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Cisco™ CCNA : Spanning Tree Protocol

* Best printed in landscape mode

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STP – Points to Remember

1. STP is a layer 2 protocol that runs on switches and bridges, the purpose of STP is to remove

switching loops. By default, STP is enabled on cisco switches.

2. All switches participating in STP exchange info with other switches in the network

Through messages known as BPDUs (Sent out at a frequency of 2 sec on every port)

3. STP port states are Blocked, Listen, Learn, Forward, Disabled

4. The command “show spanning-tree” includes the following info

i. VLAN number

ii. Root bridge priority, MAC address

iii. Bridge timers (Max Age, Hello Time, Forward Delay)

Selection Criteria

Root Bridge Selection

The switch with the lowest Bridge ID is chosen as

root.

Bridge ID is a combination of switch priority (32768

by default and the range is 0 to 65535 with

increments of 4096) and switch's MAC address

STP Port Roles

1. Root : A bridge can have only one root port. The root port is the port that leads to the root

bridge. All bridges except the root bridge will have a root port. the root port is in the STP

forwarding state.

2. Designated : One designated port is elected per link (segment). The designated port is

the port closest to the root bridge. Each designated port is in the STP forwarding state

3. Alternate : Alternate ports lead to the root bridge, but are not root ports. The alternate

ports maintain the STP blocking state.

4. Backup: This is a special case when two or more ports of the same bridge (switch) are

connected together, directly or through shared media. In this case, one port is designated,

and the remaining ports block. The role for this port is backup.

Root Port Selection

i . If there are 2 or more paths to reach the Root Bridge, select the bridge port associated with

the lowest accumulated path cost. OR

ii. If the path cost to reach the root bridge over 2 or more bridge ports is same, then: select the

neighboring switch with the lowest Switch ID value to reach the Root Bridge OR

iii. If there are two or more ports on the same bridge with the lowest path cost, then:

* Select the port with the lowest Port Priority value, if you have multiple paths to reach the

Root Bridge via same neighbor switch. OR

* If all the ports are configured with same priority number (32 by default), select the lowest

port number on the switch.

Designated Bridge Selection

i. In a LAN segment, the bridge with the lowest

path cost to the Root Bridge will be the DB OR

ii. If there are two bridges in the LAN segment

with equal path cost to the Root Bridge, then the

Bridge with the lowest Bridge ID becomes the

DB.

Designated Port Selection

i. The switch port (associated with the DB) on the LAN segment with the lowest accumulated path

cost to the Root Bridge will be selected as DP for the given segment. OR

ii. If a switch has redundant connections to the network segment, the switch port with the lowest

port priority (32 by default) is selected. OR

iii. If there is again a tie (it can happen if the priorities of the ports on this switch are the same), then

the lowest numbered port on the switch is selected.

Default Timers

Hello-----------------> 2s

Forward Delay-----> 15s

Max Age-------------> 20s

Link Costs

Bandwidth Cost

10 Mbps-----------> 100

100 Mbps----------> 19

1 Gbps---------------> 4

10 Gbps-------------> 2