protocol- Chapter 5
CCNA Exploration Semester 3
Modified by Profs. Ward
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
Basic Switch Wireless
VLANs STP- Ch 5
Redundancy accomplished at
What is redundancy?
One link or device fails –
another takes over.
Redundancy allows flexibility
but does have some
Issues with Redundancy
Create problems when all the links are active:
See examples on following slides…
2. Flooda 3. And so on
There’s B with nothing to
through non- stop it
1. Send A C
Duplicate Unicast Frame Transmissions
A is on port 3
Don’t know B
frame to Frame
A B And
Loops by mistake
Even if there are no deliberate loops for
redundancy, there can be loops set up by
For example, by improper or no labeling of
cables in the wiring closet
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.
with the combined bandwidth.
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.
What is a spanning tree?
A tree (extended star) topology
Spanning all devices
Spanningtree is _____________
Not a spanning tree
Not a tree - it has loops.
Not a spanning tree
Not spanning. Device left out.
No loops. Includes all devices.
Spanning tree protocol
Used by switches to __________________
________ unwanted links by _____________
STP defined by ________________
_____________ defined by _____________
Switches __________________ –
Spanning tree algorithm
The switches use this algorithm to
configure the ports _______
1. Choose ___________ to be
2. Choose a “_______” on ______
_____________ closest to the root
3. Choose a “__________” which are
all _______________ that are still
as “non-designated” ports
Example of port designations
Designated port Designated port
Root port Root port
Root port Designated port
Designated port Non-
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.
Choose the root bridge (cont…)
___________ Bridge Protocol Data Units (______)
The switch with the __________________________
As a switch receives a BPDU, it ___________________
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
The BPDU message is encapsulated in an
The ______________ MAC address in the
BPDU frame is a _________________ for
BPDU Frame contains 12 fields:
2 bytes Protocol ID Administrative
1 byte Version uses
1 byte Message type
1 byte Flags
8 bytes Root ID BID and path
4 bytes Cost of path information- used
to identify the root
8 bytes Bridge ID bridge and the
2 bytes Port ID cost
2 bytes Message age Timer fields used
2 bytes Max age to determine how
2 bytes Hello time messages are sent
2 bytes Forward delay 21
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
Best to configure the desired root bridge and not count on MAC
Set a _____________________ if a specific switch
is to become the root bridge
SW1#spanning-tree vlan 1 root primary
Sets value to 24576 or 4096 less than lowest priority
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?
2. Select root ports
After root bridge designation _________________________
__________________________ in the broadcast domain.
Every non-root bridge (Switch) selects a root port
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?...
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
Changing the cost of a link
SW1(config-if)#spanning-tree cost 25
To revert back to default value use “no”…
SW1(config-if)#no spanning-tree cost
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
Because path 1 has a
lower overall path cost to
the root bridge, it is the
STP then configures the
redundant path to be
blocked, preventing a loop
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…
Configure port priority
SW2(config-if)#spanning-tree port-priority 112
Priority values range from 0 - 240, in
increments of 16.
_____________________________ by STA to
3. Select designated/non-
____________ ports - All _______________
For _____ bridges, ______________________
For ___________bridges, a designated port is the
________________________ as needed.
______________ ports - All ports configured
to be in a _______________ to prevent loops
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…
Summary of Port Roles
Root ports- forwarding
Designated ports- forwarding
Non-designated ports- shut down or blocked
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 – ___________________________
States and timers
Amount of time
that a port stays in
the various port
states depends on
During a topology
A port temporarily Possibly forward or remain in
_______________ blocked state
a specified period
Up to ____________________
called the "forward
delay interval.“ __________ again- full
Timers are optimised for a 7-switch diameter
_______________________ 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
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:
Verify spanning tree
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
________________ can be implemented.
Separate instance of spanning tree runs for
The VLAN needs to be identified, so each
BID has 3 fields:
Original BID just had bridge priority and MAC
PVST+ is the default spanning-tree
configuration for a Catalyst 2960 switch.
When the priority and extended system ID
are prepended to the switch MAC address,
each VLAN on the switch can be represented
by a _________
Rapid Spanning Tree Protocol
__________________________________ with it.
RSTP (IEEE 802.1w) is an evolution of STP (IEEE 802.1D)
2 in version field indicates it is RSTP
Sends BPDUs with its current information every 2
Does not use timers in the same way as STP
3 missed BPDUs taken to mean loss of the link. (6
RSTP does not have a __________________________
_______________ 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.
___________ are categorized into 2 link
A _________________ between two switches is
regarded as a _________________
A link ____________________ is regarded as a
Ports on a ________________ are able to
Make the most use of the link type parameter.
• 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
RSTP Port Roles
____ - A ___________
that has been elected
for the spanning-tree
_______________ - 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
Root bridge should be a ______________ in
the ______________ of the network.
________ the number of _______________
Use VTP ____________.
_________________ even if no blocked
ports or physical redundancy exist
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
How the network looks when it works correctly
Use the “_________” command as needed