Chapter 5 - Spanning Tree Protocol CCNA pdf

Chapter 5
-
Spanning Tree Protocol
Chapter

5

Spanning

Tree

Protocol
CCNA Exploration 4.0
Objectives
• Explain the role of redundancy in a converged
network
network
.
• Summarize how STP works to eliminate Layer 2
loo
p
s in a conver
g
ed network.
pg
• Explain how the STP algorithm uses three steps to
converge on a loop-free topology.
• Implement rapid PVST+ in a LAN to prevent loops
between redundant switches.
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Layer 2 redundancy

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Redundancy in a hierarchical network
• Layer 2 redundancy improves the availability of the network by
implementing alternate network paths by adding equipment and
cabling.
cabling.
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Examine a redundant design
• In a hierarchical design, redundancy is achieved at the distribution and
core layers through additional hardware and alternate paths through
the additional hardware
the

additional

hardware
.
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Examine a redundant design
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Examine a redundant design
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Examine a redundant design
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Examine a redundant design
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Types of Traffic
Uk Ui
T
yp

es of traffic
(
La
y
er 2
p
ers
p
ective
)
U
n
k
nown
U
n
i
cast
yp ( y p p )
• Known Unicast: Destination addresses are in Switch Tables
• Unknown Unicast: Destination addresses are not in Switch Tables
• Multicast: Traffic sent to a
g
rou
p
of addresses
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gp
• Broadcast: Traffic forwarded out all interfaces except incoming
interface.

Issues with Redundancy
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Issues with Redundancy
A redundant switched topology (STP disabled) may cause:
• La
y
er 2 Loo
p
s
yp
• Broadcast Storms
• Duplicate Unicast Frames
• MAC address table instability
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Layer 2 Loops
• When multiple paths exist between two devices on the network and
STP has been disabled on those switches, a Layer 2 loop can occur.
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Broadcast Storms
• A broadcast storm occurs when there are so many broadcast frames
caught in a Layer 2 loop that all available bandwidth is consumed.
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Duplicate Unicast Frames
• Unicast frames sent onto a looped network can result in duplicate
frames arriving at the destination device.
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Issues with Redundancy
• Incorrectly learn the MAC address
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Real-world Redundancy Issues

Loops in the Wiring Closet
• If the network cables are not properly labeled when they are terminated
ith th lith ii l tNt kl tht lt
i
n
th
e pa
t
c
h
pane
l

i
n
th
e w
i
r
i
ng c
l
ose
t
.
N
e
t
wor
k

l
oops
th
a
t
are a resu
lt

of accidental duplicate connections in the wiring closets are a common
occurrence.
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Real-world Redundancy Issues
Loops in the Cubicles
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Prevent loop, storm bordcast…?
• Block redundant link and auto unblock redundant link when
primary link down.
• Via Spanning Tree Protocol
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S
p
annin
g
Tree Protocol
pg
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The Spanning Tree Algorthm
• STP ensures that there is only one logical path between all destinations on the
network by intentionally blocking redundant paths that could cause a loop. This

is Critical.
• A port is considered blocked when network traffic is prevented from entering or
leaving that port.
• The physical paths still exist to provide redundancy, but these paths are
di bl d t t th l f i
di
sa
bl
e
d

t
o preven
t

th
e
l
oops
f
rom occurr
i
ng.
• If the path is ever needed to compensate for a network cable or switch failure,
STP recalculates the paths and unblocks the necessary ports to allow the
redundant path to become active
redundant

path

to

become

active
.
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Spanning-Tree Protocol
• Layer 2 protocol
•
STP executes an algorithm called
Spanning Tree Algorithm (STA)
•
STP

executes

an

algorithm

called

Spanning

Tree

Algorithm

(STA)

.
• STA chooses a reference point, called a root bridge, and then
determines the available paths to that reference point.
–
If m
o
r
e

t
h
a
n
t
w
o

pat
h
s

e
xi
sts,

S
TA
p
i
c

k
s

t
h
e

best

pat
h
a
n
d

b
l
oc
k
s

o e t a t o pat s e sts, S p c s t e best pat a d b oc s
the rest
• STP calculations make extensive use of two key concepts in creating a
loop-free topology:
Bridge ID
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–
Bridge

ID
– Path Cost
STP Algorithm
• STP uses the Spanning Tree Algorithm (STA) to determine which
switch ports on a network need to be configured for blocking to prevent
loops from occurring.
• Root Bridge – The lowest BID
• Root Ports - Switch ports closest to the root bridge.
• Designated ports - All non-root ports that are still permitted to forward traffic
on the network.
• Non-designated ports - All ports configured to be in a blocking state to
prevent loops.
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The Root Bridge
• Every spanning-tree instance (switched LAN or broadcast domain) has
a switch designated as the root bridge. The root bridge serves as a
reference point for all spanning
tree calculations to determine which
reference

point

for

all

spanning
-
tree

calculations

to

determine

which

redundant paths to block.
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Bridge ID (BID)
• For each Network, the switch with the highest switch priority (the
lowest numerical priority value) is elected as the root switch.
•
Th dtil f hBIDfild di dlt btiti flt
•
Th
e
d
e
t
a
il
s o
f
eac
h

BID

fi
e
ld
are
di
scusse
d

l
a
t
er,
b
u
t

it

i
s use
f
u
l

t
o
know now that the BID is made up of a priority value, an extended
system ID, and the MAC address of the switch.
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