Spanning Tree
protocol- Chapter 5
CCNA Exploration Semester 3
Modified by Profs. Ward
and Cappellino
1
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
2
Semester 3
LAN Design
Basic Switch
Concepts
VLANs
VTP
Wireless
STP- Ch 5
Inter-VLAN
routing
3
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
4
Issues with Redundancy

Multiple ___________________

Create problems when all the links are active:



____________________
_____________________
____________________
See examples on following slides…
5
Broadcast storm
2.
Flooda
There’s
broadcast
switching loop
through nonsource ports
1. Send
ARP
request
B
A
3. And so on
with nothing to
stop it
C
D
6
Duplicate Unicast Frame Transmissions
B
A is on port 3
Don’t know B
So flood
A
Send
frame to
B
C
Frame
arrives
A
B
D
And
again
7
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
____________
____________
8
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.
9
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.
10
What is a spanning tree?
A

tree (extended star) topology
A ______________________
 Spanning

all devices
__________________________
 Spanning
tree is _____________
immediately __________________
_______________________
11
Not a spanning tree

Not a tree - it has loops.
12
Not a spanning tree

Not spanning. Device left out.
13
Spanning tree

No loops. Includes all devices.
14
Spanning tree protocol





Used by switches to __________________
_________________________________
________ unwanted links by _____________
STP defined by ________________
_____________ defined by _____________
Switches __________________ –
________________________________
15
Spanning tree algorithm
1.
2.
3.
The switches use this algorithm to
configure the ports _______
____________________
Choose ___________ to be
“____________”
Choose a “_______” on ______
_____________ closest to the root
bridge
Choose a “__________” which are
all _______________ that are still
___________________________
___________________________
_____________________ known
as “non-designated” ports
16
Example of port designations
Root bridge
Designated port
Root port
Root port
Designated port
Root port
Designated port
Designated port
Nondesignated
port
17
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.
18
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
19
BPDU



The _______________________________
________________________________
The BPDU message is encapsulated in an
Ethernet frame.
The ______________ MAC address in the
BPDU frame is a _________________ for
the ___________________________
20
BPDU Frame contains 12 fields:
2 bytes
Protocol ID
1 byte
Version
1 byte
Message type
1 byte
Flags
8 bytes
Root ID
4 bytes
Cost of path
8 bytes
Bridge ID
2 bytes
Port ID
2 bytes
Message age
2 bytes
Max age
2 bytes
Hello time
2 bytes
Forward delay
Administrative
uses
BID and path
information- used
to identify the root
bridge and the
cost
Timer fields used
to determine how
frequently BPDU
messages are sent
21
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
22
address
Configure priority


Set a _____________________ if a specific switch
is to become the root bridge
Method 1:



Method 2:


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.
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?
23
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?...
24
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
10 Gbps
1 Gbps
100 Mbps
10 Mbps
Revised cost
2
4
19
100
Previous cost
1
1
10
100
25
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
26
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.
27
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
F0/1 would be the root port…
F0/2
X
F0/1
28
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
29
3. Select designated/nondesignated 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
30
Select designated/nondesignated 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…
31
Example…
32
Summary of Port Roles

STP makes:



Root ports- forwarding
Designated ports- forwarding
Non-designated ports- shut down or blocked
33
Five Port states in traditional STP





Blocking – _______________ BPDU frames. Nondesignated port which _________________
___________________________________
Listening - ____________________ BPDU frames.
Learning - _____________________ BPDU frames.
_____________________ in preparation of
participating in frame forwarding
Forwarding – Fully active, _________________
Disabled – ___________________________
34
States and timers


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
Up to ____________________
__________ again- full
convergence
35
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
36
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.


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.


The switch port can be configured with _____________
____________________________________________
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
37
Verify spanning tree
Root bridge
This switch
38
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
39
STP developments
40
PVST+


________________ can be implemented.
Separate instance of spanning tree runs for
each VLAN
41
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.
42
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 _________
43
Rapid Spanning Tree Protocol

__________________________________ with it.



Much _____________________
____________________________



2 in version field indicates it is RSTP
Sends BPDUs with its current information every 2
seconds.


RSTP (IEEE 802.1w) is an evolution of STP (IEEE 802.1D)
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 __________________________
44
_______________ in RSTP

A port that will ______________________
__________________________




Immediately goes to forwarding state.
Same idea as Cisco’s PortFast.
An edge port becomes a normal spanningtree port if it receives a BPDU
Configuring an edge port uses the PortFast
keyword as before.

spanning-tree portfast
45
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.
46
Port states
• There are only _____________ in RSTP that
correspond to the three possible operational states.
• The 802.1D ________________ states are ________
into a unique 802.1w ________________________
Operational
STP
RSTP
Enabled
Blocking
Discarding
Enabled
Listening
Discarding
Enabled
Learning
Learning
Enabled
Forwarding
Forwarding
Disabled
Disabled
Discarding
47
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
___________________
48
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
49
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
50