STP helps to prevent loops in switched network. It works by selecting one root bridge, every non root bridge will select one root port and each network segment will select one designated port. For selecting the root bridge it has some criteria’s first it will check the priority, if the priority is same it will check the Lowest MAC address and it will elected as a root bridge.
Root Port: Lowest path cost to root bridge, if cost is same lowest sender bridge ID (MAC)
Designated Port: Lowest path cost to root bridge, if cost is same lowest sender bridge ID (MAC)
1. Lowest Bridge ID
2. Lowest path cost to root bridge
3.Lowest sender bridge ID
4.lowest port priority
5.Lowest Port ID.
Default Priority is 32768
STP and RSTP Difference
States:
STP RSTP
Disabled
Blocking Discarding
Listening
Learning Learning
Forwarding Forwarding
RSTP will wait for 3 missed BPDU’s then it will go to learning to forwarding state
STP Convergence:
(Hello = 2 Sec)
Max Age (Blocking) = 20 secs
Listening = 15 secs
Learning = 15 secs
2*forward Delay +Max Age (2*15+20) = 50 secs to convergence
Max Age (Blocking) = 20 secs
Listening = 15 secs
Learning = 15 secs
2*forward Delay +Max Age (2*15+20) = 50 secs to convergence
RSTP Convergence:
3 missed BDPUs @ 2 sec each = 6 secs
Learning (no listening) = 15 secs
= 21 secs to convergence
Learning (no listening) = 15 secs
= 21 secs to convergence
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