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.
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.
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