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Spanning Tree protocol- Chapter 5

CCNA Exploration Semester 3

Modified by Profs. Ward

and Cappellino

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Topics

Redundancy in a converged network How Spanning Tree Protocol (STP)

eliminates layer 2 loops The STP algorithm and its 3 steps Rapid spanning tree protocol

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Semester 3

LAN Design

Basic Switch Concepts

VLANs

VTP

STP- Ch 5

Inter-VLAN routing

Wireless

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We want:

Redundancy accomplished at the ____________________ ______________________________________________ What is redundancy?

Multiple ________________ _________________

One link or device fails – another takes over.

Redundancy allows flexibility but does have some challenges

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Issues with Redundancy

Multiple ___________________ Create problems when all the links are active:

____________________ _____________________ ____________________

See examples on following slides…

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Broadcast storm

1. Send ARP

request

There’s a switching loop

2. Flood broadcast through non-source ports

3. And so on with nothing to stop it

A

D

C

B

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Duplicate Unicast Frame Transmissions

A B

Send frame to

B

A is on port 3Don’t know B

So flood

Frame arrives

And againD

A

B

C

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Loops by mistake

Even if there are no deliberate loops for redundancy, there can be loops set up by mistake. For example, by improper or no labeling of

cables in the wiring closet ____________

____________

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Etherchannel – the exception

EtherChannel is a grouping of Ethernet ports on a switch that ____________________ ____________________________ Multiple connections do not make a loop where

Etherchannel is used. The _________________________________

with the combined bandwidth.

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Redundancy without loops

There needs to be just ________________ _________________________, but _______

_________________ when they are needed. This must be done quickly and automatically. ______________________ does this. A blocked port does not include bridge

protocol data unit (BPDU) frames that are used by STP to prevent loops.

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What is a spanning tree?

A tree (extended star) topologyA ______________________

Spanning all devices__________________________

Spanning tree is _____________ immediately __________________ _______________________

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Not a spanning tree

Not a tree - it has loops.

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Not a spanning tree

Not spanning. Device left out.

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Spanning tree

No loops. Includes all devices.

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Spanning tree protocol

Used by switches to __________________ _________________________________

________ unwanted links by _____________ STP defined by ________________ _____________ defined by _____________ Switches __________________ –

________________________________

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Spanning tree algorithm

The switches use this algorithm to configure the ports _______ ____________________

1. Choose ___________ to be “____________”

2. Choose a “_______” on ______ _____________ closest to the root bridge

3. Choose a “__________” which are all _______________ that are still ___________________________________________________________________________ known as “non-designated” ports

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Example of port designations

Root bridge

Root port

Root port

Root port

Designated portDesignated port

Designated port

Designated port Non-designated port

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1. Choose the Root Bridge

Root bridge serves as a __________________ ____________________________

______________ in the broadcast domain __________________________________

Each switch has a bridge ID (___) containing priority value, extended system ID, followed by the MAC address of the switch- more to come on BID …

A switch starts up. It sends out BPDU frames more to

come on BPDU … containing the switch BID and the root ID every 2 seconds. At first each switch identifies itself as the root bridge.

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Choose the root bridge (cont…)

___________ Bridge Protocol Data Units (______) to _______________________

The switch with the __________________________ ________________________ As a switch receives a BPDU, it ___________________

________________________________________ through which the BPDU was received. It passes on this information in its own BPDUs.

Eventually all switches agree that the switch with the lowest BID is the root bridge.

Administrator can set the priority to fix the selection

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BPDU

The _______________________________ ________________________________

The BPDU message is encapsulated in an Ethernet frame.

The ______________ MAC address in the BPDU frame is a _________________ for the ___________________________

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BPDU Frame contains 12 fields:

2 bytes Protocol ID Administrative uses1 byte Version

1 byte Message type

1 byte Flags

8 bytes Root ID BID and path information- used to identify the root bridge and the cost

4 bytes Cost of path

8 bytes Bridge ID

2 bytes Port ID

2 bytes Message age Timer fields used to determine how frequently BPDU messages are sent

2 bytes Max age

2 bytes Hello time

2 bytes Forward delay

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Bridge ID

Used to ______________________ on a network The bridge ID consists of:

____________________________ Lowest priority- ie. Lowest BID- becomes root bridge By default the priority is 32768

Value range is 1 - 65536 ___________________________________

Identifies the VLAN with which the BPDU is associated ________________________

MAC address used as ___________________________ MAC address with the lowest hexadecimal value has the

lower BID Best to configure the desired root bridge and not count on MAC

address

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Configure priority

Set a _____________________ if a specific switch is to become the root bridge

Method 1: SW1#spanning-tree vlan 1 root primary

Sets value to 24576 or 4096 less than lowest priority detected.

SW1#spanning-tree vlan 1 root secondary Sets value to 28672. This switch should become the root

bridge if the primary root bridge fails. Method 2:

SW1#spanning-tree vlan 1 priority 24576 command gives more granular control over the bridge

priority value by allowing the NA to configure the priority How would we verify bridge priority?

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2. Select root ports

After root bridge designation _________________________ ___________________________________________________________________________ in the broadcast domain.

Every non-root bridge (Switch) selects a root port Only _________________________________ This is the _____________________________ to the root bridge

Path information is determined by _______________________ _________ along the path from the destination to the root bridge.

What is cost?...

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Finding the cost of a link

Default port costs are defined by the speed at which the port operates

Set by IEEE. Costs may change as newer, faster Ethernet is developed.

Port cost is configurable by the NA

Link speed Revised cost Previous cost

10 Gbps 2 1

1 Gbps 4 1

100 Mbps 19 10

10 Mbps 100 100

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Changing the cost of a link

SW1(config)#int fa0/1 SW1(config-if)#spanning-tree cost 25 SW1(config-if)#end

To revert back to default value use “no”… SW1(config)#int fa0/1 SW1(config-if)#no spanning-tree cost SW1(config-if)#end

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What is the best path?

In the example, the path cost from switch S2 to the root bridge switch S1, over path 1 is 19, while the path cost over path 2 is 38. Because path 1 has a

lower overall path cost to the root bridge, it is the preferred path.

STP then configures the redundant path to be blocked, preventing a loop from occurring.

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What if root ports have the same cost?

Switch uses the customizable port priority value. If those are the ____________?

_____________________________ The port ID is ________________________________

port. This gives each port a unique number Example: by default

F0/1 has port priority value of 128 and .1 as

port ID giving: 128.1

F0/2 has 128.2

F0/1F0/2F0/1 would be the root port… X

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Configure port priority

SW2(config-if)#spanning-tree port-priority 112 Priority values range from 0 - 240, in

increments of 16. The ____________________________ ________________________

Becomes ________________________ ____________________________________

_____________________________ by STA to prevent looping

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3. Select designated/non-designated ports

____________ ports - All _______________ __________________________________ For _____ bridges, ______________________

ports. For ___________bridges, a designated port is the

_______________________________________________________________ as needed.

Only ___________________________________ ______________ ports - All ports configured

to be in a _______________ to prevent loops

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Select designated/non-designated ports cont…

If two switches are connected to the same LAN segment, which switch port will be the designated and which the non-designated port? Switches on the LAN segment in question exchange BPDU

frames, which contain the switch BID. Lowest path cost to the root bridge will be the designated

port. If _____________________? Switch with the ____________ has its port configured as a

_______________________ Switch with the higher BID- non-designated port

See example on next slide…

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Example…

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Summary of Port Roles

STP makes: Root ports- forwarding Designated ports- forwarding Non-designated ports- shut down or blocked

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Five Port states in traditional STP

Blocking – _______________ BPDU frames. Non-designated port which _________________ ___________________________________

Listening - ____________________ BPDU frames. Learning - _____________________ BPDU frames.

_____________________ in preparation of participating in frame forwarding

Forwarding – Fully active, _________________ Disabled – ___________________________

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States and timers

Up to ____________________ __________ again- full convergence

Amount of time that a port stays in the various port states depends on the ___________

During a topology change A port temporarily

__________________________________________ for a specified period called the "forward delay interval.“

Possibly forward or remain in blocked state

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BPDU timers

Timers are optimised for a 7-switch diameter network. _______________________________________

_______________________ to travel from the ________________ on the broadcast domain

The network has __________________ before switches forward user data.

Typically timers and the diameter should not be adjusted though they are configurable

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Cisco PortFast

An access port leading to a workstation or server does not need to go through the STP modes because it will not be closed down. The switch port can be configured with _____________

____________________________________________ If a switch is connected later and the port receives a

BPDU, STP can put the port into a blocking state and then through the modes. Feature called ______________________

To configure portfast, enter interface config mode for the port where portfast is going to be used and then the command:

spanning-tree portfast

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Verify spanning tree

Root bridge

This switch

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Topology change notification (TCN)

After the network converges, the root bridge sends out BPDUs, but the other switches do not normally send BPDUs back.

If there is a ______________________________ ______________ called the topology change notification (________) ______________________

Each switch that receives the TCN sends an ____________________ and sends a TCN towards the root bridge until the root bridge receives it.

The ____________________________ with the topology change (TC) bit set into the whole network

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STP developments

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PVST+

________________ can be implemented. Separate instance of spanning tree runs for

each VLAN

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PVST+ cont…

The VLAN needs to be identified, so each BID has 3 fields: Bridge Priority _____________________________________ MAC address.

Original BID just had bridge priority and MAC address

PVST+ is the default spanning-tree configuration for a Catalyst 2960 switch.

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PVST+ cont…

When the priority and extended system ID are prepended to the switch MAC address, each VLAN on the switch can be represented by a _________

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Rapid Spanning Tree Protocol __________________________________ with it.

RSTP (IEEE 802.1w) is an evolution of STP (IEEE 802.1D) Much _____________________ ____________________________

2 in version field indicates it is RSTP Sends BPDUs with its current information every 2

seconds. Does not use timers in the same way as STP

3 missed BPDUs taken to mean loss of the link. (6 seconds)

_____________________________ RSTP does not have a __________________________

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_______________ in RSTP

A port that will ______________________ __________________________ Immediately goes to forwarding state.

Same idea as Cisco’s PortFast. An edge port becomes a normal spanning-

tree port if it receives a BPDU Configuring an edge port uses the PortFast

keyword as before. spanning-tree portfast

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Link types

___________ are categorized into 2 link types, ____________________________ A _________________ between two switches is

regarded as a _________________ A link ____________________ is regarded as a

____________________ Ports on a ________________ are able to

_________________________ Designated ports

Make the most use of the link type parameter.

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Port states

Operational STP RSTP

Enabled Blocking Discarding

Enabled Listening Discarding

Enabled Learning Learning

Enabled Forwarding Forwarding

Disabled Disabled Discarding

• There are only _____________ in RSTP that correspond to the three possible operational states.

• The 802.1D ________________ states are ________ into a unique 802.1w ________________________

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RSTP Port Roles ____ - A ___________

that has been elected for the spanning-tree topology

_______________ - A ____________________________________

Alternate - An _______ ___________________ This path is different than using the root port.

Backup - A ___________________ to a segment where _____________________________________

_________ - Not strictly part of STP, a network administrator can ___________________

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Design considerations

Root bridge should be a ______________ in the ______________ of the network.

________ the number of _______________ Use VTP ____________. Use ________________________ _________________ even if no blocked

ports or physical redundancy exist

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Troubleshooting Suggestions

Before you troubleshoot a bridging loop, you need to know at least these items: Topology of the bridge network Location of the root bridge Location of the blocked ports and the redundant

links How the network looks when it works correctly

Use the “_________” command as needed