STP


Redudant links are good for network.There are two links between switches(SW1 and SW2). These multiple links between swicthes are good idea but it can create many problems if we do not use loop avoidance method in our network.

Suppose in our network we have not use loop avoidance method and there are multiple links between swicthes SW1 and SW2.
Users are connected on both switches. If user on sw1 want to connect with user on sw2 there will be loop. Frame will continously circulate between switches. User will recieve same frame multiple time. Swith will update its mac address databse based on frame it received from different interfaces.

To overcome loop problem we use STP in our network.


802.1d is default version on cisco swithes. The new version is 802.1w.
The main role of STP is to prevent loop in network. It monitors network to find all links and make sure no loop occurs by blocking redudant links.STP uses STA(spanning tree algorithm) to provide loop avoidance network.
There are tasks STP perform
The first task is to select root bridge in the network.

Selection of Root Bridge.
Root bridge is the bridge with the best bridge id. Root bridge serve as the focal point in the network. All the decision in the network such as which port to block and which port to put in forwarding state are made from prespective of root bridge.STP uses bridge id to determine root brdige in the network. Bridge id 8 bytes which include bridge priority and bridge mac address.


Two switches are connected with multiple links.To determine root bridge, switches will send BPDU messages every 2 second. These bpdu include bridge id. Bridge id include bride priority and bridge mac address. The default bridge priority is 32768. Two swicthes will compare their bridge id. The switch which has lowest bridge id will become root bridge. first bridge priority is compare from bridge id then mac address are compare if both switches has same priority.

SW1 bridge priority 32768 and mac address 1111.1111.1111 and SW2 bridge priority 32768 and mac address 2222.2222.2222. Both switches sw1 and sw2 has same priority so their mac addresses will be compared. SW1 has lowest mac address so it will become root bridge in the netwok. SW2 is non root switch.All the ports on root bridge are designated ports.





Next task is selection of Root Port

Root port is the best path from non root bridge to reach Root bridge.To select root port
Port oppiste ot root port are always dessignated ports.