What is STP (Spanning Tree Protocol) and How to Configure STP
What is STP
A well-built communications network characteristic is that it is resilient, which means that through redundancy the network needs to be able to handle a link failure or a device. Failure of a single point can be eliminated through a redundant topology by using multiple devices, multiple links, or both. In a redundant switched network the STP (Spanning Tree Protocol) is used to prevent loops.
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| STP Configuration on Old and Latest Switches |
In a switched network the redundancy would introduce the following issues without STP:
v MAC database instability: Content instability of the table results in MAC address from the same frame copies being received on different switch ports.v Broadcast storms: It means endlessly the broadcasts floods by each switch.
v Transmission of multiple frames: Multiple unicast frame copies may be delivered to the destination causing unrecoverable errors.
The standard of STP is 802.1d defined by the committee
standard IEEE, and it works on OSI Data-Link
Layer. In the locking state, the STP places certain ports so that they don’t
listen to flood or forward data frames. A tree is created by the STP to ensure
that in each network segment, there is only one path at any one time. Then in
connectivity, if any segment experiences a disruption, a new tree the STP
builds by activating the inactive previously, but path redundant.
STP Algorithm
The algorithm of the
STP is, the interface chosen by the STP that should be placed into a Forwarding
state. The interface that is not chosen to be in a Forwarding state, then the
STP places the interface in a blocking state. By default, every 2 seconds the STP configuration messages
are exchanged by the switches using a multicast frame called the BPDU (Bridge
Protocol Data Unit). The BID (Bridge ID) is the piece of information included
in the BPDU.
In each switch, the BID
is unique and is composed of the bridge MAC address (6 bytes) and the priority
value (2 bytes), and 32768 is the default priority. The switch with the lowest
BID will be the root bridge, and if the switch default value is not changed the
switch with the lowest MAC address will become root.
The interfaces that choose by the STP are placed into a Forwarding state this process is sometimes called the STA (Spanning-Tree Algorithm), and the interfaces that are not chosen by the STP will be in a blocking state, which means STP simply picks which interface should be placed to be in a Forwarding state.
Three
criteria are used by the STP to choose whether to put an interface in a forwarding
state:
v The ports of each NonRoot switch consider having the least administrative cost between the root switch and itself.
Note:
To reach the root bridge, the cost is determined by the Port bandwidth. All
working interfaces of the root will forward frame because these working
interfaces are placed in a Forwarding state by the root, and all other
interfaces are placed in a blocking state.
If there are multiple switches in the network, then only one switch will become the Root Bridge and all others will be a NonRoot bridge.
Root Bridge
The Root Bridge is the bridge in the network with the best bridge ID. For example, if there are two switches in the network then the election of Root Bridge depended on two decisions. First, if both switches are from the same company model then the decision will be made on priority numbers, by default VLAN 1 is configured on every switch and the priority number of the switch will be 32768 plus the VLAN by default configured, the priority number will become 32769.
The switch with the
lowest priority number will become the Root Bridge in the network and the
second switch will become the NonRoot
Bridge. If both switches have the same priority numbers then the decisions
will be depended on the switch’s MAC addresses. The switch with the lowest MAC address will become the Root Bridge
in the network and the second one will be NonRoot Bridge. So, the Root Bridge
will decide which port is to be blocked and which port is to be put in a Forwarding State in the network.
It means the Bridge ID is the combination of bridge priority and MAC address in the network for Root Bridge selection.
Identify Root Bridge in STP protocol
The STP protocol
selects the root bridge device in the network according to the lowest Bridge ID value.
The Bridge ID consists
of two sections which are as follows:
The first section is
"Bridge Priority". The second section is the physical address (MAC)
of the Switch device.
In the virtual settings
of Switch devices, the Bridge Priority value for all devices is equal and it is: - 32768.
Therefore the root
bridge will be selected according to the Mac address value; the device with
less address will become the root bridge.
In most medium to large
networks, the network is designed in a way that we want to place a
"Switch" device with high specifications according to the needs of the
network to be the main device (Root Bridge) and we do not leave the matter
because another device is selected automatically.
To do this, we can
change the Bridge ID value of the
device we want to become the main device (Root Bridge), making its value the
lowest among other devices.
Also, if we want to
prevent a device from becoming the main device (Root Bridge), we can change the
value of the Bridge ID of the device
and make it the largest among the other devices, thus it will not be elected to
be the main device (Root Bridge).
We notice here that we
can change the value of the "Bridge ID" by changing the "Bridge
Priority" part of this ID since the physical address part (Mac) cannot be
changed.
To change "Bridge Priority" we can use:-
Spanning-Tree VLAN
[VLAN ID] Priority [Value]
Where is the [VLAN ID]
FAKE NETWORK ID (VLAN). And [Value] is the value of priority, and this priority
value ranges from 0 to 61440 with an
increase of 4096 each time.
An example of this:-
Spanning-Tree
VLAN 1 Priority 28672
We notice that instead
of selecting a certain priority value for the Switch device and entering this
value by changing the priority to
make the device the main Root Bridge
device in the network, we can use a simpler and clearer thing to do it, which
is:-
Spanning-Tree VLAN
[VLAN ID] Root Primary
Where is the [VLAN ID]
FAKE NETWORK ID (VLAN). This and we can set a device to be a secondary, a
backup, or an alternative device to become the main device (Root Bridge) if the
main device (root bridge) malfunctions in the network using:-
Spanning-Tree VLAN
[VLAN ID] Root Secondary
Where is the [VLAN ID]
FAKE NETWORK ID (VLAN).
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| Identify Root Bridge in STP protocol |
NonRoot Bridge
The NonRoot bridges are not the root bridge
and BPDUs are exchanged by the NonRoot bridges with all bridges and update the
topology of the STP database on all switches, providing a measure of defense
and preventing loops against link failures.
Root Port
It is the port that connects
the NonRoot Bridge with the root bridge, and it always exists on NonRoot
Bridge. In the above example, one switch is a root bridge and the second is a NonRoot
bridge.
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| Root Port Selection |
If there are two links that are connected from
the Root Bridge to NonRoot Bridge, which interface will be the root port on the
NonRoot Bridge switch, the decision will be made on NonRoot Bridge interfaces
through Root Port Selection.
Root Port Selection
It means if the speed
of the interface and wire link is fast the cost will be low. The interface with
the lowest cost number will become
the root port and the other interface will be on standby means a shutdown. For
example, if both links are the same means working on 100 Mbps speed then the
cost number will be the same as 19. Now, if the cost number of both links is the
same then the decision will be on the lowest port number means the lowest
interface number. The lowest port number will become the root port and the
second will be on standby or shutdown.
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| IEEE Port Cost and Speed Table |
The shutdown or standby interface will be active if there is a problem in the root port then the standby interface will become the root port. If both interfaces are active the switching loops will be created and the purpose of STP is to prevent switching loops so one interface will be shut down.
Designated Port
Every connected computer
in the Root Bridge will be considered a client, and these connected computer
ports will be called designated ports. The Root Bridge all ports will be designated
ports because the Root Bridge switch has been considering everyone as a client.
The designated port with the lowest cost will be marked as a forwarding port.
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| Root Bridge Designated Port |
Non-Designated Port
The port with a higher
cost will be marked as a blocking port. For example in the NonRoot Bridge, if
the link of the root port fails then the non-designated port will be active
from standby, and also it will be considered a root port and the failing link
port will be considered non-designated and will remain shut down until the link
cannot troubleshoot and error fixed.
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| Non -Designated Port |
Forwarding Port
Frames will be
forwarded through the forwarding port.
Blocked Port
The port will not
forward the frames in order to prevent the switching loops, but the port will
always listen to frames.
The default port costs
defined by IEEE are listed in the below tables, which had to be revised with
the advent of 10 Gbps ports:
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| IEEE Default Port Cost Table |
STP best path selection criteria
The STP protocol implements its work in three steps, which are as follows:-
Step 1: Select a main body Root Bridge Election.
Step 2: Choosing the best path for this main device Best Path Selection.
Step 3: Block Additional Redundant Links Blocking.
In the second step when choosing the best Path, every Non-Root Bridge device chooses the best path for the main device Root Bridge according to a value called Path Cost. The least expensive path is the best path. If paths cost the same Path Cost, the device with the least Sender Bridge ID will choose the path told by it.
If the Sender Bridge ID equals more than one path to the same device, the path that connects to the port on the sender device will choose the least Sender Port ID.
If the Sender Port ID is equal to the same sender port using the Hub device, it will choose the path that connects the local port that has the least Local Port ID.
That is, the criteria used by the STP protocol to choose the best Path are:-
v Path
Cost
v Sender
Bridge ID
v Sender
Port ID
v Local
Port ID
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| STP best path selection criteria |
Basic Configurations on Oldest Model Switches
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| STP Configuration on Old Switch |
How to Configure STP
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| How to Configure STP |
Switch 1
Switch>enable
Switch# configure terminal
Enter configuration commands, one
per line. End with CNTL/Z.
Switch (config) #hostname SW1
SW1
(config) #end
SW1#show spanning-tree
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| Show Spanning-Tree |
SW1#
Switch 2
Switch>enable
Switch# configure terminal
Enter configuration commands, one
per line. End with CNTL/Z.
Switch (config) #hostname SW2
SW2
(config) #end
SW2#show spanning-tree
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| Show Spanning-Tree |
SW2#
Basic Configurations on latest Switches
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| STP Configuration on Latest Switch |
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| How to Configure STP on Latest Switches |
Switch 1
Switch>enable
Switch# configure terminal
Enter configuration commands, one
per line. End with CNTL/Z.
Switch (config) #hostname SW1
SW1
(config) #end
SW1#show spanning-tree
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| Show Spanning-Tree |
SW1#
Switch 2
Switch>enable
Switch# configure terminal
Enter configuration commands, one
per line. End with CNTL/Z.
Switch (config) #hostname SW2
SW2
(config) #end
SW2#show spanning-tree
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| Show Spanning-Tree |
SW2#
Search Tags
- What is STP protocol and how it works?
- What is STP Spanning Tree protocol used for?
- What is STP and its types?
- How long does it take to configure STP?
- How to configure STP in Cisco switch
If you want to configure RSTP, PRSTP, AND PVSTP then visit below link: 👇
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