Tag Archives: SWITCH 300-115 1.6b

SWITCH 300-115 1.6 Configure and verify spanning tree

1.6.b Switch priority, port priority, path cost, STP timers

STP summary
1.  all bridge (switch ports) stabilize at forwarding or blocking.  Forwarding ports are considered part of the spanning tree.
2.  one switch is elected root, and its ports will all move to forwarding state.
3.  each switch receives hellos from the root, directly or through another switch. The port that receives the least cost BPDU is placed in forwarding and becomes that switch’s root port
4.  for each segment one switch forwards the BPDU with the lowest cost.  That switch becomes that segment’s designated bridge.
5.  the other interfaces are placed in blocking
6.  the root sends BPDU’s every 2 seconds. This time interval can be modified and will be noted in the BPDU.
7.  if max-age elapses (20 seconds, default) and no BPDU, panic ensues and the spanning tree changes.
8.  forward delay (default 15 seconds) is the time it takes for a port to transition through the dumbass states to forwarding. (listening, 15 seconds, learning 15 seconds)
9.  when a switch goes into listening, it sends a TCN BPDU (topology change notification) through the new path to the root. Other switches will refresh their tables with the new entry.

10. spanning tree creates these delays to prevent transitional loops

SWITCH 300-115:1.6 Configure and verify spanning tree

1.6.b Switch priority, port priority, path cost, STP timers

click to enlarge.

rp cost and designated ports

my switchlab

sw1#sh ver | i Base
Base ethernet MAC Address       : 00:16:47:9E:45:00

sw2#sh ver | i Base
Base ethernet MAC Address       : 00:16:47:9E:5F:80

sw3#sh ver | i Base
Base ethernet MAC Address       : 00:16:C7:06:96:80

sw4#sh ver | i  Base
Base ethernet MAC Address       : A8:B1:D4:D3:A9:00

the 4 hallmarks of the root switch

this bridge is the root

the rid and the bid match

port roles are all designated

port status’s are all forwarding

sh spann sw1root and designated on sw 2 and 3, and root and block on sw4, below. particularly note sw4’s cumulative cost.

sw2 spann sw3 spann sw4 spann