Ccna3-Stp

Front 1

why does ethernet frame loops happen?

Back 1

-there is not TTL like in IP for ethernet packets. So, if they are not properly terminated, they continue to bounce.

Front 2

loops are possible for

Back 2

broacast packets
unicast packets
when a cable is connected to ports on the same switch

Front 3

EtherChannel

Back 3

grouping of ethernet ports on a switch that act as a single logical network connection. Because the switch treats the ports configured for the EtherChannel as a single network link, loops are not possible.

Front 4

STP

Back 4

ensures that there is only one logical path between all destinations on the network by intentionally blocking redundant paths that could cause a loop.

Front 5

BPDU

Back 5

-bridge protocol data unit.
sent even if the port is blocked.

Front 6

STP algorithm

Back 6

SPA (spanning tree algorithm)
-determine which switch ports on a network need to be configured for blocking

Front 7

root bridge

Back 7

designates a single switch as the root bridge and uses it as a reference for all calculations.

Front 8

how is root bridge chosen?

Back 8

all switches using STP exchange BPDU and determine which has the lowest bridge ID. The lowest BID becomes the root.

Front 9

STA process

Back 9

-after root is selected,
STA calculates the shortest path to the root.
-while STA determines the best path to root, all traffic is blocked.
-STA considers both path and port costs to determine which to block.

Front 10

path costs

Back 10

pasth costs are based on port cost based on port speed. The sum of port costs dtermine the overall path cost to the root.

Front 11

root port

Back 11

switch ports closest to the root bridge.

Front 12

designated ports

Back 12

all non-root ports that are permitted to forward traffic.

Front 13

non-designated ports

Back 13

all ports configured to be in a blocking state to prevent loops.

Front 14

BPDU fields

Back 14

12 fields

Front 15

Bridge id consists of

Back 15

Bridge Priority
Extended system ID
MAC address

Front 16

configure bridge priority

Back 16

spanning-tree vlan vlan-id root primary

Front 17

extended system id

Back 17

id of the vlan with which the BPDU is associated.

Front 18

broadcast storm

Back 18

occurs when there are so many broadcast frames caught in layer 2 loop that all available bandwidth is consumed.

Front 19

BPDU

Back 19

is the message frame exchanged by siwtches for STP

Front 20

BID (bridge ID)

Back 20

contained in a BPDU.
contains a priority and MAC address of the sender and optional extended system id

Front 21

switch bootup behavior

Back 21

all switches in a broadcast domain participate in the election process.
-when a switch boots, it sends a bpdu with bid and root ID every 2 secs.
-root id matches the local bid
-the root id is for the root bridge on the netowrk
-each switch identifes as root bridge after bootup.
-if the adjacent switches root id is less that it's own root id, the root id for itself is updated with the neighbor's root id.
-the switch with the lowest bid ends up as the root bridge

Front 22

STP BPDU

Back 22

12 fields
-1st 4 fields have the path and priority info
-protocol id, version, message, flags
4-8 determines the root bridge and cost of the path to the root bridge
-8-12 determines timer fields that determine the frequency of messages

Front 23

default frequency of BPDU

Back 23

2 seconds

Front 24

local switch info

Back 24

it's own BID,root ID and path cost to root

Front 25

BPDU process

Back 25

priority is the first factor for picking the root bridge.
if priority is the same, mac address is used.

Front 26

BID fields

Back 26

contains priority, extended system id and mac

Front 27

bridge priority

Back 27

can be changed to influence the selection of the root bridge. 1 is the highest priority

Front 28

extended system id

Back 28

id of the vlan on which the BPDU is associated.

Front 29

to ensure that the switch has the lowest bridge priority

Back 29

configure :
spanning-tree vlan vlan-id root primary
this will be set to a predefine value or to the next 4096 decremented value below the lowest bridge priority detected on the network

Front 30

to ensure that a switch is picked as the secondary root bridge

Back 30

configure:
spanning tree vlan vlan-id root secondary

Front 31

second method for configuring the primary`

Back 31

spanning-tree vlan vlan-id priority value
-a value configured in increments of 4096

Front 32

port roles

Back 32

location of the root bridge in the network topology determines how port roles are calculated.

Front 33

root port

Back 33

-root port exists on non-root bridges and is the switch port with the best parth to the root bridge.
-root ports forward traffic toward the root bridge
-the source mac received on the root port are populated in the mac table.
-only one root port is allowed per bridge.

Front 34

designated port

Back 34

-exists on root and non-root bridges
-for a root bridge, all ports are designated ports
-only one designated port is allowed per segment.
-if multiple switches are on the same segment, an election process is done to determine the deisgnated switch.

Front 35

non-designated port

Back 35

- is a switch port that is blocked.

Front 36

disabled port

Back 36

-is a switch port that is administratively shutdown.

Front 37

root port

Back 37

switch port with the lowest cost to the root bridge.

Front 38

root port

Back 38

exists on non-root bridges and is the switch port with the best path to the root bridge.

Front 39

designated port

Back 39

-only one allowed per segment.
-for non-root bridges, a designated port is the switch port that receives and forwards frames toward the root bridge as needed.
-capable of populating the MAC table

Front 40

non designated port

Back 40

blocked
-not fowarding
-not populating the mac address table
-also called the alternate port

Front 41

disabled port

Back 41

switch that's administratively down

Front 42

port roles

Back 42

when there are two ports with the same lowest path cost to the root bridge, switch uses the port priority value or the port ID

Front 43

port ID

Back 43

the interface ID of the switch port
-ex: priority.id

when one port is chosen over the other, the other is marked as non-designated port

Front 44

configure port priority

Back 44

configure terminal
interface ifid
spanning-tree port-priority <priority>

Front 45

port role decisions

Back 45

-root bridge automatically configures all of its ports as designated ports. Other ports have to decide on their non root ports

Front 46

how to decide which is the designated port and which is the non-designated port

Back 46

- the two switches with ports on the same LAN that are not root ports, they exchange BPDUs. The switch with lowest path cost get the designated port. If they are both equal, the switch with the lower BID gets the designated port.

Front 47

each port transitions through 5 states and 3 timers

Back 47

-blocking
-listening
-learning
-forwarding
-disabled

Front 48

blocking

Back 48

non designatedp ort. receives BPDUs to determin the location and the root id

Front 49

listening

Back 49

stp determined the port can participate in frame forwarding. it is transmitting it's BPDU frames to tell others that it can participate in active topology.

Front 50

learning

Back 50

prepares to participate in frame forwarding and begins to populate the MAC address table

Front 51

forwarding

Back 51

the port is considered part of the active topology

Front 52

disabled

Back 52

does not participate in spanning tree and does not forward frames.

Front 53

hello time

Back 53

-the hello time is the time between each BPDU frame that is sent on a port.
-2 sec by default, can be between 1 and 10

Front 54

forward delay

Back 54

-the forward delay is the time spent in the listening and learning state.
-the default is 15 secs. can be between 4 and 30

Front 55

maximum age

Back 55

the max age timer controls the maximum length of time a switch port saves a configuration BPDU info.
20 secs by default

Front 56

switch diameter

Back 56

number of switches a frame has to traverse to travel from the two farthest point on the broadcast domain

Front 57

configure switch diameter

Back 57

spanning-tree vlan 1 root primary diameter 5. 7 is the maximum allowed.

Front 58

convergence

Back 58

is the time it takes to recalculate the spanning tree if a switch or a link fails.

Front 59

cisco portfast

Back 59

a switchport configured with portfast is configured as an access port, the port transitions from blocking to forwarding state immediately.

Front 60

purpose of portfast

Back 60

minimizes the time that access ports must wait for spanning tree convergence
-should only be used on access ports

Front 61

configure portfast

Back 61

spanning-tree portfast in the interface config mode

Front 62

convergence steps

Back 62

1. elect a root bridge
2. elect root ports
3. elect the designated and non-designated ports

Front 63

electing a root bridge

Back 63

trigged after boot up or if a path failure is detected.

Front 64

initial state of a port

Back 64

blocking and lasts 20 secs

Front 65

election time for a root bridge

Back 65

14 seconds due to the diameter max being 7.

Front 66

after boot up

Back 66

-BPDU is sent at every hello timer expiry by default 2 secs.

Front 67

TCN

Back 67

topology change notification

Front 68

tcn

Back 68

-happens when a port that was forwarding goes down
-happens when a port transitions to forwarding and the switch has a designated port.

Front 69

what does the switch that detects tcn do with this info

Back 69

notifies the root bridge of the STP.
root bridge broadcasts the information to the network

Front 70

normal stp operation

Back 70

a switch gets BPDU from the root bridge on the root port, but never sends a BPDU to the root bridge.

Front 71

TCN

Back 71

is the only BPDU that a switch sends to the root bridge.

Front 72

TCN format

Back 72

contains no infol
sent at the time of the hello interval expiry.

Front 73

receiver of the TCN

Back 73

is called the designated bridge.
-it acks the TCN by sending back a normal BPDU with a TCA bit set. This happens until one designated bridge forwards the BPDU to the root bridge and root bridge responds.

Front 74

root bridge broadcast of TCN

Back 74

sends a broadcast BPDU with the Topology change bit set. The switch can receive this both on the forwarding and the blocking ports.

Front 75

TC bit

Back 75

set by the root for a period of
max age + forward delay seconds
20+15=35 sec by default.

Front 76

per-vlan spanning tree (PVST)

Back 76

maintains spanning tree instance for each vlan

Front 77

PVST

Back 77

-uses Cisco proprietary ISL trunking protocol that allows a VLAN trunk to be forwarding for some VLANs while blocking other vlans.

Front 78

PVST

Back 78

-treats each vlan as a separate network
-load balance traffic at layer 2
-each vlan has an instance of spanning tree
-includes extenstions

Front 79

PVST +

Back 79

support IEEE 802.1Q trunking
-includes extensions
-includes portfast enhancements called BPDU guard and root guard.

Front 80

Rapid-PVST+

Back 80

-based on IEEE 802.1w
-faster convergence than 802.1d

Front 81

RSTP

Back 81

-faster than 8021.d
-implements generic version of cisco propiertary STP extensions

Front 82

MSTP

Back 82

-multiple vlans mapped into the same spanning-tree instance
-provides multiple forwarding paths for data traffic and enables load balancing.

Front 83

PVST +

Back 83

-all switches are engaged in converging the network.
-all switch ports have to accomodate the additional bandwdth used for each PSVST+ to send it's own BPDUs

Front 84

PVST+ root bridge implentations

Back 84

-half the vlans can forward on each uplink trunk.
-configure on switch to be elected the root bridge for half of the total number of vlans in the network
-configure a second switch to be elected the root bridge for tthe other half of the vlans.

Front 85

PVST+ bridge id

Back 85

-extneded system Id is added to the BID to indiate the vlan id. When this happens, the priority becomes 4 bits insead of 16.

Front 86

PVST+ priority field

Back 86

increments of 4096 instead of 1.
The default priority is 32768

Front 87

configure 2960 series for PVST+

Back 87

enable state - enabled on vlan 1
STP mode - PVST+
switch priority -
port priority - 128
port cost - 4 for 1000, 19 for 100
and 100 for 10

Front 88

PVST +

Back 88

-all switches are engaged in converging the network.
-all switch ports have to accomodate the additional bandwdth used for each PSVST+ to send it's own BPDUs

Front 89

PVST+ root bridge implentations

Back 89

-half the vlans can forward on each uplink trunk.
-configure on switch to be elected the root bridge for half of the total number of vlans in the network
-configure a second switch to be elected the root bridge for tthe other half of the vlans.

Front 90

PVST+ bridge id

Back 90

-extneded system Id is added to the BID to indiate the vlan id. When this happens, the priority becomes 4 bits insead of 16.

Front 91

PVST+ priority field

Back 91

increments of 4096 instead of 1.
The default priority is 32768

Front 92

configure 2960 series for PVST+

Back 92

enable state - enabled on vlan 1
STP mode - PVST+
switch priority -
port priority - 128
port cost - 4 for 1000, 19 for 100
and 100 for 10
vlan port priority - 128
vlan port cost - same as port cost
timers - hello 2secs
forward delay - 15
max age time 20
transmit hold count - 6 BPDUs

Front 93

configure PVST +

Back 93

-select the switch to be primary and secondary root bridges for each vlan.
-configure a switch to be primary for one vlan
-configure a switch to be secondary root bridge for a vlan.
-set the spanning tree priority to be low enough so the configured switches will assume it's roles

Front 94

configure primary root bridge for PVST+

Back 94

spanning-tree mode pvst
spanning-tree extended system-id
spanning-tree vlan vlan-id root primary

Front 95

configure secondary root bridge for PVST+

Back 95

spanning-tree vlan vlan-id root secondary

Front 96

configure priority

Back 96

spanning-tree vlan vlan-id priority <increments of 4096)

Front 97

show active spanning tree config

Back 97

show spanning-tree active

Front 98

RSTP port states

Back 98

discarding, learning or forwarding
-discarding port is the alternate

Front 99

RSTP charasteristics

Back 99

-speeds up convergence
-redefines the type of port states
-preferred protocol
-cisco proprietary protocols are not compatible
-backward compatible to STP
-BPDU is the same STP, version is incremented to 2.
-actively confimr that a port can transition to forwarding without relying on timer configuration.

Front 100

RSTP flag field

Back 100

-indicates the port role, state, TCA, TCN etc.

Front 101

RSTP BPDU

Back 101

-info can be immediately aged on a port is a hello is not received for 3 consecutive hello times or if the max age expires

Front 102

edge port

Back 102

can never be connected another switch.
immediately transitions to forwarding

Front 103

edge port

Back 103

an RSTP edge port that receives a BPDU loses edge port status and becomes part of the normal spanning tree port.

Front 104

edge port configuration

Back 104

spanning-tree portfast

Front 105

link type

Back 105

-predetermine the active role that the port plays

Front 106

non-edge ports link types

Back 106

point-to-point and shared.

Front 107

edge port link type

Back 107

point-to-point

Front 108

port roles

Back 108

-before link type is considered, RSTP determines port roles
-root ports do not have link type parameter
-root ports can transition to forwarding as the port is in sync
-alternate and backup ports do not link type
-designated ports use link type. Transition of a DP to forwarding occurs when the link type indicates a point-point link

Front 109

RSTP port states

Back 109

discarding - prevents forwarding of data frames. Can happen in the active state or topology sync state.
learning - accepts and populates the MAC table
forwarding - only during stable active topology. Forwarding of data frames occurs only after a proposal and agreement process.

Front 110

difference between STP and RSTP port states

Back 110

stp rstp
blocking discarding
listening discarding
learning learning
forwarding forwarding
disabled disabled

Front 111

rstp port roles

Back 111

defines the purpose of a switch port and how it handles the data frames.
-root port
-designated port
-alternate port

Front 112

proposal and agreement process

Back 112

in stp, when a port becomes the designated port, it has to wait 2 times the forward delay before moving to forwarding state.
in rstp, convergence is done on a link by link basis.

Front 113

Rapid PVST-+

Back 113

RSTP version to use on Cisco systems

Front 114

Rapid PVST+ configuration

Back 114

- spanning tree instance is created when an interface is assigned to a vlan and it is removed when the last interface is moved to another vlan.

Front 115

cisco 2960

Back 115

supports PVST+, RPVST+ and MSTP

Front 116

configuration commands

Back 116

configure terminal
spanning-tree mod rapid-pvst
interface interface-id
spanning-tree link-type point-to-point
clear spanning-tree detected protocols