Switch Looping:

• Cause broadcast storm
• MAC database instability

STP (Spanning Tree Protocol)   (802.1D)

• L2 protocol
• by default enable
• Provides a loop free redundant topology by placing certain port in a blocking state

Root Bridge:

• Chosen based on least priority /mac address                                                                
• one root bridge in a domain
• one designated port (forwarding port) per segment (link)

 

10 Mbps cost = 100
100 Mbps cost = 19
1 Gbps cost = 4
10 Gbps cost = 2

Switch elect the best path to the Root bridge based on lowest cost

Root port:most suitable port to reach Root Bridge. (Root Bridge will never have root port)
Designated port: choosen based on lower mac address

STP state:      (convergence time 50 sec)

Convergence time: the time of bringing to the forwarding state

RSTP (Rapid Spanning Tree Protocol): 802.1W
convergence time – 10 sec

 

Conf) # int fa 0/6
# spanning-tree portfast

(portfast is used for port that connects the computers. Don’t run portfast on trunking port. It is always for non-trunking port, if implemented on trunking port I will start looping)

# show spanning-tree
(config)# spanning-tree mode rapid-pvst
# show spanning-tree    (shows the RSTP)

Note: In order to run RSTP, first enable portfast on those ports connected to computers and then enable RSTP in entire (mandatory) switches in the network.

     # intfa 0/6
     # spanning-tree bpduguard enable
       (only will be implemented on those port which is PORTFAST to block BPDU on particular port)