Spanning Tree Protocol (802.1D)

STP is enabled by Default in Cisco Switches. STP constructs a loop-free shortest path network and allows for duplicate or redundant paths.

 

There is a separate STP in each VLAN

 

States of STP

Status

Frame Processing

Time

Blocked (max age)

(Discarding State for RSTP)

No frames forwarded, Receiving BPDUs

20 Sec

Listening

No frames forwarded, Listening frames, BPDUs processed (sent/recd) to determine paths and build topology

15 Sec

Learning

No frames forwarded, Learning addresses, builds bridging table

15 Sec

Forwarding

Frames forwarded. Sending/Receiving data

 

Disabled

No frames forwarded, No BPDUs heard. Administratively down

 

 

Note:BPDUs sent out every 2 seconds by default.

 

Note: Rapid STP (802.1w) can bypass Listening and Learning states because it can skip the process on Edge Type shared

and Edge Type Point-to-point links.

 

Switches selected based on BID (Bridge ID), and the lowest will become the Root Bridge.

Bridge ID is made up of a Bridge Priority and the MAC address.

 

Priority can be changed with the following switch command

Switch(config)#spanning-tree vlan 1 priority <number> (number can be any value from 0-65535. Default is 32768)

 

Once converged, BPDUs are only sent out from Root Switch (except RSTP). If no BPDUs are heard when Max Age is reached

Then switch initiates negotiations to rebuild Spanning Tree. It does this by transitioning to the Listening State, and follows

the same process as the startup.

 

During the Election process, the root bridge, root ports and designated ports are elected. Root bridge is decided as above, by BID. Root ports and designated ports are selected based on cumulative path costs, based on the following table:

Bandwidth

Total or cumulative path cost

4 Mbps

250

10 Mbps

100

16 Mbps

62

45 Mbps

39

100 Mbps

19 (previously 10)

155 Mbps

14

622 Mbps

6

1 Gbps

4 (previously 1)

10 Gbps

2 (previously 1)

 

 

 

STP prevents three problems in switch networks:

         Broadcast Storms

         Multiple Frame Transmissions

         Unstable MAC Databases

 

 

 

Switch show and debug commands

show spanning-tree

show run