Spanning Tree Protocol

Spanning Tree Protocol

1. Versions

   • IEEE 802.1D

2. What Problem(s) can happen without STP

   • LOOP///// Some Ethernet frames would loop around the network for a long time (hours, days, literally forever if the LAN devices and links never failed).
     • unknown-destination unicast frames
     • broadcast frames
       • Broadcast Storm
         • MAC table instability
         • Multiple frames received to host
     • multicast frames
     • STP prevents loops by placing each switch port in either:
       • Blocking State
         • do not process any frames except STP messages (and some other overhead messages). Interfaces that block do not forward user frames, do not learn MAC addresses of received frames, and do not process received user frames.
       • Forwarding State
         • act as normal, forwarding and receiving frames.

3. STP convergence

   • process by which the switches collectively realize that something has changed in the LAN topology and determine whether they need to change which ports block and which ports forward.

4. How STP works

   • STP uses 3 criteria to choose whether to put an interface in forwarding state:
     • 3. The switch with the lowest root cost, as compared with the other switches attached to the same link, is placed in forwarding state.
       • That switch is the designated switch, and that switch’s interface, attached to that segment, is called the designated port (DP).
     • 2. Non-root switches select one of its ports as root port.
       • Root port (RP) means that it has lowest cost (or nearest path) back to the Root bridge
       • root port (RP) is in forwarding state.
     • 1. Select Root switch and put all its working ports in forwarding state