Ccnp Switch - Spanning Tree Protocol

Front 1

How does spanning tree prevent problems related to bridging loops?

Back 1

STP solves the problem by blocking redundant paths and allowing only a single active path.

Front 2

How does STP work?

Back 2

1. Selects a root switch
2. selects a loop-free path to each switch

Front 3

How does STP choose which route to take?

Back 3

It chooses

• a single root bridge
• one root port for each non-root switch
• one designated port for each network segment

Front 4

Name 5 types of STP

Back 4

• Common Spanning Tree (CST)
• Per VLAN spanning Tree Plus (PSVT+)
• Rapid STP (RSTP)
• PVRST+ (RPVST+)
• Multiple Spanning Tree (MST)

Front 5

What standard is STP?

Back 5

IEEE 802.1D

Front 6

What is CST?

Back 6

Common Spanning Tree

An instance of STP that runs for the entire switched network resulting in low CPU requirements
Suboptimal traffic paths when multiple VLANs are used.
Slow to converge time
Bridge ID does not include System if Extension field (VLAN ID on 12 bits)

Front 7

What is PVST+?

Back 7

Per VLAN Spanning Tree Plus

one instance of STP per VLAN,
more resources required,
slow convergence still
includes portfast
backbone fast
uplinkfast
BPDU filter
BPDU Guard and Loop Guard Enhancements
Interoperates with CST

Front 8

What is RSTP?

Back 8

Rapid Spanning Tree

• Fast Convergence
• suboptimal traffic
• Backbone fast
• UplinkFast
• PortFast

Front 9

Why does RSTP have suboptimal traffic?

Back 9

Only a single instance of the the entire switched network runs at a time.

Front 10

If RSTP is properly configured, how much faster is the convergence time?

Back 10

a few hundred miliseconds

Front 11

If everything in RSTP is working properly, what should be used as a backup or if there are no legacy devices?

Back 11

802.1D timers such as

• Forward Delay
• Max Age

Front 12

What is PVRST+ aka RPVST+?

Back 12

Behaves similarly to PVST+
supports separate instances of RSTP for each VLAN
BPDU Filter
BPDU Guard
Root Guard
Loop Guard Enhancements
Largest CPU and memory requirments

Front 13

What is MST?

Back 13

Multiple Spanning Tree

• Map multiple VLANs
• similar traffic to RSTP
• Supports RSTP for fast convergence
• BPDU filter
• BPDU Guard
• Root Guard
• Loop Guard Enhancements

Front 14

What is the dominate spanning-tree protocols of choice in Cisco Switches?

Back 14

Multiple Spanning Tree (MST)
Per VLAN Spanning Tree (PVRST+)

Front 15

What is the default flavor of STP that is enabled when a VLAN is created on most switches?

Back 15

Per VLAN Spanning Tree (PVRST+)

Front 16

How does spanning tree build the tree structure?

Back 16

By using the fastest links it has available for the active path.

Front 17

What makes a BPDU Superior to another?

Back 17

Lowest Bridge ID (BID)
Lowest path cost to the root
Lowest updtream bridge ID
Lowest updtream port ID (PID)

Front 18

What is a bridge ID?

Back 18

Bridge Priority and MAC address

Front 19

What is the Bridge Priority?

Back 19

0-65,535

Front 20

What is the default Priority?

Back 20

32,768

Front 21

What is Path Cost?

Back 21

The cumulative cost of all links between the switch and the root bridge.

Front 22

What is the Port Priority range?

Back 22

0-240

Front 23

What is the default port priority?

Back 23

128

Front 24

In what increments do port priority increase?

Back 24

increments of 16

Front 25

What is STA?

Back 25

Spanning Tree Algorithm

Implements distributed variations of the Bellman Ford iterative algorithm.

Front 26

How is the Bellman Ford Iterative Algorithm described?

Back 26

A gradient process.

NOTE:
Meaning It repeatedly looks for the optimal solution selecting an optimal candidate every time.

Front 27

What is Step One in the STP Selection?

Back 27

Each VLAN elects one root bridge. The Bridge with the lowest BID becomes the root.

Front 28

What happens to the ports on the root bridge?

Back 28

They become designated ports.

Front 29

How are root ports determined on all non-root brdges?

Back 29

• The root port is chosen based on the port with the lowest-cost path between the non-root bridge and root bridge.
• Lowest upstream Bridge ID
• Lowest upstream port ID (PID)

Front 30

How do you choose a designated port?

Back 30

• They are chosen between switches from non-root ports that have the lowest path cost to the root bridge.
• If tied: Bridge ID works as a tie breaker (lowest winds)
• If Tied: Port ID (lowest wins)

Front 31

Where is an RP?

Back 31

Root Port

Always on an upstream facing interface and always facing a DP

Front 32

Where are Root Ports Located?

Back 32

on non-root bridges only

Front 33

What does a (Root Port) RP do with traffic?

Back 33

Receives and forwards traffic toward the bridge as needed.

Front 34

What does a (Root Port) RP mostly receive?

Back 34

BPDU's

NOTE:

Sends very few of them

Front 35

How many Root Ports per switch?

Back 35

one

Front 36

Does a root port populate on a MAC table?

Back 36

It can

Front 37

Where is a DP?

Back 37

Designated Port

Always a downstream facing Interface.

Front 38

Where are designated ports (DP) located?

Back 38

On root and non-root bridges.

Front 39

All the ports on a root bridge are ___________?

Back 39

Designated

Front 40

What does a Designated Port (DP) do with traffic?

Back 40

Receives and forwards them as needed.

Front 41

What does a Designated port mostly send?

Back 41

BPDUs

NOTE:

Receives very few of them.

Front 42

How many designated ports are there?

Back 42

Only one per segment and per VLAN in PVST+

Front 43

Can Designated Port (DP) Populate in a MAC table?

Back 43

Yes

Front 44

Where is a Non designated port located?

Back 44

Always an upstream facing interface

Front 45

Does a non designated port mostly send or receive BPDUs?

Back 45

Receives

NOTE:

It sends very few

Front 46

What is a disabled Port?

Back 46

A port that is shut down.

Front 47

What are the 5 STP port states?

Back 47

1. Disable
2. Blocking
3. Listening
4. Learning
5. Forwarding

Front 48

What does a disabled port do?

Back 48

• Does not participate in STP
• Does not forward nor receive frames, including BPDUs

Front 49

Where is a blocking port located?

Back 49

Always an upstream facing interface

Front 50

What does a blocking port do with BPDUs?

Back 50

Absoluteely needs to receive BPDUs to remain blocking and to determin root switch.

Front 51

Can blocking port be a Root Port or a Designated Port?

Back 51

No

Front 52

What happens to a blocking port in a non designated role?

Back 52

Does not forward or receive traffic.

Front 53

What is the convergence time for a blocking port?

Back 53

20 seconds in this state before trying to converge (max-age)

Front 54

What is a Listening Port?

Back 54

It receives and sends BPDUs

Front 55

What is the forward delay on a listening port?

Back 55

15 seconds

Front 56

What type of port is a forwarding port?

Back 56

Root Port or Designated Port

Front 57

What port state is part of the active topology?

Back 57

Forwarding

Front 58

What does a forwarding port do to frames?

Back 58

Forwards and receves frames.

Front 59

What does a forwarding port do to BPDUs?

Back 59

Sends and Receives

Front 60

What metric does spanning tree use to determine the Root Port and Designated Port?

Back 60

Cost

Front 61

How is cost calculated in STP?

Back 61

By adding all of the links between the root bridge and the local switch.

Front 62

What is the cost for 10 MBits/s?

Back 62

100

Front 63

What is the cost for 100 MBits/s?

Back 63

19

Front 64

What is the cost for 1 Gbit/s?

Back 64

4

Front 65

What is the cost for 10 Gbits/s?

Back 65

2

Front 66

What are the 3 BPDU types?

Back 66

1. Configuration
2. Topology Change Notification (TCN)
3. Topology Change Acknowledgement (TCA)

Front 67

What is a Configuration BPDU?

Back 67

Used for STP Computation

Front 68

How does a configuration BPDU work?

Back 68

Sourced by the root bridge to advertise Bridge and link attributes

Front 69

What is a TCN?

Back 69

Topology Change Notification

Used to advertise network topology notification

Front 70

What does a Topology Change Notification (TCN) quickly age?

Back 70

quickly age out the CAM table of all switches in case of a port state topology change.

Front 71

What does TCN Configure?

Back 71

Topology Change Notification

Configures BPDUs with Topology Change Flag Set

Front 72

What is the default hello timer?

Back 72

2

Front 73

What is the default Max age timer?

Back 73

20 s (10 Hellos)

Front 74

What is the default forward delay?

Back 74

15s

Front 75

How are STP timers advertised and set?

Back 75

By the current root bridge for the whole layer 2 (VTP)domain

NOTE:

This may change as the root bridge fails over to another switch

Front 76

How does STP Reconverge

Back 76

-The BPDU (Bridge Protocol Data Unit) TCN is sent by a switch through its RP a soon as it detects a port state/topology change.
-Upstream switch sends a TCA back.
-Upstream switch sends a TCN out its RP.
-Next upstream switch sends a TCA back.
-Next upstream switch sends a TCN out its RP.
-Process goes on until the RB receives the TCN.
-The RB replies with a TCN out all its ports for a certain period of time (max_age + forward_delay), relayed by all downstream switches.
The result is that the CAM aging time is reduced to "Forward delay" in all switches: from 5 mn to 15 seconds during 35 seconds (by default).
-This whole process does not happen if the state of a port changes while it is configured with the portfast feature.

Front 77

What are the 3 major implications by default of PSVT+

Back 77

Per VLAN Spanning Tree Plus

· All VLANS will elect the same root bridge which will be the one with the lowest MAC address, generally the oldest switch!
· A port state / topology change will impact all VLANs in exactly the same way
· All redundant links will also be blocked in exactly the same manner

Front 78

In STP load balancing what happens to the VLANs if the topology or port state change?

Back 78

They will be impacted the same way.

Front 79

How are redundant links blocked?

Back 79

In the same Manner.

Front 80

How can you force VLANs to prefer one link and others prefer other links?

Back 80

Change the local port cost or upstream port priority for the active VLANs

Front 81

What happens if you change the local port cost or upstream priority?

Back 81

Traffic would begin to use that link and place the other uplink into the blocking state

Front 82

What is Portfast?

Back 82

This causes layer 2 switch interfaces to enter forwarding state immediately, bypassing the listening and learning states,

Front 83

Where should Portfast be used?

Back 83

Ports directly connected to end hosts like servers or workstations.

Front 84

What happens if the switch does not generate any TCN?

Back 84

Topology Change Notification

When something changes on a portfast access port, leaving the CAM tables unchanged in the entire layer 2 domain.

Front 85

What happens if portfast is not enabled?

Back 85

DHCP timeouts can occure while STP converges, causing more problems

Front 86

What is UplinkFast applied?

Back 86

At the access Layer.

Front 87

What does UplinkFast do?

Back 87

Provides a mechanism to very quickly (saving 30 sec) have a secondary uplink to the distribution layer if the current STP uplink fails.

NOTE:

This is similar to RSTP alternate port role and only address a direct link failure

Front 88

With UplinkFast after the alternate port takes over what happens?

Back 88

The switch floods its CAM table out the alternate port.

Front 89

What is CAM?

Back 89

Content Addressable Memory

This removes the need for the dummy multicast generation process of UplinkFast.

Front 90

What does UplinkFast do to the cost of the interfaces?

Back 90

It changes them to a very high value.

Front 91

Why does UplinkFast change the cost values of the interfaces?

Back 91

In order to persuade downstream switches to use another switch for their path to the root since it appears to have at least one unstable link.

Front 92

What is BackboneFast?

Back 92

It is often applied at the distribution and core layers and was developed to fix STP convergence slowness on indirect backbone links.

Front 93

What is designed to detect failures?

Back 93

UplinkFast

Front 94

What is designed to detect indirect failures?

Back 94

BackboneFast

Front 95

What is an indirect failure?

Back 95

When detection is not immediate it depends on the max age timer.

Front 96

What does BackboneFast eliminate?

Back 96

The max age timeout period associated with an indirect failure, lowering convergence from the default 50 seconds to 30 seconds.

Front 97

What are BPDUs?

Back 97

Bridge Protocol Data Unit

BPDUs are data messages that are exchanged across the switches within an extended LAN that uses a spanning tree protocol topology.

Front 98

What doe BPDU packets contain?

Back 98

Bridge Protocol Data Unit

BPDU packets contain information on ports, addresses, priorities and costs and ensure that the data ends up where it was intended to go