Spanning-Tree,spt process explained, LAN switching

[wpanchor id=”stpup”]

1.  Election of a root bridge
2. Election of a root port for each non-root switch
3. Election of designated port for each segment
4. Port transition to forwarding or blocking state.

In a layer 2 switched LAN, all links are running at 100 Mbps. STP uses all default port cost configuration.

1- Election of a root bridge. [wpanchor id=”rootbridge”]
1.1 The switch with the lowest bridge ID. You can have ONLY one root bridge by broadcast domain.
This example

From the table above you can noticed that  all the switches have the same bridge priority(the default value:32769), so the priority is tied, so check the next condition

“Lowest MAC address will be the root bridge”

1.2 The switch with lowest MAC address will be the Root Bridge. Again from the table above, the lowest MAC address of the 4 switches is switch SW4 with MAC address of 0007.EC41.0465,

The switch SW4 is the root bridge for the VLAN 1, because it has the lowest MAC address(0007.EC41.0465)

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2- Election of root port. [wpanchor id=”electionrooport”]The root port is that used by the rest of switches in the STP to reach the switch that is elected as root bridge.

Every non-root switch(SW1,SW2,and SW3) will have a root.

SW2’s Fa04 and SW1’s Fa0/4 all receive a zero-cost BPDU from the root switch SW4, making them a root port each switch.

The determine to root port with lowest cost back to the root bridge. For example here is the process to election the root port on switch 1, from the image of network diagram the switch SW1 has connected to LAN

All links are FastEthernet running at 100 Mbps, with default port cost of 19

The port FastEthernet 0/4 will be the root port, because it has the lowest root cost

SW1#show spanning-tree interface FastEthernet 0/4
Vlan Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
VLAN0001 Root FWD 19 128.4 P2p

Finally, the show spanning-tree vlan 1
SW1#show spanning-tree vlan 1
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address 0007.EC41.0465
Cost 19
Port 4(FastEthernet0/4)
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address 00D0.D37C.B651
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 20

Interface Role Sts Cost Prio.Nbr Type
—————- —- — ——— ——– ——————————–
Fa0/11 Desg FWD 19 128.11 P2p
Fa0/3 Desg FWD 19 128.3 P2p
Fa0/4 Root FWD 19 128.4 P2p
Fa0/2 Altn BLK 19 128.2 P2p

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3-Election of Designated port by segment.[wpanchor id=”electiondesignatedport”] Is the port with lowest cost to reach the root bridge.

The ports of the switch elected as root bridge are automatically elected as Designated for their segment.

4- Port transition to forwarding or blocking. [wpanchor id=”porttransition”]The designated and root ports will pass to forwarding state and the rest of the ports will be blocked for sending and receiving frames.

Spanning-Tree,spt process explained, LAN switching
Interface Role Sts Cost Prio.Nbr Type
SW3
Fa0/1 Altn BLK 19 128.1 P2p
Fa0/2 Root FWD 19 128.2 P2p
Fa0/13 Desg FWD 19 128.13 P2p

SW1
Fa0/2 Altn BLK 19 128.2 P2p
Fa0/3 Desg FWD 19 128.3 P2p
Fa0/4 Root FWD 19 128.4 P2p
Fa0/11 Desg FWD 19 128.11 P2p

SW2
Fa0/4 Root FWD 19 128.4 P2p
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/3 Desg FWD 19 128.3 P2p
Fa0/12 Desg FWD 19 128.12 P2p
Fa0/24 Desg FWD 19 128.24 P2p

SW4
Fa0/1 Desg FWD 19 128.1 P2p
Fa0/2 Desg FWD 19 128.2 P2p
Fa0/14 Desg FWD 19 128.14 P2p
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Download a lab used to explain the spanning-tree process or practice the CCNA exam online
[download id=”4462″]