Spanning tree protocol (STP) is used to preventing network
loops on layer 2 network switch. STP
Monitor the network to track all links and shut down the
redundant links.
Redundant links between switches provides us redundancy but it’s
also possibility to create loops when switches do broadcasts.
Spanning tree prevent loops when you have multiple redundant
links between switches. Spamming tree also help us to avoid broadcast storm,
frame copies and instability database. Spanning is a open standard protocol
(IEEE 802.1D). Spanning tree is enable by default on all Cisco catalyst
switches.
Spanning-tree terms
Root Bridge is the bridge with the lowest best bridge ID. Bridge
ID is priority + MAC address of the switch. The switches within the STP network
elect a root bridge, and it’s become central point in the network and all the
remaining switches will be considered as Non-root Bridge.
Non-root Bridge is the bridge which is not a Root Bridge. There
is only one Root Bridge in each LAN and all the remaining bridge will be considered
as Non-root Bridge. Non root bridge exchange BPDUs with all the other bridges
and update the Spanning tree topology database.
BPDU all switches exchange information to use for the
subsequent configuration of the network is called bridge protocol data units
(BPDUs). BPDU are sent every 2 seconds and maximum age 20 seconds, inside the
BPDU is the bridge ID , BPDU contains information regarding ports, switches,
port priority and addresses.
Bridge ID is a combination of the bridge priority 32,768 by
default on all Cisco switches and the MAC address. The bridge with the lowest
bridge ID becomes the root bridge in the LAN.
Port cost is determine the best path when we are using
multiple links between two switches, every bridge find the most efficient path
to the root bridge.
Path cost is calculated for each unique path by adding the
individual port cost encountered on the way to the root bridge.
Bridge port
Root port is the link with the lowest path cost to the root
bridge.
Designating port is lowest cost to get on a given network segment.
Designating port is a forwarding port and we can have only one forwarding port
per network segment.
Non-designating port is one with a higher cost then the
designating port. These ports are put in blocking or discarding mode.
Forwarding port is either a root port or a designating port.
Its forwards frames.
Blocking port are not forward frames but its still listen to
BPDU frames from its neighbors but never transmit frames.
Spanning-tree Port States.
Disable state is manually disable by administrator.
Blocking state the purpose of this state is preventing
loops. When the switch powers up all the ports is in blocking state 20 seconds
or no limits.
Listening state the
port in this state listen the BPDU just to ensure there is no loops occur on
the network before passing data frames. It takes 15 seconds.
Learning state this state listens to BPDU and learns all the
paths in the switch network its take 15 seconds.
Forwarding state sends and receives all the data frames on
the bridge port.
lets see the CLI
- first we will find which switch is root bridge , root port, block port, and forwarding port.
- second step we will change the root bridge and we will see changes to STP ports.
sw-1#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.C70C.320A
Cost 19
Port 1(FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 00D0.BCC3.256A
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
Fa0/1 Root FWD 19 128.1 P2p
sw-2#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.C70C.320A
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 0001.C70C.320A
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/1 Desg FWD 19 128.1 P2p
(switch 2 is root bridge in our topology)
sw-3#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.C70C.320A
Cost 19
Port 2(FastEthernet0/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0060.2F17.343E
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2 Root FWD 19 128.2 P2p
Fa0/3 Desg FWD 19 128.3 P2p
Switch#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.C70C.320A
Cost 38
Port 3(FastEthernet0/3)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0060.7062.2A37
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/3 Root FWD 19 128.3 P2p
Fa0/4 Altn BLK 19 128.4 P2p
let change the root bridge for vlan 1, we are going to configure switch 1 as a root bridge
sw-1(config)#spanning-tree vlan 1 priority 0
sw-1(config)#end
sw-1#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 1
Address 00D0.BCC3.256A
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 1 (priority 0 sys-id-ext 1)
Address 00D0.BCC3.256A
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
Fa0/1 Desg FWD 19 128.1 P2p
sw-3#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 1
Address 00D0.BCC3.256A
Cost 38
Port 2(FastEthernet0/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0060.2F17.343E
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2 Root FWD 19 128.2 P2p
Fa0/3 Altn BLK 19 128.3 P2p
now switch 3 fastethernet 0/3 become alternative block state.
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.C70C.320A
Cost 19
Port 1(FastEthernet0/1)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 00D0.BCC3.256A
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
Fa0/1 Root FWD 19 128.1 P2p
sw-2#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.C70C.320A
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 0001.C70C.320A
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/1 Desg FWD 19 128.1 P2p
(switch 2 is root bridge in our topology)
sw-3#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.C70C.320A
Cost 19
Port 2(FastEthernet0/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0060.2F17.343E
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2 Root FWD 19 128.2 P2p
Fa0/3 Desg FWD 19 128.3 P2p
Switch#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0001.C70C.320A
Cost 38
Port 3(FastEthernet0/3)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0060.7062.2A37
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/3 Root FWD 19 128.3 P2p
Fa0/4 Altn BLK 19 128.4 P2p
let change the root bridge for vlan 1, we are going to configure switch 1 as a root bridge
sw-1(config)#spanning-tree vlan 1 priority 0
sw-1(config)#end
sw-1#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 1
Address 00D0.BCC3.256A
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 1 (priority 0 sys-id-ext 1)
Address 00D0.BCC3.256A
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
Fa0/1 Desg FWD 19 128.1 P2p
sw-3#show spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 1
Address 00D0.BCC3.256A
Cost 38
Port 2(FastEthernet0/2)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 0060.2F17.343E
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20
Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/2 Root FWD 19 128.2 P2p
Fa0/3 Altn BLK 19 128.3 P2p
now switch 3 fastethernet 0/3 become alternative block state.
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