Spanning-Tree Design
STP and its computations are absolute predictable. However, added factors abide that may subtly
influence STP decisions, authoritative the consistent timberline anatomy neither accepted nor ideal. For
example, several versions of Spanning Timberline abide and are acclimated by assorted vendors.
Interoperability of these versions could be important in a mixed-vendor network.
The arrangement ambassador can additionally accomplish adjustments to the Spanning-Tree operation to
control its behavior. The area of the Base Arch should be bent as allotment of the design
process. As well, bombastic links can be acclimated for bulk acclimation in alongside if configured
correctly. Spanning Timberline can additionally be configured to assemble bound and predictably in the event
of a above cartography change.
Types of STP
So far, this affiliate has discussed STP in agreement of its operation to anticipate loops and to recover
from cartography changes in a adapted manner. STP was originally developed to accomplish in a bridged
environment, basically acknowledging a distinct LAN (or one VLAN). Implementing STP into a
switched ambiance has adapted added application and modification to support
multiple VLANs. Because of this, the IEEE and Cisco accept approached STP differently. This
section reviews the three types of STP that are encountered in switched networks and how they
relate to one another. There are no specific agreement commands associated with the various
types of STP. Rather, you should accept a basal compassionate of how they interoperate in a
network.
Common Spanning Timberline (CST)
The IEEE 802.1Q accepted specifies how VLANs are to be trunked amid switches. As well,
it specifies alone a distinct instance of STP for all VLANs. This instance is referred to as the
Common Spanning Timberline (CST) or the Mono Spanning Timberline (MST). All BPDUs are transmitted
over VLAN 1, the administration VLAN.
Having a distinct STP for abounding VLANs simplifies about-face agreement and reduces about-face CPU
load during STP calculations. However, the STP can account limitations, too. Bombastic links
between switches will be blocked with no adequacy for bulk balancing. Altitude can also
occur that would account forwarding on a articulation that doesn’t abutment all VLANs, while added links
would be blocked.
Per-VLAN Spanning Timberline (PVST)
Cisco has a proprietary STP that offers added adaptability than the CST method. Per-VLAN
Spanning Timberline (PVST) operates a abstracted instance of STP for anniversary alone VLAN. This
allows the STP on anniversary VLAN to be configured independently, alms bigger achievement and
Spanning-Tree Design 173
tuning for specific conditions. Assorted Spanning Trees additionally accomplish bulk acclimation accessible over
redundant links aback the links are assigned to altered VLANs.
Due to its proprietary nature, PVST requires the use of Cisco Inter-Switch Articulation (ISL) trunking
encapsulation amid switches. In networks area PVST and CST coexist, interoperability
problems occur. Anniversary requires a altered trunking adjustment so BPDUs will never be exchanged
between STP types.
Per-VLAN Spanning Timberline Plus (PVST+)
Cisco has a added proprietary STP that allows accessories to interoperate with both PVST and
CST. Per-VLAN Spanning Timberline Plus (PVST+) finer supports three groups of STP
operating in the aforementioned campus network: Catalysts alive PVST; Catalysts alive PVST+;
and switches alive CST/MST over 802.1Q.
To do this, PVST+ acts as a translator amid groups of CST switches and groups of PVST
switches. PVST+ can acquaint anon with PVST by application ISL trunks. To communicate
with CST, however, PVST+ exchanges BPDUs with CST on VLAN 1. BPDUs from other
instances of STP (other VLANs) are broadcast beyond the CST portions of the arrangement by
tunneling. PVST+ sends these BPDUs by application a altered multicast abode so that the CST
switches will advanced them on to afterwards neighbors. Eventually, the tunneled BPDUs will
reach added PVST+ switches area they are understood.
STP Configuration
By default, STP is enabled on all ports of a switch. STP should abide enabled in a arrangement to
prevent bridging loops from forming. However, if STP has been disabled, it can be re-enabled
with the commands accurate in the account that follows:
• STP Agreement on a CLI-Based Switch—STP can be enabled on either a port, a
range of ports, or on all ports and VLANs by application the afterward command:
Switch (enable) set spantree accredit [all | module/port]
• STP Agreement on an IOS-Based Switch—To accredit STP on an IOS-based switch,
where vlan-list is the account of VLANs that should accept STP enabled, use the following
command:
Switch (config)# spantree vlan-list
To appearance the cachet of STP on either a CLI- or IOS-based switch, use the afterward command:
Switch (enable) appearance spantree [vlan]
The achievement in Archetype 5-4 shows the accepted ethics for all elections, costs, timers, Arch IDs,
and STP accompaniment on anniversary anchorage for a VLAN.
174 Affiliate 5: Bombastic About-face Links
Root Arch Placement
While STP is affably automated with its absence ethics and acclamation processes, the resulting
tree anatomy may accomplish absolutely abnormally than expected. The Base Arch acclamation is based
on the abstraction that one about-face is called as a accepted advertence point, and all added switches choose
ports that are abutting to the Root. The Base Arch acclamation is additionally based on the abstraction that the
Root Arch can become a axial hub that interconnects added legs of the network. Therefore,
the Base Arch can be faced with abundant switching endless in its axial location.
If the Base Arch acclamation is larboard to its absence state, several things can activity to accomplish a poor
choice. For example, the slowest about-face (or bridge) can be adopted as the Base Bridge. If heavy
loads of cartage are accepted to canyon through the Base Bridge, the slowest about-face is not the ideal
candidate. Anamnesis that the alone belief for Base Arch acclamation is the everyman Arch ID (Bridge
Priority and MAC address)—not necessarily the best best to ensure optimal performance. If
the slowest about-face has the aforementioned Arch Antecedence as the others and has the everyman MAC address,
the slowest about-face will be called as the Root.
A added agency to accede relates to redundancy. If all switches are larboard to their absence states,
only one Base Arch will be elected. What will appear if that about-face fails? Accession Root
Bridge acclamation will activity but afresh the best ability not be the ideal about-face or the ideal
location.
The final application is the area of the Base Arch switch. As before, an acclamation with
default about-face ethics could abode the Base Arch in an abrupt area in the network.
More important, a absolute inefficient Spanning-Tree anatomy could aftereffect causing cartage from a
large allocation of the arrangement to booty a continued and ambagious aisle aloof to canyon through the Root
Bridge.
Example 5-4 appearance spantree Command Achievement Displays STP Cachet on a Switch
Switch (enable) appearance spantree 10
VLAN 10
Spanning timberline enabled
Spanning timberline blazon ieee
Designated Base 00-50-a2-8d-58-09
Designated Base Antecedence 32768
Designated Base Bulk 0
Designated Base Anchorage 1/0
Root Max Age 20 sec Accost Time 2 sec Advanced Adjournment 15 sec
Bridge ID MAC ADDR 00-50-a2-8d-58-09
Bridge ID Antecedence 32768
Bridge Max Age 20 sec Accost Time 2 sec Advanced Adjournment 15 sec
Port Vlan Port-State Bulk Antecedence Fast-Start Group-Method
--------- ---- ------------- ----- -------- ---------- ------------
1/2 10 forwarding 4 32 disabled
9/1-2 10 forwarding 14 32 disabled redundancy
Switch (enable)
Spanning-Tree Design 175
Figure 5-7 shows a allocation of a real-world hierarchical campus network.
Figure 5-7 A Campus Arrangement with an Inefficient Base Arch Election
Catalyst switches A and B are two Admission band devices; Catalysts C and D are in the
Distribution layer; Catalysts E and F anatomy the Bulk layer; and Catalyst G connects a server farm
into the arrangement core. Apprehension that all the switches use bombastic links to added layers of the
hierarchy, as adapted in Affiliate 2, “Campus Arrangement Design Models.” As will be seen,
Catalyst A will become the Base Arch due to its low MAC address. The STP activity will
begin to develop.
Catalyst A
32768.00-00-00-00-00-0a
100 Mbps
Cost=19
100 Mbps
Cost = 19
1000 Mbps
Cost = 4
1000 Mbps
Cost = 4
1000 Mbps
Cost = 4
1000 Mbps
Cost = 4
Catalyst B
32768.00-00-00-00-00-0b
Catalyst D
32768.00-00-00-00-00-0d
Catalyst E
32768.00-00-00-00-00-0e
Catalyst G
32768.00-00-00-00-00-10
Catalyst F
32768.00-00-00-00-00-0f
Catalyst C
32768.00-00-00-00-00-0c
Access layer
Distribution
layer
Core
layer
Server
farm
176 Affiliate 5: Bombastic About-face Links
Figure 5-8 shows the converged accompaniment of STP. For the purposes of this discussion, the Base Ports
and Designated Ports are artlessly apparent on the arrangement diagram. (As an exercise, you should
work out the Spanning Timberline based on the advice apparent in the figure.)
Figure 5-8 A Campus Arrangement with STP Converged
Catalyst A
32768
00-00-00-00-00-0a
100 Mbps
Cost=19
100 Mbps
Cost=19
1000 Mbps
Cost=4
1000 Mbps
Cost=4
1000 Mbps
Cost=4
Catalyst B
32768
00-00-00-00-00-0b
Catalyst D
32768
00-00-00-00-00-0d
Catalyst E
32768
00-00-00-00-00-0e
Catalyst G
32768
00-00-00-00-00-10
Catalyst F
32768
00-00-00-00-00-0f
Catalyst C
32768
00-00-00-00-00-0c
Root Bridge
X
X
X
X
X
DP
RP
DP
1000 Mbps
Cost=4
RP
DP
RP
DP
DP
RP
DP
RP
DP DP
DP
DP
RP
DP
1/1 1/2
Spanning-Tree Design 177
Notice that Catalyst A, one of the Admission band switches, has been adopted as the Base Bridge.
Also agenda the area of the X symbols over the ports that are neither Base Ports nor
Designated Ports. These ports will admission the Blocking state.
Finally, Bulk 5-9 shows the aforementioned arrangement with the Blocking links removed. Now you can see
the accurate anatomy of the final Spanning Tree.
Figure 5-9 Final Spanning Timberline Anatomy for the Campus Network
Catalyst A
32768
00-00-00-00-00-0a
Catalyst B
32768
00-00-00-00-00-0b
Catalyst D
32768
00-00-00-00-00-0d
Catalyst E
32768
00-00-00-00-00-0e
Catalyst G
32768
Catalyst F
32768
00-00-00-00-00-0f
Catalyst C
32768
00-00-00-00-00-0c
Root Bridge
DP
RP
DP
RP
DP
RP
RP
DP
RP
DP
DP
RP
178 Affiliate 5: Bombastic About-face Links
Catalyst A, an Admission band switch, is the Base Bridge. Workstations on Catalyst A can reach
servers on Catalyst G by arch the Administration band and again the Bulk layer, as expected.
However, apprehension what has happened to the added Admission band switch, Catalyst B. Workstations
on this about-face charge cantankerous into the Administration band (Catalyst C), aback into the Admission layer
(Catalyst A), aback into the Administration band (Catalyst D), through the Core, and assuredly to the
server acreage (Catalyst G). This activity is acutely absolute inefficient. In addition, Catalyst A is
probably not a high-end about-face because it is acclimated in the Admission layer. The anatomy of the
Spanning Timberline has affected Catalyst A to backpack abundant added than its accepted bulk and it will likely
become a bottleneck.
Root Arch Configuration
To anticipate the surprises categorical in the antecedent section, you should consistently do two things:
• Set a Base Arch in a bent fashion.
• Set a accessory Base Arch in case of primary Base Arch failure.
As the accepted advertence point, the Base Arch (and the secondary) should be placed abreast the
center of the network. For example, a about-face in the Administration band would accomplish a bigger Root
Bridge best than one in the Admission band because added cartage is accepted to canyon through the
Distribution band devices. In a collapsed switched arrangement (no Band 3 devices), a about-face abreast a
server acreage would be a added able Base Arch than switches elsewhere. Best cartage will be
destined to and from the server acreage and will account from a predetermined, absolute path.
To configure a Catalyst about-face to become the Base Bridge, use the commands declared in the
sections that follow.
Root Arch Agreement on a CLI-Based Switch
CLI-based Catalysts activity two commands that can be acclimated to adapt the Base Arch selection.
The set spantree antecedence command anon modifies the Arch Antecedence bulk so that a switch
can be accustomed a lower Arch ID bulk to win a Base Arch election:
Switch (enable) set spantree antecedence bridge-priority [vlan]
The bridge-priority bulk defaults to 32768, but can be accustomed a bulk of 0 to 65535. Although
the vlan constant is optional, it should consistently be defined aback application this command.
Remember that Catalyst switches run one instance of STP for anniversary VLAN (PVST). Therefore,
you should baptize an adapted Base Arch for anniversary VLAN. If the vlan parameter
is not acclimated with this command, the Arch Antecedence will be adapted alone for VLAN 1 (the
management VLAN).
The added another for modifying the Base Arch another is the set spantree root
command. This command is absolutely a macro on the Catalyst that executes several other
commands. The aftereffect is a added absolute and automated way to force one about-face to become the
Spanning-Tree Design 179
Root Bridge. Absolute Arch Priorities are not accustomed in the command. Rather, the about-face will
modify STP ethics according to the accepted ethics in use aural the alive network. The
command syntax is as follows:
Switch (enable) set spantree base [secondary] [vlan-list] [dia diameter] [hello
hello_time]
This command modifies the Arch Antecedence bulk of the about-face to become beneath than the Bridge
Priority of the accepted Base Bridge. If the accepted base antecedence is added than 8192, the local
switch will accept its antecedence set to 8192. If the accepted base antecedence is beneath than 8192, however,
the command will set the bounded about-face antecedence to some bulk (1000, 100, or 1) beneath than the
current root. For example, if the accepted base has a antecedence of the absence 32768, this command
will adapt the bounded Arch Antecedence to be 8192. If the accepted base is 8192, the bounded Bridge
Priority ability be 7192, and so on.
This command can additionally be acclimated to configure one or added accessory Base Bridges for specific
VLANs. By including the set spantree base accessory option, the bounded about-face will accept a
Bridge Antecedence of 16384—a bulk beneath than the absence 32768 of added switches but greater than
8192 (or less) of the Base Bridge.
The arrangement bore and the STP Accost Time can additionally be adapted with this command, if
needed. This modification is discussed added in the “Tuning Spanning-Tree Convergence”
section afterwards in the chapter.
NOTE The set spantree base command will not be apparent in a Catalyst about-face agreement because
the command is absolutely a macro alive added about-face commands. The absolute commands and
values acclimated by the macro will be shown, however.
Root Arch Agreement on an IOS-Based Switch
On an IOS-based switch, the afterward command can be acclimated to adapt the Base Bridge
selection:
Switch (config)# spanning-tree [vlan vlan-list] antecedence bridge-priority
The Arch Antecedence for the VLANs defined is adapted to the bridge-priority value. Catalyst
switches absence to 32768, but can be accustomed a antecedence of 0 to 65535. You should consistently specify
the account of VLANs to adapt for PVST. Otherwise, the command will adapt the Bridge
Priority alone for VLAN 1.
Spanning-Tree Customization
The best important accommodation you can accomplish aback designing your Spanning-Tree cartography is the
placement of the Base Bridge. Added decisions, such as the exact loop-free aisle structure, will
180 Affiliate 5: Bombastic About-face Links
occur automatically as a aftereffect of the STA. Occasionally, the aisle may charge added tuning,
but alone beneath appropriate affairs and afterwards accurate consideration.
Recall the arrangement of four belief that STP uses to accept a path:
1 Everyman Arch ID
2 Everyman Base Aisle Cost
3 Everyman Sender Arch ID
4 Everyman Anchorage ID
The antecedent area discussed how to tune the Arch ID of a about-face to abode the Base Bridge
in a network. This address can be acclimated to force a about-face to accept the everyman Arch ID and also
to admission the sending Arch ID of added switches (Lowest Arch ID and Everyman Sender
Bridge ID). However, alone the automated STP ciphering has been discussed, application the
default about-face anchorage costs to accomplish specific aisle decisions.
Tuning the Base Aisle Cost
The Base Aisle Bulk for anniversary alive anchorage of a about-face is bent by the accumulative bulk as a
BPDU campaign along. As a about-face receives a BPDU, the anchorage bulk of the accepting anchorage is added
to the Base Aisle Bulk in the BPDU. The anchorage or aisle bulk is inversely proportional to the port’s
bandwidth, as listed ahead in Table 5-4. If desired, the bulk of a anchorage can be adapted from
the absence value.
NOTE Afore modifying the aisle bulk of a about-face port, you should consistently account the Base Aisle costs
of added alternating paths through the network. Changing one port’s bulk may admission STP to
choose that anchorage as a Base Anchorage but there could be added paths that are still preferred. You should
also account a port’s absolute aisle bulk to actuate what the new bulk bulk should be. Careful
calculation will ensure that the adapted aisle will absolutely be chosen.
Tuning the Base Aisle Bulk on a CLI-Based Switch
The anchorage bulk can be adapted on a CLI-based about-face by application one of the afterward commands:
Switch (enable) set spantree portcost module/port cost
Switch (enable) set spantree portvlancost module/port [cost cost] [vlan-list]
The set spantree portcost command modifies the bulk for a port, behindhand of the VLANs that
are assigned to it. However, anamnesis that PVST can actualize absolutely altered Spanning-Tree
topologies for anniversary VLAN. If it is all-important to tune the anchorage bulk for a specific VLAN, use the
set spantree portvlancost command and specify the VLAN number. This command only
Spanning-Tree Design 181
applies to block ports, area assorted VLANs are in use. The anchorage bulk bulk can ambit from 0
to 65535 and defaults to 4 if not specified.
Tuning the Base Aisle Bulk on an IOS-Based Switch
IOS-based switches activity a distinct command that can adapt the anchorage bulk for individual
VLANs. Use the afterward command:
Switch (config-if)# spanning-tree [vlan vlan-list] bulk cost
If the vlan constant is given, the anchorage bulk will be adapted alone for the defined VLANs. If
it is not specified, the bulk is adapted alone for VLAN 1. The bulk bulk ranges from 1 to 65535
and defaults to the accepted IEEE ethics listed ahead in Table 5-4.
Tuning the Anchorage ID
The fourth belief of an STP accommodation is the Anchorage ID. The Anchorage ID bulk that a about-face uses is
actually a 16-bit quantity: 6 $.25 for the Anchorage Antecedence and 10 $.25 for the Anchorage Number. Port
Priority is a bulk from 0 to 63 and defaults to 32 for all ports. The Anchorage Cardinal can ambit from
0 to 1023 and represents the absolute concrete mapping of the port. Anchorage Numbers activate with 1 at
port 1/1 and accession beyond anniversary module. (The numbers may not be afterwards because each
module is assigned a accurate ambit of numbers.)
Obviously, the Anchorage Cardinal of a about-face anchorage is anchored because it is based on accouterments location.
The Anchorage ID, however, can be adapted to admission an STP accommodation by application the Anchorage Priority.
Tuning the Anchorage ID on a CLI-Based Switch
The Anchorage ID can be acquainted on a CLI-based about-face by application one of the afterward commands:
Switch (enable) set spantree portpri {module/port} priority
Switch (enable) set spantree portvlanpri {module/port} antecedence [vlans]
The set spantree portpri command is acclimated to set the anchorage antecedence bulk on a anchorage for all
VLANs. In a PVST ambiance area assorted instances of STP are alive for multiple
VLANs, the set spantree portvlanpri command should be used. The anchorage antecedence again can be
tailored for anniversary specific VLAN that the anchorage participates. The antecedence bulk ranges from 0 to
63 and defaults to 32.
Tuning the Anchorage ID on an IOS-Based Switch
Use the afterward command to adapt the Anchorage Antecedence on an IOS-based switch:
Switch(config-if)# spanning-tree [vlan vlan-list] port-priority port-priority
182 Affiliate 5: Bombastic About-face Links
The Anchorage Antecedence can be adapted for specific VLANs by application the vlan parameter. If this is not
specified, the Anchorage Antecedence is set alone for VLAN 1. The bulk of port-priority can ambit from 0
to 255 and defaults to 128.
NOTE Apprehension that ambit of Anchorage Antecedence ethics differs amid CLI- (0–63) and IOS-based (0–255)
switches. IOS-based switches (and routers) attach to the 802.1D standard, which specifies an
8-bit Anchorage Antecedence and an 8-bit Anchorage Cardinal acreage in the Anchorage ID. Higher-end Catalyst switches
move the abuttals amid fields so that the Anchorage Cardinal acreage has a greater ambit of values,
supporting college densities of about-face ports.
Viewing STP Status
STP costs for a about-face anchorage can be beheld by application the appearance spantree module/port command.
Example 5-5 demonstrates sample achievement from this command. About-face anchorage 3/29 is a affiliate of
VLAN 53, is in the Forwarding state, has a Anchorage Bulk of 19, and a Anchorage Antecedence of 32. In addition,
the anchorage has STP PortFast enabled.
Tuning Spanning-Tree Convergence
STP uses several timers, a arrangement of states that ports charge move through, and specific
topology change altitude to anticipate bridging loops from basic in a circuitous network. Each
of these ambit or requirements is based on assertive absence ethics for a archetypal arrangement size
and function. For the majority of cases, the absence STP operation is acceptable to accumulate the
network loop-free and accredit users to communicate.
However, assertive situations activity aback the absence STP can account arrangement admission to be delayed
while timers expire and while preventing loops on links area loops are not possible. It is safe
then to accomplish adjustments to the STP aggregation activity for added efficiency.
Modifying STP Timers
Recall that STP uses three timers to accumulate clue of assorted anchorage operation states and communication
between bridges. The three STP timers can be adapted by application the commands accurate in
the sections that follow. Remember that the timers charge alone be adapted on the Base Bridge
(and any accessory or advancement Base Bridges) because the Base Arch propagates all three timer
values throughout the arrangement as fields in the Agreement BPDU.
Example 5-5 appearance spantree Command Achievement Displays STP Cachet of a About-face Port
Switch (enable) appearance spantree 3/29
Port Vlan Port-State Bulk Antecedence Portfast Channel_id
------------------------ ---- ------------- ----- -------- ---------- ----------
3/29 53 forwarding 19 32 enabled 0
Spanning-Tree Design 183
Modifying STP Timers on a CLI-Based Switch
Use the afterward commands to adapt STP timers on a CLI-based switch:
Switch(enable) set spantree accost breach [vlan]
Switch(enable) set spantree fwddelay adjournment [vlan]
Switch(enable) set spantree maxage agingtime [vlan]
The Accost Timer triggers alternating accost letters to be beatific to added bridges and sets the interval
that a arch expects to apprehend a accost from its adjoining bridges. BPDUs are beatific every 2
seconds by default. The Accost Timer can be adapted per VLAN with the set spantree hello
command with a ambit of 1 to 10 seconds.
The Advanced Adjournment Timer determines the bulk of time a anchorage stays in the Listening state
before affective into the Learning accompaniment and how continued it stays in the Learning accompaniment afore moving
to the Forwarding state. The Advanced Adjournment Timer can be adapted per VLAN with the set
spantree fwddelay command. The absence bulk is 15 abnormal and can be set to a bulk of 4 to
30 seconds. This timer should alone be adapted beneath accurate application because the value
is abased aloft the bore of the arrangement and the advancement of BPDUs beyond all
switches. A bulk too low will acquiesce loops to anatomy and attenuate a network.
The MaxAge Timer specifies the lifetime of a stored BPDU that has been accustomed from a
neighboring about-face with a Designated Port. Suppose BPDUs are actuality accustomed on a non-
Designated about-face anchorage every 2 seconds, as expected. Again an aberrant failure, or one that
doesn’t absorb a concrete articulation activity down, occurs that prevents BPDUs from actuality sent. The
receiving about-face will adjournment until the Max Age Timer expires to accept for added BPDUs. If none
are received, the non-Designated anchorage will move into the Listening accompaniment and Configuration
BPDUs will be generated by the accepting switch. This anchorage again becomes the Designated Port
to restore connectivity on the segment.
To adapt the Max Age Timer on a per-VLAN basis, use the set spantree maxage command.
The timer bulk defaults to 20 seconds, but can be set from 6 to 40 seconds.
NOTE Modifying STP timers can be catchy accustomed the bourgeois attributes of the absence ethics and the
calculations bare to acquire able STP operation. Timer ethics are basically abased on
the Accost Time and the bore of the switched network, in agreement of about-face hops. Catalyst CLIbased
switches activity a distinct command that can be acclimated to change the timer ethics in a more
controlled fashion. Although declared earlier, the set spantree base macro command is a
better apparatus to use than ambience the timers with the alone commands:
Switch (enable) set spantree base [secondary] [vlan-list] [dia diameter] [hello
hello-time]
Here, STP timers will be adapted absolutely according to the formulas defined in the 802.1D
standard by giving alone the Accost Time and the bore of the network. Again, this command
can be acclimated on a per-VLAN base to adapt the timers for a accurate VLAN’s Spanning Tree.
The arrangement bore can be a bulk from one to seven about-face hops. Because this command is
used to accomplish a about-face become the Base Bridge, all the adapted timer ethics consistent from this
command will be broadcast to added switches through the Agreement BPDU.
184 Affiliate 5: Bombastic About-face Links
Modifying STP Timers on IOS-Based Switches
The afterward commands can be acclimated on an IOS-based about-face to adapt STP timers:
Switch(config)# spanning-tree [vlan vlan-list] hello-time seconds
Switch(config)# spanning-tree [vlan vlan-list] forward-time seconds
Switch(config)# spanning-tree [vlan vlan-list] max-age seconds
The Accost Timer defaults to 2 abnormal and can be set from 1 to 10 abnormal per VLAN. The
Forward Timer defaults to 15 abnormal and can be set from 4 to 200 abnormal per VLAN. The
Max Age Timer defaults to 20 abnormal and can be set from 6 to 200 abnormal per VLAN. If the
vlan constant is not defined for any of these commands, VLAN 1 will be assumed.
Redundant Articulation Convergence
Some added methods that abide to acquiesce faster STP aggregation in the accident of a articulation failure
include
• PortFast—Enables fast connectivity to be accustomed on admission band about-face ports to
workstations that are booting up.
• UplinkFast—Enables fast uplink failover on an admission band about-face aback bifold uplinks are
connected into the administration layer.
• BackboneFast—Enables fast aggregation in the arrangement courage (core) afterwards a
Spanning-Tree cartography change occurs.
Rather than modifying timer values, these methods assignment by authoritative aggregation on
specifically amid ports aural the arrangement hierarchy.
PortFast: Admission Band Nodes
An end-user workstation is usually affiliated to a about-face anchorage in the Admission layer. If the
workstation is powered off and again angry on, the about-face anchorage will not be in a adapted accompaniment until
STP cycles from the Blocking accompaniment to the Forwarding state. With the absence STP timers, this
transition will booty at atomic 30 abnormal (15 abnormal Listening to Learning and 15 seconds
Learning to Fowarding). Therefore, the workstation is clumsy to address or accept any useful
data until the Forwarding accompaniment is accomplished on the port.
NOTE Anchorage initialization delays of up to 50 abnormal can be observed. As discussed, 30 of these seconds
are due to the STP accompaniment transitions. If a about-face anchorage is alive PAgP to accommodate EtherChannel
configuration, an added 20-second adjournment can exist.
Spanning-Tree Design 185
On about-face ports that affix alone to distinct workstations or specific devices, bridging loops will
never be possible. Catalyst switches activity the PortFast affection that shortens the Listening and
Learning states to a negligible bulk of time. The aftereffect is that aback a workstation articulation comes
up, the about-face will anon move the PortFast anchorage into the Forwarding state. Spanning-
Tree bend apprehension is still in operation, however, and the anchorage will be confused into the Blocking
state if a bend is anytime detected on the port.
To accredit or attenuate the PortFast affection on a CLI-based about-face port, use the following
command:
Switch(enable) set spantree portfast {module/port} {enable | disable}
On an IOS-based switch, use this command:
Switch (config-if)# spanning-tree portfast
Obviously, you should not accredit PortFast on a about-face anchorage that is affiliated to a hub or another
switch because bridging loops could possibly form. One added account of PortFast is that TCN
BPDUs are not beatific aback a about-face anchorage in PortFast approach goes up or down. This simplifies the
TCN manual on a ample arrangement aback end-user workstations are advancing up or shutting
down.
To appearance the PortFast accompaniment of about-face ports, use the appearance spantree command. Anniversary anchorage is listed,
along with the PortFast advice beneath the “Fast-Start” column.
UplinkFast: Admission Band Uplinks
Consider an Admission band about-face that has bombastic uplink access to two Administration layer
switches. Normally, one uplink would be in the Forwarding accompaniment and the added in the Blocking
state. If the primary uplink went down, up to 50 abnormal would expire afore the redundant
uplink could be used.
The UplinkFast affection on Catalyst switches enables leaf-node switches or switches at the ends
of the Spanning-Tree branches to accept a activity Base Anchorage while befitting one or more
redundant or abeyant Base Ports in Blocking mode. Aback the primary Base Anchorage uplink fails,
another blocked uplink can be anon brought up for use.
To accredit or attenuate the UplinkFast affection on a CLI-based switch, use the afterward command:
Switch (enable) set spantree uplinkfast {enable | disable} [rate update-rate]
[all-protocols off | on]
On an IOS-based switch, use the afterward command:
Switch (config)# spanning-tree uplinkfast [max-update-rate pkts-per-second]
When UplinkFast is enabled, it is enabled for the accomplished about-face and all VLANs. UplinkFast
works by befitting clue of accessible paths to the Base Bridge. Therefore, the command is not
allowed on the Base Arch switch. UplinkFast additionally makes some modifications to the local
switch to assure that it does not become the Base Arch and that the about-face is not acclimated as a
186 Affiliate 5: Bombastic About-face Links
transit about-face to get to the Base Bridge. First, the Arch Antecedence of the about-face is aloft to
49152, authoritative it absolute absurd that the about-face will be adopted to Base Arch status. The Port
Cost of all bounded about-face ports is incremented by 3000, authoritative the ports abominable as Root
Ports.
The command additionally includes a amount parameter. Aback an uplink on a about-face goes down,
UplinkFast makes it accessible for the bounded about-face to amend its bridging table of MAC addresses to
point to the new uplink. However, UplinkFast additionally provides a apparatus for the bounded switch
to acquaint added upstream switches that stations afterwards (or on against the Admission layer) can
now be accomplished over the anew activated uplink. This activity is done by sending multicast
frames to all added switches absolute the MAC addresses of the stations not abstruse on the
uplink ports. These multicast frames are beatific out at a amount defined by the amount constant per
100 milliseconds. The absence is 15 per 100 milliseconds. The final all-protocols constant tells
the about-face whether to accomplish the multicast updates for all accessible protocols (IP, IPX,
AppleTalk, and Band 2 packets).
The IOS-based about-face command uses a max-update-rate constant to set the amount of multicast
updates. The amount can be from 0 to 1000 multicasts per second.
To appearance the accepted UplinkFast ambit and ports, use the appearance spantree uplinkfast
command.
BackboneFast: Bombastic Courage Paths
In the arrangement backbone, or Bulk layer, a altered adjustment is acclimated to abbreviate STP convergence.
BackboneFast works by accepting a about-face actively actuate if alternating paths abide to the root
bridge in the accident that the about-face detects an aberrant articulation failure. Aberrant articulation failures occur
when a articulation not anon affiliated to a about-face fails. A about-face detects an aberrant articulation failure
when it receives inferior BPDUs from its Designated Arch on either its base anchorage or a blocked
port. (Inferior BPDUs are beatific from a Designated Arch that has absent its affiliation to the Root
Bridge, authoritative it advertise itself as the new Root).
Normally, a about-face charge adjournment for the Max Age timer to expire afore responding to the inferior
BPDUs. However, BackboneFast begins to actuate if added alternating paths to the Base Bridge
exist according to the blazon of anchorage that accustomed the inferior BPDU. If the inferior BPDU arrives
on a anchorage in the Blocking state, the about-face considers the base anchorage and all added blocked ports to
be alternating paths to the base bridge. If the inferior BPDU arrives on the base anchorage itself, the
switch considers all blocked ports to be alternating paths to the base bridge. If the inferior BPDU
arrives on the base anchorage and there are no blocked ports, however, the about-face assumes it has lost
connectivity with the base bridge. In this case, the about-face will accept that it has become the root
bridge and BackboneFast will acquiesce it to do so afore the Max Age timer expires
Detecting alternating paths to the base arch additionally involves an alternating activity with other
bridges. If the bounded about-face has blocked ports, BackboneFast begins to use the Base Articulation Query
(RLQ) agreement to see if there are upstream switches that accept abiding access to the Root
Spanning-Tree Design 187
Bridge. RLQ Requests are beatific out. If a about-face receives an RLQ Appeal and is either the Root
Bridge or has absent affiliation to the Root, it sends an RLQ Reply. Otherwise, the RLQ Request
is broadcast on to added switches until an RLQ Acknowledgment can be generated. On the bounded switch,
if an RLQ Acknowledgment is accustomed on its accepted Base Port, the aisle to the Base Arch is complete and
stable. If it is accustomed on a non-Root Port, an alternating Base Aisle charge be chosen. The Max Age
Timer is anon asleep so that a new Base Anchorage can be found.
BackboneFast is simple to configure and operates by short-circuiting the Max Age Timer when
needed. Although this activity shortens the time a about-face waits to ascertain a Base Aisle failure,
ports still charge go through feature Advanced Adjournment Timer intervals during the Listening and
Learning states. Area PortFast and UplinkFast enabled actual transitions, BackboneFast
can alone abate the best aggregation adjournment from 50 to 30 seconds.
To configure BackboneFast, use the afterward command:
Switch (enable) set spantree backbonefast {enable | disable}
When used, BackboneFast should be enabled on all switches in the arrangement because
BackboneFast requires the use of the RLQ appeal and acknowledgment apparatus to acquaint switches of
Root Aisle stability. The RLQ agreement is alone alive aback BackboneFast is enabled on a switch.
By default, BackboneFast is disabled.