STP States
To participate in STP, anniversary anchorage of a about-face charge advance through several states. A anchorage begins
its activity in a Disabled accompaniment affective through several acquiescent states and assuredly into an alive state
if accustomed to advanced traffic. The STP anchorage states are as follows:
• Disabled—Ports that are administratively shut bottomward by the arrangement ambassador or by
the arrangement due to a accountability action are in the Disabled state. This accompaniment is appropriate and is not
part of the accustomed STP progression for a port.
• Blocking—After a anchorage initializes, it begins in the Blocking accompaniment so that no bridging loops
can form. In the Blocking state, a anchorage cannot accept or abode abstracts and cannot add
MAC addresses to its abode table. Instead, a anchorage is alone accustomed to accept BPDUs so
that the about-face can apprehend from added adjoining switches. In addition, ports that are put
into standby approach to abolish a bridging bend access the Blocking state.
• Listening—The anchorage will be confused from Blocking to Alert if the about-face thinks that
the anchorage can be alleged as a Basis Anchorage or Designated Port. In added words, the anchorage is on
its way to activate forwarding traffic. In the Alert state, the anchorage still cannot accelerate or
receive abstracts frames. However, the anchorage is accustomed to accept and accelerate BPDUs so that it can
actively participate in the Spanning-Tree cartography process. Here the anchorage is assuredly allowed
to become a Basis Anchorage or Designated Anchorage because the about-face can acquaint the anchorage by
sending BPDUs to added switches. Should the anchorage lose its Basis Anchorage or Designated Port
status, it is alternate to the Blocking state.
• Learning—After a aeon of time alleged the Advanced Delay in the Alert state, the port
is accustomed to move into the Acquirements state. The anchorage still sends and receives BPDUs as
before. In addition, the about-face can now apprentice new MAC addresses to add into its address
table. This gives the anchorage an added aeon of bashful accord and allows the about-face to
assemble at atomic some abode table information.
• Forwarding—After accession Advanced Delay aeon of time in the Acquirements state, the port
is accustomed to move into the Forwarding state. The anchorage can now accelerate and accept data
frames, aggregate MAC addresses into its abode table, and accelerate and accept BPDUs. The
port is now a absolutely activity about-face anchorage aural the Spanning-Tree topology.
Spanning-Tree Protocol 169
NOTE Remember that a about-face anchorage is alone accustomed into the Forwarding accompaniment if there are no redundant
links (or loops) and if the anchorage has the best aisle to the basis arch as the Basis Anchorage or Designated
Port.
Example 5-3 shows the achievement of a about-face as one of its ports progresses through the STP port
states.
Example 5-3 A Anchorage Progressing Through the STP Anchorage States
Console> (enable) set anchorage attenuate 4/10
This command may abstract your telnet session.
Do you appetite to abide (y/n) [n]?y
Port 4/10 disabled.
Console> (enable) set anchorage accredit 4/10
Port 4/10 enabled.
Console> (enable) appearance spant 4/10
Port Vlan Port-State Cost Priority Fast-Start
-------- ---- ------------- ----- -------- ----------
4/10 1 alert 10 32 disabled
Console> (enable) sh spant 4/10
Port Vlan Port-State Cost Priority Fast-Start
-------- ---- ------------- ----- -------- ----------
4/10 1 alert 10 32 disabled
Console> (enable) sh spant 4/10
Port Vlan Port-State Cost Priority Fast-Start
-------- ---- ------------- ----- -------- ----------
4/10 1 alert 10 32 disabled
Console> (enable) sh spant 4/10
Port Vlan Port-State Cost Priority Fast-Start
-------- ---- ------------- ----- -------- ----------
4/10 1 acquirements 10 32 disabled
Console> (enable) sh spant 4/10
Port Vlan Port-State Cost Priority Fast-Start
-------- ---- ------------- ----- -------- ----------
4/10 1 acquirements 10 32 disabled
Console> (enable) sh spant 4/10
Port Vlan Port-State Cost Priority Fast-Start
-------- ---- ------------- ----- -------- ----------
4/10 1 acquirements 10 32 disabled
Console> (enable) sh spant 4/10
Port Vlan Port-State Cost Priority Fast-Start
-------- ---- ------------- ----- -------- ----------
4/10 1 acquirements 10 32 disabled
Console> (enable) sh spant 4/10
Port Vlan Port-State Cost Priority Fast-Start
-------- ---- ------------- ----- -------- ----------
4/10 1 forwarding 10 32 disabled
Console> (enable)
170 Affiliate 5: Redundant About-face Links
The archetype begins as the anchorage is administratively disabled from the command line. Back the
port is enabled, alternating appearance spantree module/port commands affectation the Port-State as
Listening, Learning, and again Forwarding. Because this anchorage was acceptable as a Basis Port, the
show command was never able to assassinate fast abundant to appearance the anchorage in the Blocking state.
STP Timers
STP operates as switches accelerate BPDUs to anniversary added in an accomplishment to anatomy a loop-free topology.
The BPDUs booty a bound bulk of time to biking from about-face to switch. In addition, account of a
topology change (such as a articulation or Basis Arch failure) can ache from advancement delays as
the advertisement campaign from one ancillary of a arrangement to the other. Because of the achievability of
these delays, befitting the Spanning-Tree cartography from clearing out or advancing until all
switches accept had time to accept authentic advice is important.
STP uses three timers to accomplish abiding that a arrangement converges appropriately afore a bridging loop
can afield form. The timers and their absence ethics are as follows:
• Hello Time—The time breach amid Configuration BPDUs beatific by the Basis Bridge.
The Hello Time amount configured in the Basis Arch about-face will actuate the Hello Time
for all non-root switches because they aloof broadcast the Configuration BPDUs as they are
received from the root. However, all switches accept a locally configured Hello Time that is
used to time TCN BPDUs back they are retransmitted. The IEEE 802.1D standard
specifies a absence Hello Time amount of two seconds.
• Advanced Delay—The time breach that a about-face anchorage spends in both the Alert and
Learning states. The absence amount is 15 seconds.
• Maximum (max) Age—The time breach that a about-face food a BPDU afore discarding
it. While alive the STP, anniversary about-face anchorage keeps a archetype of the “best” BPDU that it has
heard. If the antecedent of the BPDU loses acquaintance with the about-face port, the about-face will notice
that a cartography change has occurred afterwards the Max Age time elapses and the BPDU is aged
out. The absence Max Age amount is 20 seconds.
The STP timers can be configured or adapted from the about-face command line. However, the
timer ethics should never be afflicted from the defaults after authentic consideration. Then, the
values should alone be afflicted on the Basis Arch switch. Recall that the timer ethics are
advertised in fields aural the BPDU. The Basis Arch will accomplish abiding that the timer ethics are
propagated to all added switches.
NOTE The absence STP timer ethics are based on some assumptions about the admeasurement of the arrangement and
the breadth of the Hello Time. A advertence archetypal of a arrangement accepting a bore of seven
switches is acclimated to acquire these values. The bore is abstinent from the Basis Arch switch
outward, including the Basis Bridge. A Hello Time of two abnormal is acclimated in this computation.
Spanning-Tree Protocol 171
The arrangement bore can be configured on the Basis Arch about-face to added accurately reflect
the authentic admeasurement of the concrete network. Making that amount added authentic will abate the absolute STP
convergence time during a cartography change. As well, Cisco recommends that if changes need
to be made, alone the arrangement bore amount should be adapted on the Basis Arch switch.
When the bore is changed, the about-face will account new ethics for all three timers. This
option is discussed in the “Selecting the Basis Bridge” area in this chapter.
Topology Changes
To advertise a change in the alive arrangement topology, switches accelerate a Cartography Change
Notification (TCN) BPDU. Table 5-5 shows the architecture of these messages.
A cartography change occurs back a about-face either moves a anchorage into the Forwarding accompaniment or moves
a anchorage from Forwarding or Acquirements into the Blocking state. In added words, a anchorage on an active
switch comes up or goes down. The about-face sends a TCN BPDU out its Designated Anchorage so that
ultimately the Basis Arch will accept account of the cartography change. Apprehension that the TCN
BPDU carries no abstracts about the change, but alone informs recipients that a change has occurred.
Also apprehension that the about-face will not accelerate TCN BPDUs if the anchorage has been configured with
PortFast enabled.
The about-face will abide sending TCN BPDUs every Hello Time breach until it gets an
acknowledgement from an upstream neighbor. As the upstream neighbors accept the TCN
BPDU, they will bear it on against the Basis Bridge. Back the Basis Arch receives the
BPDU, the Basis Arch additionally sends out an acknowledgement. However, it additionally sends out the
Topology Change banderole in a Configuration BPDU so that all added bridges will abbreviate their
bridge table crumbling times bottomward from the absence (300 seconds) to aloof the Advanced Delay value
(default 15 seconds).
This action causes the abstruse locations of MAC addresses to be ablaze out abundant sooner
than they commonly would, abatement the arch table bribery that ability action due to the change
in topology. However, any stations that are actively communicating during this time will be kept
in the arch table. This action lasts for the sum of the Advanced Delay and the Max Age
(default 15 + 20 seconds).