The calculation of a unique path between nodes on the network starts by electing a Root bridge which is the logical centre of a network. The bridge with the lowest mac-address or priority is elected the Root. Once the network converge, the root bridge generates and sends configuration BPDUs at specific intervals. Other devices process and forward the configuration BPDU to communicate the topology changes, ensuring a stable network topology.
Spanning-tree is calculated based on two metrics ID's and Path cost:
ID's are classified into Bridge ID (BID) and Port ID (PID)
- BID - According to IEEE 802.1D, a BID is composed of a bridge priority (leftmost 16 bits) and a bridge MAC address (rightmost 48 bits). On an STP network, the device with the smallest BID is elected as the root bridge.
- PID - A PID is composed of a port priority (leftmost 4 bits) and a port number (rightmost 12 bits). The PID is used to select the designated port.
- The path cost is a port variable used for link selection. STP calculates path costs to select robust links and blocks redundant links, and finally trims the network into a loop-free tree topology. On an STP network, a port's path cost to the root bridge is the sum of the path costs of all ports between the port and the root bridge. This path cost is the root path cost.
Spanning-tree uses three elements to prune a ring network in a tree network: Root bridge, Root port and designated port.
Lets have a look to the following topology to understand better how spanning-tree works. In order to follow this lab you must configure the legacy spanning-tree on all switches, by issuing the command: "stp mode stp".
[LSW1] display stpThe command "display stp" displays the global spanning-tree information and also per port information. The following output is a sample of the "display stp" where we can see the global Spanning-tree information on LSW1.
-------[CIST Global Info][Mode STP]------- CIST Bridge :32768.4c1f-cc17-62ac Config Times :Hello 2s MaxAge 20s FwDly 15s MaxHop 20 Active Times :Hello 2s MaxAge 20s FwDly 15s MaxHop 20 CIST Root/ERPC :32768.4c1f-cc12-4c05 / 40000 CIST RegRoot/IRPC :32768.4c1f-cc17-62ac / 0 CIST RootPortId :128.2 BPDU-Protection :Disabled TC or TCN received :139 TC count per hello :0 STP Converge Mode :Normal Time since last TC :0 days 0h:0m:35s Number of TC :6 Last TC occurred :GigabitEthernet0/0/2
The Root Bridge:
From the "display stp" command output on LSW1 we can see the Root Bridge priority (32768), the Root Bridge mac-address (4c1f-cc12-4c05), which corresponds to switch LSW3 and the accumulative path cost of 40000 from LSW1 perspective.
CIST Root/ERPC :32768.4c1f-cc12-4c05 / 40000
When Spanning-tree starts the Root Bridge election process it first checks for the bridge priority value (in red), if the bridge priority value is the same on all switches spanning-tree will use the lowest mac-address as a tie breaker. Therefore on our topology since all the switches have the same Bridge priority (32768) Spanning-tree elects the switch with the lowest mac address, which is LSW3.The Root Bridge election can be influenced by configuring a Bridge priority on the switch we want as the Root Bridge. Lets configure LSW1 to be the Root Bridge, by configuring a lower Bridge priority then the default value (32768).
[LSW1] stp priority 4096On LSW3 run the command "display stp" to confirm that LSW1 is now the Root Bridge.
[LSW3]display stp
-------[CIST Global Info][Mode STP]-------
CIST Bridge :32768.4c1f-cc12-4c05
Config Times :Hello 2s MaxAge 20s FwDly 15s MaxHop 20
Active Times :Hello 2s MaxAge 20s FwDly 15s MaxHop 20
CIST Root/ERPC :4096 .4c1f-cc17-62ac / 40000
CIST RegRoot/IRPC :32768.4c1f-cc12-4c05 / 0
CIST RootPortId :128.2
BPDU-Protection :Disabled
TC or TCN received :90
TC count per hello :0
STP Converge Mode :Normal
Time since last TC :0 days 0h:1m:52s
Number of TC :9
Last TC occurred :GigabitEthernet0/0/2
The Root Port:
From the following output, on LSW3, we can see that the accumulative path cost value is 40000 and the Root Port is GigabitEthernet 0/0/2 with path cost value of 20000.[LSW3]display stp -------[CIST Global Info][Mode STP]------- CIST Bridge :32768.4c1f-cc12-4c05 Config Times :Hello 2s MaxAge 20s FwDly 15s MaxHop 20 Active Times :Hello 2s MaxAge 20s FwDly 15s MaxHop 20 CIST Root/ERPC :4096 .4c1f-cc17-62ac / 40000 CIST RegRoot/IRPC :32768.4c1f-cc12-4c05 / 0 CIST RootPortId :128.2 BPDU-Protection :Disabled TC or TCN received :90 TC count per hello :0 STP Converge Mode :Normal Time since last TC :0 days 0h:1m:52s Number of TC :9 Last TC occurred :GigabitEthernet0/0/2 ----[Port2(GigabitEthernet0/0/2)][FORWARDING]---- Port Protocol :Enabled Port Role :Root Port Port Priority :128 Port Cost(Dot1T ) :Config=auto / Active=20000 Designated Bridge/Port :32768.4c1f-cc24-6747 / 128.1 Port Edged :Config=default / Active=disabled Point-to-point :Config=auto / Active=true Transit Limit :147 packets/hello-time Protection Type :None Port STP Mode :STP Port Protocol Type :Config=auto / Active=dot1s BPDU Encapsulation :Config=stp / Active=stp PortTimes :Hello 2s MaxAge 20s FwDly 15s RemHop 0 TC or TCN send :37 TC or TCN received :3 BPDU Sent :61 TCN: 0, Config: 61, RST: 0, MST: 0 BPDU Received :18 TCN: 3, Config: 15, RST: 0, MST: 0
The Root Port is the port that has the lowest accumulative path cost value to the Root Bridge, on Spanning-tree every port has a path cost value, the accumulative path cost value is the sum of all path cost values that lead to the Root Bridge.
Why did Spanning-tree elected GigabitEthernet0/0/2 as the Root Port?
The first step that Spanning-tree takes to elect a Root Port is to compare the accumulative path cost value of the ports that lead to the Root Bridge, on our topology, on LSW3, the accumulative path cost value of GigabitEthernet0/0/2 and GigabitEthernet0/0/1 is the same, in the event of equal accumulative path cost values Spanning-tree carries on to step two, where it compares the mac-addresses of the neighbouring switches, since the mac-addresses are different, Spanning-tree chooses the port that connects to the neighbouring switch with the lowest mac-address as the Root Port and the election process stops. (remember that Spanning-tree always choose the lowest value)
Lets influence the Root Port election on LSW3 by configuring the path cost value on port GigabitEthernet0/0/1 to be lower than 20000.
[LSW3]interface g0/0/1 [LSW3-GigabitEthernet0/0/1]stp cost 10000We can confirm on LSW3 that the accumulative path cost value is now 30000 and Root Port is GigabitEthernet 0/0/1 with a path cost of 10000.
[LSW3]display stp -------[CIST Global Info][Mode STP]------- CIST Bridge :32768.4c1f-cc12-4c05 Config Times :Hello 2s MaxAge 20s FwDly 15s MaxHop 20 Active Times :Hello 2s MaxAge 20s FwDly 15s MaxHop 20 CIST Root/ERPC :4096 .4c1f-cc17-62ac / 30000 CIST RegRoot/IRPC :32768.4c1f-cc12-4c05 / 0 CIST RootPortId :128.1 BPDU-Protection :Disabled TC or TCN received :47 TC count per hello :0 STP Converge Mode :Normal Time since last TC :0 days 0h:3m:32s Number of TC :9 Last TC occurred :GigabitEthernet0/0/1 ----[Port1(GigabitEthernet0/0/1)][FORWARDING]---- Port Protocol :Enabled Port Role :Root Port Port Priority :128 Port Cost(Dot1T ) :Config=10000 / Active=10000 Designated Bridge/Port :32768.4c1f-cc94-1c38 / 128.2 Port Edged :Config=default / Active=disabled Point-to-point :Config=auto / Active=true Transit Limit :147 packets/hello-time Protection Type :None Port STP Mode :STP Port Protocol Type :Config=auto / Active=dot1s BPDU Encapsulation :Config=stp / Active=stp PortTimes :Hello 2s MaxAge 20s FwDly 15s RemHop 0 TC or TCN send :52 TC or TCN received :19 BPDU Sent :137 TCN: 0, Config: 137, RST: 0, MST: 0 BPDU Received :115 TCN: 2, Config: 113, RST: 0, MST: 0Till now we have seen how Spanning-tree elects a Root Port when up links go through different up link switches, let's understand now how Spanning-tree elects a Root Port when up links go through the same switch.
The elected Root Port by Spanning-tree on switch LSW6 is GigabitEthernet 0/0/2, lets confirm it with the command "display stp" or "display stp brief".
[LSW6]display stp brie
MSTID Port Role STP State Protection
0 GigabitEthernet0/0/1 ALTE DISCARDING NONE
0 GigabitEthernet0/0/2 ROOT FORWARDING NONE
0 GigabitEthernet0/0/3 DESI FORWARDING NONE
0 GigabitEthernet0/0/4 ALTE DISCARDING NONE
Why did Spanning-tree elect GigabitEthernet 0/0/2 on LSW6 as the Root Port? Let's once again go through the steps that Spanning-tree takes to elect a Root Port.1 - Spanning-tree compares the accumulative path cost value of each up link port and elects the port with the lowest accumulative path cost value as the Root Port.
Confirm the accumulative path cost value with the command "display stp".
[LSW6]display stp | in CIST Root/ERPC
CIST Root/ERPC :4096 .4c1f-cc17-62ac / 40000
LSW6 has 4 up-links of which GigabitEthernet 0/0/2 and 0/0/1 have the same accumulative path cost value, since ports GigabitEthermet 0/0/2 and 0/0/1 are equal Spanning-tree needs to find a tie breaker.
2 - Spanning-tree compares the mac-addresses of the neighbouring switches that are connected through the up links, the lowest mac-address will win the election and break the tie.
Since the up-links GigabitEthernet 0/0/2 and 0/0/1 have the lowest accumulative path cost we only need to confirm the mac-address of the neighbouring switch connected to these up-links by issuing command "display lldp neighbor interface GigabitEthernet [Slot number]". (note that the LLDP protocol is out of the scope of this tutorial, if you want to learn more about it go here)
[LSW6]display lldp neighbor interface GigabitEthernet 0/0/2 | in Chassis ID GigabitEthernet0/0/2 has 1 neighbors: Neighbor index : 1 Chassis ID :4c1f-cc24-6747 [LSW6]display lldp neighbor interface GigabitEthernet 0/0/1 | in Chassis ID GigabitEthernet0/0/1 has 1 neighbors: Neighbor index : 1 Chassis ID :4c1f-cc24-6747Spanning-tree has found that the mac-address of the neighbouring switch is the same on GigabitEthernet 0/0/2 and 0/0/1 and needs to find a tie breaker.
3 - Spanning-tree compares the port priority values received from the neighbouring switch, the port that receives the lowest port priority value will win the election and become the Root Port.
Let's look at the port priority values for GigabitEthernet 0/0/2 and 0/0/1 on LSW6 "display stp interface [port]".
[LSW6]display stp interface GigabitEthernet 0/0/1 | in Designated Designated Bridge/Port :38768.4c1f-cc24-6747 / 128.5 [LSW6]display stp interface GigabitEthernet 0/0/2 | in Designated Designated Bridge/Port :38768.4c1f-cc24-6747 / 128.4Spanning-tree has found the the priority values received on both GigabitEthernet 0/0/2 and 0/0/1 are the same and will go through a final step to find a tie breaker
4 - Spanning-tree will compare the received port ID values form the neighbouring switch and finally will elect a Root Port since there are no two identical port ID values. We can confirm this with the command used on step 3.
[LSW6]display stp interface GigabitEthernet 0/0/1 | in Designated Designated Bridge/Port :38768.4c1f-cc24-6747 / 128.5 [LSW6]display stp interface GigabitEthernet 0/0/2 | in Designated Designated Bridge/Port :38768.4c1f-cc24-6747 / 128.4Spanning-tree elects port GigabitEthernet 0/0/2 as the Root Port since its receiving the lowest port ID from the neighbouring switch.
To influence the election of the Root port on LSW6 let's configure the port GigabitEthernet 0/0/5 priority value to be lower then 128 on switch LSW4.
[LSW4]interface GigabitEthernet 0/0/5 [LSW4-GigabitEthernet0/0/5]stp port priority 64Now confirm that port GigabitEthenet 0/0/1 is the Root port on LSW6 with the command "display stp interface GigabitEthernet 0/0/1".
[LSW6]display stp interface GigabitEthernet 0/0/1 ----[Port1(GigabitEthernet0/0/1)][FORWARDING]---- Port Protocol :Enabled Port Role :Root Port Port Priority :128 Port Cost(Dot1T ) :Config=auto / Active=20000 Designated Bridge/Port :32768.4c1f-cc24-6747 / 64.5
We can also confirm that the priority value received on port GigabitEthernet 0/0/1 on LSW6 is the priority value configured previously on port GigabitEthernet 0/0/5 on LSW4.
Designated ports:
When electing a Designated Port Spanning-tree will take into consideration the bridge mac-address and as you know with Spanning-tree the lowest value always win. On the network segment between LSW3 and LSW6 the Designated port elected by Spanning-tree is GigabitEthernet 0/0/3 on switch LSW3 since this switch as the lowest mac-address.
We can confirm the port roles/states on LSW3 with the command "display stp brie".
[LSW3]display stp brie MSTID Port Role STP State Protection 0 GigabitEthernet0/0/1 ALTE DISCARDING NONE 0 GigabitEthernet0/0/2 ROOT FORWARDING NONE 0 GigabitEthernet0/0/3 DESI FORWARDING NONE
