show spanning-tree interface Spanning tree interface parameters for instance 0 Interface ge-0/0/10.0 ge-0/0/12.0

Port ID 128:523 128:525

Designated port ID 128:523 128:525

Designated bridge ID 4096.002688027490 16384.0019e2516580

Port Cost 20000 20000

State

Role

FWD BLK

ROOT ALT

State

Role

FWD BLK

ROOT DIS (Loop-Incon)

When BPDUs are not received on protected interface: {master:0} user@Switch-3> show spanning-tree interface Spanning tree interface parameters for instance 0 Interface ge-0/0/10.0 ge-0/0/12.0

Port ID 128:523 128:525


Designated port ID 128:523 128:525

Designated bridge ID 4096.002688027490 32768.0019e2553600

Port Cost 20000 20000



Agenda: Spanning Tree  Spanning Tree Protocol  Rapid Spanning Tree Protocol  Configuring and Monitoring RSTP Protection Features

•BPDU Protection •Loop Protection Root

Protection




What If…?  Given the topology and details below, what if a rogue

switch with a bridge priority of 4K was connected to the Layer 2 network? Switch-1 (Root Bridge) Priority = 8k

Switch-2 Priority = 32k

New root bridge


Switch-2

Switch-1

Switch-3

BPDUs would be exchanged, a new STP calculation would occur, and the rogue switch would become the new root bridge potentially leading to a network outage © 2011 Juniper Networks, Inc. All r ights reserved.



Root Protection  Enable root protection to avoid unwanted STP

topology changes and root bridge placement •If a superior BPDU is received on a protected interface, the interface is disabled and transitions to the blocking state Switch-1 (Root Bridge) Priority = 4k


Root protection is typically configured on the ports of aggregation switches that connect to access switches







Configuring Root Protection  Enable root protection on ports that should not

receive superior BPDUs from the root bridge and should not be elected as the root port: {master:0}[edit protocols rstp] user@Switch-1# show bridge-priority 4k; interface all { no-root-port; }

Switch-1 (Root Bridge) Priority = 4k ge-0/0/12.0 ge-0/0/13.0





{master:0}[edit protocols rstp] user@Switch-2# show bridge-priority 8k; interface ge-0/0/6.0 { no-root-port; } interface ge-0/0/7.0 { no-root-port; } interface ge-0/0/8.0 { no-root-port; }

Switch-5 Priority = 32k Worldwide Education Services


Monitoring Root Protection Before superior BPDU is received on protected interface {master:0} user@Switch-1> show spanning-tree interface

Spanning tree interface parameters for instance 0 Interface

ge-0/0/6.0 ge-0/0/7.0 ge-0/0/8.0

Port ID

128:519 128:520 128:521

Designated port ID 128:519 128:520 128:521

Designated bridge ID 4096.0019e2516580 4096.0019e2516580 4096.0019e2516580

ge-0/0/12.0 ge-0/0/13.0

128:525 128:526

128:525 128:526

4096.0019e2516580 4096.0019e2516580

Port State Cost 20000 FWD 20000 FWD 20000 FWD 20000 20000

FWD FWD

Role

DESG DESG DESG DESG DESG

Switch-1 (Root Bridge) Priority = 4k

After superior BPDU is received on protected interface {master:0} user@Switch-1> show spanning-tree interface

Spanning tree interface parameters for instance 0 Interface ge-0/0/6.0 ge-0/0/7.0 ge-0/0/8.0 ge-0/0/12.0 ge-0/0/13.0

Port ID 128:519 128:520 128:521 128:525 128:526

Designated port ID 128:519 128:520 128:521 128:525 128:526


Designated bridge ID 0.002688027490 4096.0019e2516580 4096.0019e2516580 4096.0019e2516580 4096.0019e2516580

Port Cost 20000 20000 20000 20000 20000

State BLK FWD FWD FWD FWD


Role ALT (Root-Incon) DESG DESG DESG DESG


Summary  In this chapter, we:

•Explained when a spanning tree is required •Described STP and RSTP operations •Listed some advantages of using RSTP over STP •Configured and monitored RSTP •Described the BPDU, loop, and root protection features •Configured and monitored the BPDU, loop, and root protection features




Review Questions 1. What is the purpose of STP? 2. Describe how to build a spanning tree. 3. How are STP and RSTP different? 4. What is the purpose of the BPDU protection feature?




Lab 3: Implementing Spanning Tree  Configure and monitor RSTP and protection features.





JEX 11.a C4 SpanningTree

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