3b pvst ether channel rstp new

Enhancements to 802.1D, PVST+, EtherChannel, RSTP and MST

Multilayer Switched Networks

CCNP3

Rick Graziani

Rick Graziani [email protected] 2

Additional Notes

• See Notes section for additional detailed information.


Download this file

• Download: PT-Topology-STP2.pkt


IEEE Documents

• IEEE 802.1D - Media Access Control (MAC) bridges

• IEEE 802.1Q - Virtual Bridged Local Area Networks

• IEEE 802.1w - Rapid Reconfiguration (Supp. To 802.1D)

• IEEE 802.1s - Multiple Spanning Tree (Supp. To 802.1Q)

• IEEE 802.1t - Local and Metropolitan Area Network: Common Specifications


Enhancements to STP

• PortFast

• Per VLAN Spanning Tree (PVST+)

• Rapid Spanning Tree Protocol (RSTP)

• Multiple Spanning Tree Protocol (MSTP)– MSTP is also known as Multiple Instance Spanning

Tree Protocol (MISTP) on Cisco Catalyst 6500 switches and above

• Per VLAN Rapid Spanning Tree (PVRST)

Helping STP protect your LAN from Problems

PortFast

BPDU Guard

Root Guard

UplinkFast

BackboneFast


Powercycle a host and watch link lights…

How long until switch link light turns green?


PortFast

• Host powered on.

• Port moves from blocking state immediately to listening state (15 seconds).– Determines where switch fits into spanning tree topology.

• After 15 seconds port moves to learning state (15 seconds). – Switch learns MAC addresses on this port.

• After 15 seconds port moves to forwarding state.

Powered On

Blocking StateListening StateLearning StateForwarding State I’m adding any addresses on this port to my MAC Address

Table.


PortFast – Problem DHCP

• Host sends DHCP Discovery

• Host never gets IP addressing information

Powered On

Blocking StateListening StateLearning StateForwarding State

DHCP DiscoveryTimeout

IP Address = 169.x.x.x


PortFast

• The purpose of PortFast is to minimize the time that access ports wait for STP to converge.

• The advantage of enabling PortFast is to prevent DHCP timeouts.

• Host sends DHCP Discovery

• Host can now can IP addressing information.

Powered On

Portfast enabledForwarding State

DHCP Discovery

DHCP Offer


Configuring Portfast

Access2(config)#interface range fa 0/10 - 24Access2(config-if-range)#switchport mode access

<Previously configured>Access2(config-if-range)#spanning-tree portfast

• Warning: PortFast should only be enabled on ports that are connected to a single host.

• If hubs, concentrators, switches, and bridges. are connected to the interface when PortFast is enabled, temporary bridging loops can occur.

• Use with caution.

• Use the following command to enable PortFast globally in global configuration mode:

ORAccess2(config)#spanning-tree portfast default


Powercycle the host again (portfast enabled)

How long until switch link light turns green?


Configuring Portfast

Switch(config)#interface range fa 0/10 - 24Switch(config-if-range)#switchport mode access

<Previously configured>Switch(config-if-range)#spanning-tree portfast

Configure Portfast on all Distribution and Access switches


• Enabling PortFast can create a security risk in a switched network.

• A port configured with PortFast will go into blocking state if it receives a Bridge Protocol Data Unit (BPDU).

• This could lead to false STP information that enters the switched network and causes unexpected STP behavior.

Portfast

X

Blocking and now listening to BPDUs

Forwards BPDUs to other switches.

STP Reconvergence?

BPDU

Problem: BPDUs


• When the BPDU guard feature is enabled on the switch, STP shuts down PortFast enabled interfaces that receive BPDUs instead of putting them into a blocking state.

• BPDU guard will also keep switches added outside the wiring closet by users from impacting and possibly violating Spanning Tree Protocol.

Distribution1(config)#interface range fa 0/10 - 24Distribution1(config-if-range)#spanning-tree bpduguard enable

BPDU

|

Err-Disable, Shutdown

Portfast & BPDU Guard

No BPDUs sent

Solution: BPDU Guard

Not supported with Packet Tracer


• Root Guard prevents a switch from becoming the root bridge.– Typically access switches

• Configured on switches that connect to this switch.

Potential Root

Protect Protect

Potential Root

Root Guard


Root Guard

Distribution1(config)#interface fa 0/3Distribution1(config-if-range)#spanning-tree guard rootDistribution1(config)#interface gig 0/2Distribution1(config-if-range)#spanning-tree guard rootDistribution2(config)#interface fa 0/3Distribution2(config-if-range)#spanning-tree guard rootDistribution2(config)#interface gig 0/1Distribution2(config-if-range)#spanning-tree guard root

Access2(config)#no spanning-tree uplinkfast

• UplinkFast must be disabled because it cannot be used with root guard.

• Next


• This message appears after root guard blocks a port:

%SPANTREE-2-ROOTGUARDBLOCK: Port 0/3 tried to become non-designated in VLAN 1. Moved to root-inconsistent state

Root Guard

Root Guard

Superior BPDU

I want to be root bridge!

STP Inconsistent State – no traffic is passed.

I no longer want to be root. I have

been reconfigured to be a non-root bridge.

I will now transition to listening sate, then learning state, then forwarding sate.


Root Guard (May not work with PT)

Distribution2# show spanning-treeVLAN0001 Spanning tree enabled protocol ieee Root ID Priority 24586 Address 0001.C945.A573 Cost 4 Port 26(GigabitEthernet0/2) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Core# show spanning-treeVLAN0001 Spanning tree enabled protocol ieee Root ID Priority 24586 Address 0001.C945.A573 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec


• Uplinkfast allows access layer switches the ability to converge quickly when a link has failed.– “Leafs” (end nodes) of the spanning tree.– Not for use within backbone or distribution switches

(BackboneFast).

UplinkFast


• UplinkFast must have direct knowledge of the link failure in order to move a blocked port into a forwarding state.

• This switchover occurs within 5 seconds.

Root

X

Unblock G 1/1 skips listening and learning and goes directly to forwarding

UplinkFast


• Cisco switches do not support Uplinkfast on a per-VLAN basis.

Access1(config)#spanning-tree uplinkfast

Root

X

Unblock G 1/1 skips listening and learning and goes directly to forwarding

UplinkFast

Not supported with Packet Tracer


• Backbone fast is a Cisco proprietary feature that, once enabled on all switches of a bridge network, can save a switch up to 20 seconds (max_age) when it recovers from an indirect link failure.

BackboneFast Root


• BackboneFast is initiated when a root port or blocked port on a switch receives inferior BPDUs from a designated bridge.

RP X

Inferior BPDU

BlockingForwarding

My link to the Root has gone down. I have no alternate

path to it. So, I’m the new

root and send out my BPDUs

on all ports.

This new BPDU is inferior to the

one it had stored for this port so I

will ignore it. Let me send my current Root a

query.

I just heard from Core that they are still the Root. I will:• Send BPDU to D1• Transition port immediately to listening state saving 20 seconds

Thanks for telling me Core is the Root. I

will change my RP to Fa 0/5.

RP

After 20 seconds this port will now go into Forwarding state.

Listening

Root


FYI – More Information• An inferior BPDU identifies one switch as

both the root bridge and the designate bridge.

• Distribution 1 is the Designated Bridge.• Normally, sends BPDUs with Root Bridge

as the Core BID.• Inferior BPDU – A received BPDU that

identifies the root bridge and the designated bridge as the same switch. (“I was only just the Designate Bridge, but now that I can’t get to the Root Bridge, so now I am also the Root Bridge.”)

BackboneFast

= Core

= Dist1

= Dist1

= Dist1Same Switch

Normal BPDU

Inferior BPDU


PVST+: Per-VLAN Spanning Tree

I am the Root Bridge for all VLANS: 1, 10, 20I will be the Root

Bridge for VLANs 1 and 10 I will be the

Root Bridge for VLAN 20


Core is Root for all VLANs – 1, 10, 20

Core# show spanning-treeVLAN0001 Spanning tree enabled protocol ieee Root ID Priority 16385 Address 0001.C945.A573 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 secVLAN0010 Spanning tree enabled protocol ieee Root ID Priority 24586 Address 0001.C945.A573 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 secVLAN0020 Spanning tree enabled protocol ieee Root ID Priority 24596 Address 0001.C945.A573 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

By default, a single spanning tree runs on each configured VLAN, provided STP has not been manually disabled,


PVST+

• Per VLAN Spanning Tree Plus (PVST+) maintains a separate spanning-tree instance for each VLAN. – PVST Only over ISL– PVST+ Includes ISL and 802.1Q

• Provides for load balancing on a per-VLAN basis.• Switches maintain one instance of spanning tree for each VLAN allowed

on the trunks. • Non-Cisco 802.1Q switches maintain only one instance of spanning tree

for all VLANs allowed on the trunks.


Core is Root for all VLANs


PVST+ by default

Distribution2# show spanning-tree summarySwitch is in pvst modeRoot bridge for:Extended system ID is enabledPortfast Default is disabledPortFast BPDU Guard Default is disabledPortfast BPDU Filter Default is disabledLoopguard Default is disabledEtherChannel misconfig guard is disabledUplinkFast is disabledBackboneFast is disabledConfigured Pathcost method used is short

Distribution2(config)# spanning-tree mode pvst

If modified and to re-enable PVST+


PVST+

Distribution1(config)# spanning-tree vlan 1, 10 root primary

Distribution2(config)# spanning-tree vlan 20 root primary

Core(config)# no spanning-tree vlan 1-30 root primary

Remove Core as Root Bridge for all VLANs


Distribution1 is Root for VLANs 1 and 10

Distribution1#show spanning-treeVLAN0001 Spanning tree enabled protocol ieee Root ID Priority 12289 Address 0005.5E0D.9315 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 secVLAN0010 Spanning tree enabled protocol ieee Root ID Priority 12298 Address 0005.5E0D.9315 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 secVLAN0020 Spanning tree enabled protocol ieee Root ID Priority 16404 Address 0060.47B0.5850 Cost 4 Port 26(GigabitEthernet0/2) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec


Distribution2 is Root for VLAN 20

Distribution2#show spanning-treeVLAN0001 Spanning tree enabled protocol ieee Root ID Priority 12289 Address 0005.5E0D.9315 Cost 4 Port 26(GigabitEthernet0/2) Hello Time 2 sec Max Age 20 sec Forward Delay 15 secVLAN0010 Spanning tree enabled protocol ieee Root ID Priority 12298 Address 0005.5E0D.9315 Cost 4 Port 25(GigabitEthernet0/1) Hello Time 2 sec Max Age 20 sec Forward Delay 15 secVLAN0020 Spanning tree enabled protocol ieee Root ID Priority 16404 Address 0060.47B0.5850 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec


Distribution1 is the Root for VLAN1 and 10

Root VLANs 1,10


Distribution2 is the Root for VLAN 20

Root VLAN 20


Load Balancing with 2 Root Switches

Root VLANs 1,10 Root VLAN 20

Notice that more links are being used!


EtherChannel

• Add another link between Distribution1 and Distribution2 using Fa0/6 on both switches.

• Are both links being used? Why or why not?– No, STP is blocking the redundant link to prevent a

loop.

• Can both links be used?– Yes, with EtherChannel


EtherChannel

• Allows parallel links to be treated by Spanning Tree as one physical link – Providing full-duplex bandwidth of 200 to 800 Mbps for Fast

Ethernet – All traffic is distributed across the links in the channel.

• Bundle - group of links managed by the EtherChannel process


PAgP and LACP (Management)

• The Catalyst family of switches supports both:– Port Aggregation Protocol (PAgP) - Cisco proprietary – Link Aggregation Control Protocol (LACP) - Industry standard 802.3ad-based

protocol • PAgP is a management function, which checks the parameter consistency at either

end of the link and assists the channel in adapting to link failure or addition. • PAgP packets are sent every 30 seconds using multicast group MAC address 01-00-

0C-CC-CC-CC with protocol value 0x0104. • The logical port, or Agport, is composed of all the ports that make up the

EtherChannel. For instance, the spanning tree algorithm treats Agport as a single port.• Both LACP and PAgP prevents STP loops or packet loss due to misconfigured

channels and aids network reliability.• Not many differences.• When a Cisco switch is connected to a non-Cisco switch use LACP. • Fast EtherChannel (FEC) or Gigabit EtherChannel (GEC)

– Full-duplex bandwidth up to: • 1600 Mbps (8 links of Fast Ethernet) • 16 Gbps (8 links of Gigabit Ethernet)


Best Practices for EtherChannel

• Maximum of eight ports per EtherChannel. – May be model specific.– Ports do not have to be contiguous or on the same module.

• All ports in an EtherChannel must use the same protocol (PAgP or LACP).

• All ports in an EtherChannel must have the same speed and duplex mode. – LACP full-duplex only

All PAgP or LACP

Same Speed and Duplex


Best Practices for EtherChannel

All ports must be:• Same channel group (configuration coming)• VLANs

– Single VLAN• Same access VLAN configuration

– Multiple VLANs (Trunks)• Same allowable VLAN list • Same native VLAN (untagged traffic)• Same trunk mode (ex: dot1q desirable)

• Same Spanning Tree settings.

VLAN 10 VLAN 10

VLANs 1, 10, 20 VLANs 1, 10, 20

Native VLAN 1 Native VLAN 1

802.1Q Trunk 802.1Q Trunk


• Load is not necessarily balanced equally across all the links. • Instead, frames are forwarded on a specific link as a result of a

hashing algorithm. • The algorithm can be modified using:

– source IP address– destination IP address– combination of source and destination IP addresses– source and destination MAC addresses– TCP/UDP port numbers.

Switch(config)# port-channel load-balance method

For more information: http://www.cisco.com/en/US/tech/tk389/tk213/technologies_tech_note09186a0080094714.shtml#topic1


parameters are used in configuring LACP:

• System priority: Each switch running LACP must have a system priority, which can be specified automatically or through the CLI. The switch uses the MAC address and the system priority to form the system ID.

• Port priority: Each port in the switch must have a port priority, which can be specified automatically or through the CLI. The port priority and the port number form the port identifier. The switch uses the port priority to decide which ports to put in standby mode when a hardware limitation prevents all compatible ports from aggregating.

• Administrative key: Each port in the switch must have an administrative key value, which can be specified automatically or through the CLI. The administrative key defines the ability of a port to aggregate with other ports, determined by the following: – The port’s physical characteristics, such as data rate, duplex

capability, and point-to-point or shared medium – The configuration constraints that you establish


Describing EtherChannel Configuration Commands


Etherchannel Configuration

Core(config)# interface range fa 0/x – y

Core(config-if)# channel-group number mode {active | on | {auto [non-silent]} | {desirable [non-silent]} | passive}


Describing EtherChannel Configuration Commands


Etherchannel Configuration Command Description


Exam Certification Guide

Here are some reminders about EtherChannel operation and interaction:

• EtherChannel on mode does not send or receive PAgP or LACP packets. Therefore, both ends should be set to the on mode.

• EtherChannel desirable (PAgP) or active (LACP) mode attempts to ask the far end to bring up a channel. Therefore, the other end must be set to either desirable or auto mode.

• EtherChannel auto (PAgP) or passive (LACP) mode participates in the channel protocol, but only if the far end asks for participation. Two switches in the auto or passive mode will not form an EtherChannel.

• PAgP desirable and auto modes default to the silent submode, where no PAgP packets are expected from the far end. If ports are set to non-silent submode, PAgP packets must be received before a channel will form.


Etherchannel Example using LACP

DLS1(config)# interface range fa 0/11 – 12

DLS1(config-if)# switchport trunk encapsulation dot1q

DLS1(config-if)# switchport mode trunk

DLS1(config-if)# channel-group 1 mode active

DLS1 DLS2

DLS2(config)# interface range fa 0/11 – 12

DLS2(config-if)# switchport trunk encapsulation dot1q

DLS2(config-if)# switchport mode trunk

DLS2(config-if)# channel-group 1 mode active



DLS2# show inter port-channel 1

Port-channel1 is up, line protocol is up (connected)

Hardware is EtherChannel, address is 001b.8fc8.008d (bia 001b.8fc8.008d)

MTU 1500 bytes, BW 200000 Kbit, DLY 100 usec,

reliability 255/255, txload 1/255, rxload 1/255

Encapsulation ARPA, loopback not set

Full-duplex, 100Mb/s, link type is auto, media type is unknown

input flow-control is off, output flow-control is unsupported

Members in this channel: Fa0/11 Fa0/12



DLS2# show spanning-tree vlan 1 detail

Port 72 (Port-channel1) of VLAN0001 is forwarding

Port path cost 12, Port priority 128, Port Identifier 128.72.

Designated root has priority 32769, address 001b.0c98.8100

Designated bridge has priority 32769, address 001b.9018.bc80

Designated port id is 128.72, designated path cost 24

Timers: message age 4, forward delay 0, hold 0

Number of transitions to forwarding state: 1

Link type is point-to-point by default

BPDU: sent 5005, received 6



DLS2# show etherchannel 1 port-channel

Port-channels in the group:

---------------------------

Port-channel: Po1 (Primary Aggregator)

------------

Age of the Port-channel = 00d:00h:07m:50s

Logical slot/port = 2/1 Number of ports = 2

HotStandBy port = null

Port state = Port-channel Ag-Inuse

Protocol = LACP

Ports in the Port-channel:

Index Load Port EC state No of bits

------+------+------+------------------+-----------

0 00 Fa0/11 Active 0

0 00 Fa0/12 Active 0

RSTP – IEEE 802.1w (Rapid Spanning Tree Protocol)


Rapid Spanning Tree Protocol


Rapid Spanning Tree Protocol

• The immediate hindrance of STP is convergence. • Depending on the type of failure, it takes anywhere from 30 to 50

seconds, to converge the network. • RSTP helps with convergence issues that plague legacy STP.


RSTP vs STP

• RSTP is based on IEEE 802.1w standard. • RSTP is proactive and therefore negates the need for the 802.1D

delay timers. • RSTP (802.1w) supersedes 802.1D, while still remaining backward

compatible.• RSTP BPDU format is the same as the IEEE 802.1D BPDU format,

except that the Version field is set to 2 to indicate RSTP.• The RSTP spanning tree algorithm (STA) elects a root bridge in

exactly the same way as 802.1D elects a root.

vs


Port States

Operational Port State

STP Port State RSTP Port State

Enabled Blocking Discarding

Enabled Listening Discarding

Enabled Learning Learning

Enabled Forwarding Forwarding

Disabled Disabled Discarding


RSTP Port Roles

• Root ports and Designated ports immediately transition to forwarding state.


What Are Edge Ports?

• Will never have a switch connected to it

• Never generate topology changes notifications (TCNs) when the port transitions to a disabled or enabled status.

• If an edge port receives a BPDU, it becomes a normal spanning-tree port.

• Immediately transitions to forwarding

• Functions similarly to PortFast

• Configured by issuing the spanning-tree portfast command


RSTP Link Types


RSTP Link Types

• The link type can predetermine the active role that the port plays as it stands by for immediate transition to a forwarding state, if certain parameters are met.

• These parameters are different for edge ports and non-edge ports.

• Non-edge ports are categorized into two link types.

• Link type is automatically determined but can be overwritten with an explicit port configuration.

RSTP Link Types

Link TypeDescription

Point-to-point •Port operating in full-duplex mode. •It is assumed that the port is connected to a single switch device at the other end of the link.

Shared •Port operating in half-duplex mode. •It is assumed that the port is connected to shared media where multiple switches might exist.


Topology Change Notifications

• In 802.1D, any port state change generates a TCN. • When an 802.1D bridge detects a topology change (TC), it sends TCNs toward the root

bridge. • The root bridge sends out a TC (Configuration BPDUs) that are relayed to switches down

from the root. • When a bridge receives this BPDU the switch reduces its bridge-table aging time to

forward delay seconds.– It does not wait the 5 minutes (300 seconds) to age ot the MAC Address table.

• This ensures a relatively quick flushing of the MAC address table.


RSTP Topology Change Mechanism

• In RSTP, only non-edge ports moving to the forwarding state cause a topology change.

• Loss of connectivity is not considered to be a topology change, and, under these conditions, a port moving to the blocking state does not generate a TC BDPU.

• Switches clear MAC Address tables and move to forwarding almost immediately (TC While timer, “twice the hello time”)

• The topology change propagation is now a one-step process. • The initiator of the topology change is flooding this information throughout the

network, as opposed to 802.1D, where it relies on the root. • This mechanism is much faster than the 802.1D equivalent.

Initiator floods network with change instead of communicating with Root and having Root to it.

Switches to not wait for 15 seconds (Forward Delay) or 35 seconds (Root: Max age + Forward Delay)


RSTP (802.1w) uses type 2, version 2 BPDUs

• RSTP uses the flag byte:


Synchronization

RSTP Convergence:• Nonedge ports begin in the Discarding state.• Root Bridge can be identified “BPDU”.• If a port receives a superior BPDU from a neighbor it becomes the root port.• The switch exchanges a proposal-agreement handshake for each nonedge

port to decide the state of each end of the link.• Each switch assumes its port should become the designated port for the

segment, and a proposal message “a configuration BPDU” is sent to the neighbor suggesting this.

• Proposal’s sender has a superior BPDU, the local switch realizes that the sender should be the designated and its own port must become the new root port.

• Before the switch agrees to anything, it must synchronize itself with the topology.

• All nonedge ports immediately are moved into the Discarding (Blocking) state so that no bridging loops can form.

• An agreement message “a configuration BPDU” is sent back to the sender, indicating that the switch is in agreement with tne new designated port choice.


Synchronization

•The root port immediately is moved to the Forwarding state. The sender’s port also immediately can begin forwarding•For each Nonedge port that is currently in Discarding state, a proposal is sent to respective neighbor.•An agreement message is expected and received from a neighbor on a nonedge port.•The nonedge port immediately is moved to the Forwarding state.


Rapid PVST Implementation Commands

Switch(config)# spanning-tree mode rapid-pvst Switch(config)# show spanning-tree

Cisco implements RSTP with PVST



Access1# show spanning-treeVLAN0001 Spanning tree enabled protocol rstp Root ID Priority 24577 Address 0001.C945.A573 Cost 4 Port 26(GigabitEthernet1/2) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0003.E461.46EC Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20


RSTP Summary

• RSTP provides faster convergence than 802.1D STP when topology changes occur.

• RSTP defines three port states: discarding, listening, and forwarding.

• RSTP defines five port roles: root, designated, alternate, backup, and disabled.

• Edge ports forward while topology changes occur.

• RSTP makes use of two link types – p2p and shared.

• The RSTP topology change notification process differs from 802.1D.

• Rapid PVST enables RSTP while still maintaining PVST.

Rick Graziani [email protected]

Multiple Spanning Tree Protocol – 802.1s



• Multiple Spanning Tree (MST) extends the IEEE 802.1w RST algorithm to multiple spanning trees.

• The main purpose of MST is to reduce the total number of spanning-tree instances to match the physical topology of the network and thus reduce the CPU cycles of a switch.

• PVST+ runs STP instances for each VLAN and does not take into consideration the physical topology that may not require many different STP topologies.

• MST, on the other hand, uses a minimum number of STP instances to match the number of physical topologies present.

Instance 1 maps to VLANs 1–500Instance 2 maps to VLANs 501–1000



• The 1000 VLANs map to two MST instances.

• Rather than maintaining 1000 spanning trees (like PVST+), each switch needs to maintain only two spanning trees, reducing the need for switch resources.

Instance 1 maps to VLANs 1–500Instance 2 maps to VLANs 501–1000


The Possible STP Topologies for Two VLANs using MST:


MST Regions

• The main enhancement introduced by MST is the ability to map several VLANs to a single spanning-tree instance.

• This raises the problem, however, of determining what VLAN is to be associated with what instance.

- Tagging BPDUs so that receiving devices can identify the instances and the VLANs to which they apply.

Received BPDU = Which VLANs is this associated with?


MST Regions

• Each switch that is running MST in the network has a single MST configuration that consists of three attributes:– An alphanumeric configuration name (32 bytes)– A configuration revision number (2 bytes)– A 4096-element table that associates each of the potential 4096

VLANs supported on the chassis to a given instanceThe switches only need to know whether they are in the sameregion as a neighbor:Only a digest of the VLANs-to-instance mapping table is sent, alongwith the revision number and the name.

Legacy 802.1D

BPDUs OR

Different

digests


CST, MST Instances and IST Instances


MAC address reduction

Is a feature that ensures bridge ID uniqueness for all 4000 VLANs, even when there are only 1024 or 64 MAC addresses available on the switch.

By making the 16-bit Bridge Priority field in the BPDU unique for each VLAN.


Steps to Configure MST:


Verifying MSTP


MST Regions

Switch(config)# spanning-tree mode mst Switch(config)# instance 1 vlan 1-100 Switch(config)# show spanning-tree mst configuration

Not currently supported in Packet Tracer

Mapping VLANs 1-100 to instance 1



Access1# show spanning-treeMST00 Spanning tree enabled protocol mstp Root ID Priority 24577 Address 0001.C945.A573 Cost 4 Port 26(GigabitEthernet1/2) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address 0003.E461.46EC Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 20

Enhancements to 802.1D, PVST+, EtherChannel, RSTP and MST

Multilayer Switched Networks

CCNP3

Rick Graziani