STP (spanning tree protocol)

STP (Spanning Tree Protocol)
It prevents a network from frame looping by putting some interfaces in forwarding state & some
interfaces in blocking state.
Whenever two or more switches are connected with each other for redundancy purpose loop can occur.
STP Protocol is used to prevent the loop. STP is layer 2 Protocol & by default it is enabled on switches.
 STP – IEEE 802.1D (Open Standard).
 RSTP – 802.1W (IEEE)
 MST – IEEE 802.1S (Multiple Spanning Tree)
 PVST – Cisco Proprietary (Per Vlan Spanning Tree)
 PVST+ - Cisco Proprietary
 RPVST – Cisco Proprietary
CST (Common Spanning Tree)- All Vlan will participate in the single instance number.
IST (Inter Spanning Tree)- For different-different Vlans different-different instances number.
If we don’t use STP, these problems will occur on the network:
(i) Broadcast Storms
(ii) High Processor Utilization
Figure 1 STP LOOP Problem

(iii) Mac Table instability
(iv) Multiple Frame Transmission
STP Tasks
1. Elect Root Bridge
2. Elect Designated Port
3. Elect Root Port
Root Bridge- A switch which has best bridge ID (Lower Best)
Bridge ID is a combination of Switch priority and its MAC addresses. It is 8 bytes ID. It contains 2 bytes
priority Plus 6 bytes MAC.
Switches by default Priority is 32768.
We can change the priority between 0-65535.
MAC- Each Switch has a supervisor engine. Supervisor engine has a MAC pool, the pool contain 1024
MAC addresses. When a switch wants to create Bridge ID, it borrows MAC from MAC Pool.
Figure 2 Root Bridge

Designated Port- They are those ports which advertise lowest cost BPDUs. DP sends BPDUs towards
the Non Root Bridge.
Root Port – A port which receives lowest cost BPDUs. Shortest path to Root Bridge.
Cost – An integer value. It is used for DP & RP election.
Ethernet Standard Cost
10 Mbps 100
100 Mbps 19
1 Gbps 4
10 Gbps 2
BPDU (Bridge Protocol Data Unit)
Switches sends hello to each other. This hello is called hello BPDU (in every 2 sec).
Types of BPDU
1. Configuration BPDU (Root to Non-root)
2. TCN BPDU (Topology Change Notification) – Non-root to Root
Configuration BPDU Contents
1. Protocol ID (always 0)
2. Version (always 0)
3. Message Type
4. Flag
5. Root Bridge ID
6. Root Cost
7. Sender Bridge ID
8. Sender Port Priority
9. Max-Age
10. Message Age
11. Hello
12. Forward delay
TCN Contents
1. Protocol ID
2. Version
3. Message Type

 Root bridge, after every 2 sec sends configuration BPDU.
 The root bridge always sends 0 cost BPDUs.
Requirements for Root Bridge
1. Lower Bridge Priority
2. Lower Mac Address.
(Note: Rood Bridge Always Generates 0 Cost BPDU.)
Requirements for DP and RP
1. Lower Bridge ID (Only for DP, Between Root & Non Root)
2. Lower Cost | For DP & RP
3. Lower Sender Bridge ID | B/W Root – Non Root
4. Lower Sender Port Priority | Non Root – Non Root
5. Lower Sender Port ID
(Note: All Ports of Root Bridge are DP.
Every non root bridge must have at least one root port.)
Root Bridge Election
When we start the switches then each switch consider itself as a root, and sends BPDU to its all ports.
When a switch receives a superior BPDU then it accepts the neighbor switch as a root, which is sending
superior BPDU.
(Note: Superior means lower priority.)
Figure 3

DP, RP & Blocking port selection method
1. Lower cost to Root Bridge
2. Lower Sender Bridge ID
3. Lower Port ID (Port Priority 128 + Port number)
STP Port States
1. Disable
2. Blocking
3. Listening
4. Learning
5. Forwarding
1. Disable – It means port is administratively shut down.
2. Blocking – It means neither a port is sending data nor receiving data, but still it is receiving BPDU.
3. Listening – It means a port is Preparing for forwarding state without learning MAC.
4. Learning – It means a port is preparing for forwarding state with learning MAC.
5. Forwarding – It means a port is sending or receiving Data along with BPDU.
Figure 4

STP Timers
1. Hello – 2 sec
2. Max Age – 20 sec
3. Forward Delay 15 sec
4. Listening 15 sec
5. Learning 15 sec
 Hello– It is used to send Hello BPDU. Default time 2 sec
 Max Age– This time is used to store the best BPDU when a root port goes down.
 Forward Delay– It is that time which is spent by a port in listening state and learning state.
(Conversion time till forwarding state.)
STP Convergence Time is 52 Sec
1. Max age – 20 sec
2. Listening – 15 sec
3. Learning – 15 sec
4. BPDU - 2 Sec
5. Total - 52 Sec
PVST Convergence Time is 32 Sec
(Note: Cisco switches by default uses PVST (Per VLAN Spanning Tree). It doesn’t use max age)
1. Listening - 15 sec
2. Learning - 15 sec
3. BPDU - 2 sec
4. Total - 32 Sec
5. Max-age always related to superior BPDU.
Types of Topology Changes
1. Direct Topology Change (32 sec)
2. in-Direct Topology Change (52 sec)
3. in-sufficient Topology Change
1. Direct Topology Change- When a root port of a non-root bridge goes down then it will be direct
topology change for this non-root bridge.
2. In-direct Topology Change- When an indirect link goes down that will called as in-direct link
failure.

3. In-sufficient Topology Change- When an access port goes up or down then switch sends TCN
BPDUs. These changes are called in-sufficient topology change.
Convergence Mechanism
 Port Fast- A feature that puts a port directly in forwarding state when it becomes physically
active.
(Note: Two types of Port fast
1. Globally
2. Interface
Always applied on access port.)
 Uplink Fast- A feature it enable non root switches to put their alternate port (Blocking port)
directly in forwarding state, when their root port goes down. (In case of Direct Link Failure, it will
work)
1. It sends multicast updates 0100-0CCd-CDCD
2. It sends 150 packet per/sec
3. Priority will change 32768 – 49152
4. 3000 cost will add per link cost
(Note: If you will change the priority then uplink fast will not work.
With using uplink fast load balancing will not possible.)
Figure 5

 Backbone Fast- A feature which prevents your network from inferior BPDU. (We use this for in
case of indirect link failure)
When an indirect link failure occurs in a topology then switch will receive inferior BPDUs. Due to
max-age timer of superior BPDU Switch will not take any action on inferior BPDUs until and
unless its max-age timer expire. To reduce this timer we enable back bone fast on all switches in
the network.
(Note: Backbone fast will remove max age time. Now total convergence time will 32 sec.
Using backbone fast we save 20 sec (Max-Age).)
Inferior BPDU
When a Switch announces itself as a root in the presence of root, these BPDUs are called inferior BPDU.
When we enable back bone fast and a switch root port goes down now this switch will not send inferior
BPDUs. It will send RLQ (Root Link Query) to neighbor Switch.
Figure 6

Sw1#sh cdp neighbor
Sw4 – 21/22
Sw3 – 19/20
Sw2 – 23/24
Sw1#sh spanning-tree
0012.7ffc.cw80
0012.4383. E200
0017.9581.3a00
Sw4#sh spanning tree
0012.0106.6f00
19/20 - DP, 21 – Root, 22/23/24 Alt Blk
19 – Root, 20 – Alt Blk, 21/22/23/24 – DP
19/20/21/22/24 – Blk, 23 – RP
All port DP
Figure 7

Now here we can see Sw4 is the root bridge. Now we are going to make sw1 as a root bridge.
Sw1 (config) #spanning-tree vlan 1 priority ?
0 to 61440
Sw1 (config) #spanning-tree vlan 1 priority 0
We can use the value multiply of 4096
Now if we want to make sw1 as Root Bridge for all the vlans
Sw1 (config) #spanning-tree vlan 1 – 4094 priority 0
Now switch 1 is the root bridge for all the vlans.
To remove
Sw1 (config) #no spanning-tree vlan 1 – 4094 priority 0
If we want to see particular vlan 1 information
Sw1#sh spanning-tree vlan 1
Now here if we want to load balancing of root bridges
Now if we have sw1 and sw2 is a good configuration switches.
I want to make sw1 work as a root bridge 1 for vlan 1 to 5.
And Sw2 work as root-bridge 2 for vlan 6 to 10
If incase sw1 goes down then sw2 will become root-bridge for all the switches.
If sw2 goes down then sw1 become root-bridge
Sw1 (config) #spanning-tree vlan 1 -5 root primary
Sw1 (config) #spanning-tree vlan 6 -10 root secondary
Sw2 (config) #spanning-tree vlan 1 -5 root secondary
Sw2 (config) #spanning-tree vlan 6 -10 root primary

When we add primary keyword, then switch will decrease the priority from 32768 to 24576
When we add secondary keyword, then it will make its own priority 28672 automatically
Sw2#spanning-tree vlan 1
Now if Sw1 is down, then Sw2 would become root bridge for all (1 to 10) vlans.
If Sw2 goes down then Sw1 will become root-bridge for (1 to 10) Vlans
Now if incase Sw3 will make its own priority 0 then, it would become the root bridge for all the vlans.
Primary or Secondary keywords would not work.
Now here for the safe side, we can set the priority 0 for vlan 1 to 5 and for 6 to 10 we will set the priority
4096 on Sw1
On Sw2 we will set the priority for 1 to 5 vlans 4096, and for 6 to 10 we will set 0.
Now first we remove the previous commands. When we remove primary and secondary command, Sw4
will become the root bridge.
Suppose here if we want to make int 22 as root port.
For that either we can increase the int 21s cost or we can decrease the cost of int 22.
Sw1 (config) #int fa0/21
Sw1 (config-if) #spanning-tree vlan 1 cost 20
Root port is 22
By default 100 MB links priority is 19, we increased the cost of 21s link.
Again here we will make it default
Sw1 (config-if) #no spanning-tree vlan 1 cost 20

We can change the port priority also
Sw4 (config-if) #spanning-tree vlan 1 port priority ?
Sw4 (config-if) #spanning-tree vlan 1 port priority 112
Sw1#debug spanning-tree events
When an access port goes up it takes 32 sec to start forwarding data, to stop this delay we can use Port
Fast.
We can enable this command globally or on a particular interface also
Sw1 (config-if) #spanning-tree portfast
We can use range also
Sw1 (config) #int range fa0/1 – 18
Sw1 (config-if-range) #spanning-tree portfast
Globally
Sw1 (config) #spanning-tree portfast default
Now here we will enable UplinkFast. It’s a globally enabled command.
Sw1 (config) #spanning-tree UplinkFast
Backbone Fast
Figure 8

Sw2#debug spanning-tree events
Sw3 (config-if) #shut
Now we can see on Sw2 after 20 sec wait & then listening and learning state.
Sw3 (config-if) #no shut
Sw3 (config) #spanning-tree backbonefast
Now again we can see all the events on sw2 switch
To change the Hello Timer
Sw1 (config) #spanning-tree vlan 1 hello-time ?
Between 1 to 10
To change forward Timer
Sw1 (config) #spanning-tree vlan 1 forward-time ?
Between 4 to 30 (int STP per vlan)
To change Max-age Timer
Sw1 (config) #spanning-tree vlan 1 max-age ?
Between 6 to 40
Figure 9

To Stop Spanning –tree protocol
Sw1 (config) no spanning-tree vlan 1
How to check the Root bridges through command line
Sw1#sh cdp neighbor
Sw2#sh spanning-tree root
Now here we will make Switch 1 as a root bridge.
Sw1 (config) #spanning-tree vlan 1 root priority?
If we provide 1 here then we will see some warning.
Bridge priority must be in increments of 4096.
Now we check the by default priority
Sw2#sh spanning-tree int fa0/24 detail
Here we will see Path cost 0
Path cost 19
128.19
128 is priority & 19 is port cost
For Vlan 1 it is designated forwarding.
Now here we want, if the primary root will down then secondary root will comes up.
On Switch 2
Sw2 (config) #spanning-tree vlan 1 root secondary
Sw2#sh run |be spann

Now here if we connect one more link on 23 port, then by default 23 would be the root port but if we
decrease the priority of 24 then it would become root port.
Sw2 (config-if) #spanning-tree vlan 1 cost 140
Now we will change the port priority
SW1 (config) # int f0/24
Sw1 (config-if) # spanning-tree vlan 1 port priority 120
We will get some warning message here
Port Priority in increments of 16 is required
Sw1 (config-if) #spanning-tree vlan 1 port priority 112
Here we can see port id is 112.
To create a root port manually
Figure 10

Manually change the cost
Switch (config) #int fa0/22
Switch (config-if) #spanning-tree cost 10
Switch#sh spanning-tree
We can see 22 is now root port. Because previously its cost was 19, when we decrease the cost then it
would become root port.
Another criteria is Port Priority
We will change the sender’s port priority
Switch1 (config) #int fa0/22
Switch1 (config-if) #spanning tree port priority 16
Switch4 #sh spanning-tree
Here we can see 22 is root port
To remove above command
Switch1 (config-if) #no spanning tree port priority 16
To change the Hello Timer
Sw1 (config) #spanning-tree vlan 1 hello time?
We can select from 1 to 10 sec
To change Max-age
Sw1 (config) #spanning-tree vlan 1 max-age?
We can select here between 6 to 40 sec
To change the forward delay timer
Sw1 (config) #spanning-tree vlan 1 forward-time?
Between 4 to 30 sec
By default cisco switches runs PVST. Per Vlan Spanning Tree Protocol
Switch#sh vlan brief
By default we can see only one Vlan. This is Vlan 1

Switch#sh spanning tree
One vlan, one instance
Single instance for single Vlan
Switch (config) #vlan 1 – 10
Switch #sh spanning-tree
We can see here now 10 vlan instances
Switch#debug spanning-tree events
If we are using STP & connection is lost then we will get the connection after 52 sec. If we use PVST then
we will get the connection after 32 sec.
Switch4 (config-if) #shut down
Switch4#debug spanning-tree events
We can see here the listening and learning time
Switch4 (config-if) #no shut down
Now we will implement uplink fast
SW4 #sh spanning-tree
Before enabling the uplink fast , Priority is 32768
Sw4 (config) #spanning-tree UplinkFast max-update-rate?
0 to 32000
By default are 150 packets per sec.
We can increase it also.
Command for enabling uplink fast is
Sw4 (config) #spanning-tree UplinkFast
After enabling the uplink fast we can see here the priority will change.
Previously it was 32768, now it is 49152

And 3000 will added on each links cost.
Uplink Fast is applied for all the Vlans.
Uplink Fast is a feature which enables non root switch to put there alternate port directly in forwarding
state when there root port goes down.
Before enabling Uplink fast we were getting delay of 32 sec.
If uplink fast is working then port will up instantly.
Backbone Fast
Switch (config) #spanning-tree BackboneFast
Switch#sh spanning-tree BackboneFast
BackboneFast is enabled
Port Fast
Switch# int fa0/ 22
Switch (config-if) #shut
Switch (config-if) # no shut
Switch (config-if) #do sh spanning-tree
Here we will see first listening and then learning state
15 sec for listening and 15 sec for learning then it will come in forwarding state. On access port there is
no need to listening and learning. We can enable portfast globally or locally.
Switch# int fa0/ 1
Switch (config-if) #spanning-tree portfast
Switch (config-if) #shutdown
Switch (config-if) #no shut
Now here we will see port is direct in forwarding state, no listening no learning.
Port fast is applied on Access port.
Uplink fast is applied on Non root switches.
Backbone fast is applied on all the switches.

If we will change the default priority then uplink fast will not work.
Switch (config) #spanning-tree vlan 1-4096 priority 0
R1 (config) #int fa0/0
R1 (config-if) #no shut
Sw1#sh int trunk
Sw2#sh cdp neighbor
Sw1 (config) #spanning-tree portfast default
Globally enabled
Figure 11

If we use inter vlan routing then we will enable portfast on trunk link
Sw1 (config-if) #spanning-tree portfast trunk
To enable BPDU Guard
Sw1 (config) #spanning-tree portfast BPDU guard
If we want to enable BPDU Guard per interface
Sw1 (config-if) #spanning-tree BPDU enable
Sw1#sh spanning-tree summary
R1 (config) #bridge 1 protocol IEEE
R1 (config-if) #bridge-group 1
Sw1#sh int status
Error disables recovery for BPDU Guard
Sw1 (config) #errdisable recovery cause bpduguard
Sw1 (config) #errdisable recovery interval?
R1 (config-if) #no bridge-group 1
R1 (config-if) #no bridge 1 protocol IEEE
Sw1#sh int status
Sw1 (config) #no spanning-tree portfast bpdu guard default
Sw1 (config-if) #spanning-tree BPDU guard disable
Sw1 (config-if) #spanning-tree portfast default
Sw1#sh spanning-tree int fa0/1 portfast
Sw1 (config) #spanning-tree portfast BPDU filter default
To run on interfaces
Sw1 (config-if) #spanning-tree BPDU filter enable

Sw1#sh spanning-tree summary
R1 (config) #bridge 1 protocol IEEE
R1 (config-if) #bridge-group 1
Sw1#sh int status
Sw1 (config-if) #spanning-tree bpdu filter enable
Sw2 (config) #spanning-tree uplinkfast
This command will use Globally
Sw1 (config) #spanning-tree backbone fast
Sw2 (config) #spanning-tree vlan 1 max-age ?
Sw2 (config) #spanning-tree vlan 1 forward time
STP Show Commands
Switch#sh spanning-tree
Switch#show spanning-tree blockedports
Switch#show spanning-tree inconsistentports

STP (spanning tree protocol)

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