Typically every network infrastructure has redundant path for better communication among the network, but there may be loops over the redundant paths. So STP(Spanning Tree Protocol) ensure loop-free network topology in the entire network and this helps to avoid the unwanted traffic over the network. To overcome this issue we STP can be used, before that Root Bridge should be selected in the network topology.
In root bridge selection, there are some facts need to consider. I will explain about this using the below example with that network diagram.
When Root bridge is selected other connected ports of switches are classified, those are described below.
- Root Port : Best port reach to the Root Bridge
- Designated Port : Other end of the root port connecting to the other switches.
- Non - Designated Port : This is the redundant port and it is blocked.
In Root Bridge selection, it is done with an election process by considering some facts. In a network topology every switches has Bridge ID (BID) which contains priority value(Default Value : 32769). When the every switches in the network boots up, they send BPDU(Bridge Protocol Data Unit) packets including BID and the Root ID to the entire network. Just after the booting up, every switches identifies then as Root Bridge itself. After that everyone sends out the BPDUs to the neighbouring switches and reads Root ID of everyone, then decide the Root Bridge. Root Bridge will be the switch which contains lowest BID value. So that switch will be the Root Bridge.
In my example, I’ll show the the spanning-tree details in each switches.
Switch0#show spanning-tree active
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000C.CFC2.CDEA
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 000C.CFC2.CDEA
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/4 Desg FWD 19 128.4 P2p
Fa0/6 Desg FWD 19 128.6 P2p
Fa0/7 Desg FWD 19 128.7 P2p
Fa0/5 Desg FWD 19 128.5 P2p
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/3 Desg FWD 19 128.3 P2p
Fa0/8 Desg FWD 19 128.8 P2p
Switch1#show spanning-tree active
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000C.CFC2.CDEA
Cost 19
Port 1(FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 40961 (priority 40960 sys-id-ext 1)
Address 00D0.97DE.DEEE
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/6 Altn BLK 19 128.6 P2p
Fa0/3 Altn BLK 19 128.3 P2p
Fa0/4 Altn BLK 19 128.4 P2p
Fa0/7 Desg FWD 19 128.7 P2p
Fa0/8 Desg FWD 19 128.8 P2p
Fa0/1 Root FWD 19 128.1 P2p
Fa0/2 Altn BLK 19 128.2 P2p
Fa0/5 Altn BLK 19 128.5 P2p
Switch2#show spanning-tree active
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000C.CFC2.CDEA
Cost 19
Port 1(FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 40961 (priority 40960 sys-id-ext 1)
Address 0001.6338.8C20
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/8 Desg FWD 19 128.8 P2p
Fa0/6 Desg FWD 19 128.6 P2p
Fa0/3 Desg FWD 19 128.3 P2p
Fa0/4 Desg FWD 19 128.4 P2p
Fa0/5 Desg FWD 19 128.5 P2p
Fa0/1 Root FWD 19 128.1 P2p
Fa0/2 Altn BLK 19 128.2 P2p
Fa0/7 Desg FWD 19 128.7 P2p
Switch3#show spanning-tree active
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 000C.CFC2.CDEA
Cost 19
Port 5(FastEthernet0/5)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 40961 (priority 40960 sys-id-ext 1)
Address 0001.6399.2891
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/8 Desg FWD 19 128.8 P2p
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/4 Altn BLK 19 128.4 P2p
Fa0/6 Altn BLK 19 128.6 P2p
Fa0/5 Root FWD 19 128.5 P2p
Fa0/7 Desg FWD 19 128.7 P2p
Fa0/3 Altn BLK 19 128.3 P2p
According to the results you can see the Bridge ID in Switch1, Switch2, Switch3 are same with 40961, but the Switch0 has value 32769.
40961 > 32769
Switch1, Switch2, Switch3 > Switch0
Therefor the Switch0 will be elected as Root Bridge. But if You want to change the root bridge you can change it by changing the BID.
Switch(config)#spanning-tree vlan 1 priority <value>
<value> : <0-61440> bridge priority in increments of 4096
Thank You ... :)
