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Chapter 31© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco !"lic© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco !"lic
S#I$C% v& Chapter 31
Chapter 3:
ImplementingSpanning Tree
CCNP SWITCH: Implementing IP Switching
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Chapter 32© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco !"lic
Chapter 3 Objectives
§ 'escri"e spanning tree protocols.
§ 'escri"e and con(ig!re )S$.
§ 'escri"e and con(ig!re *S$.
§ Con(ig!re S$ (eat!res to enhance resiliency and prevent
(or+arding loops.§ -plain recommended S$ con(ig!rations and practices.
§ $ro!"leshoot spanning tree iss!es.
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Chapter 33© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco !"lic
Spanning TreeProtocol Basics
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Chapter 3© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco !"lic
Spanning Tree Histor
§ S$ +as invented in 1/ "y )adia erlman at the 'igital
!ipment Corporation.
§ In 1//0, I p!"lished the (irst standard (or the protocol
as 02.1'.
§ Common Spanning $ree CS$4 56 Cisco S$8 56 )apid
S$ )S$4 or I 02.1+ 56 Cisco )S$8 56 *!ltiple
Spanning $ree *S$4 or I 02.1s 56 S$ sec!rity
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Chapter 3© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco !"lic
STP Operation ! "#eview $rom CCN%&
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STP Operation ' "#eview $rom CCN%&
%ll lin(s are !)) *b+s,
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#api- SpanningTree Protocol
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#STP Operation . Port States
ort State 'escription
'iscarding $his state is seen in "oth a sta"le active topology and d!ring topologysynchroni9ation and changes. $he discarding state prevents the (or+ardingo( data (rames, th!s :"rea;ing< the contin!ity o( a =ayer 2 loop.
=earning $his state is seen in "oth a sta"le active topology and d!ring topologysynchroni9ation and changes. $he learning state accepts data (rames topop!late the *AC ta"le to limit (looding o( !n;no+n !nicast (rames.
>or+arding$his state is seen only in sta"le active topologies. $he (or+arding s+itchports determine the topology. >ollo+ing a topology change, or d!ringsynchroni9ation, the (or+arding o( data (rames occ!rs only a(ter a proposaland agreement process.
?perational Stat!s S$ ort State )S$ ort State ort Incl!ded in Active $opology
na"led @loc;ing 'iscarding ona"led =istening 'iscarding o
na"led =earning =earning Bes
na"led >or+arding >or+arding Bes
'isa"led 'isa"led 'iscarding o
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#STP Operation . Port #oles
S$ ort )ole )S$ ort
)ole
S$ ort
State
)S$ ort
State)oot port )oot port >or+arding >or+arding
'esignated port 'esignatedport
>or+arding >or+arding
ondesignatedport
Alternate or"ac;!p port
@loc;ing 'iscarding
'isa"led 'isa"led 5 'iscarding
$ransition $ransition =istening=earning
=earning
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#STP Operation . #api- Transition to/orwar-ing . 0in( Tpe
=in;$ype
'escription
oint5to5point
ort operating in (!ll5d!ple- mode. It isass!med that the port is
connected to a singles+itch device at the otherend o( the lin;.
Shared ort operating in hal(5d!ple- mode. It isass!med that the port isconnected to sharedmedia +here m!ltiples+itches might e-ist.
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#STP Operation . #api- Transition to/orwar-ing . 1-ge Ports
§ An )S$ edge port is a s+itch port
that is never intended to "e
connected to another s+itch device.
It immediately transitions to the
(or+arding state +hen ena"led.
§ either edge ports nor ort>ast5
ena"led ports generate topologychanges +hen the port transitions
to disa"led or ena"led stat!s.
nli;e ort>ast, an edge port that
receives a @' immediately loses
its edge port stat!s and "ecomes anormal spanning5tree port. #hen
an edge port receives a @', it
generates a topology change
noti(ication $C4.
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#STP Operation . Proposal an-%greement
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Chapter 313© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco !"lic
#STP Operation . Topolog Change "TC&*echanism
§ ?nly non5edge ports that are moving to the (or+arding state
ca!se a topology change. A port that is moving to "loc;ing
does not ca!se the respective "ridge to generate a $C@'.
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#STP Operation . Bri-ge I-enti$ier $orP2#ST
§ ?nly (o!r high5order "its o( the 1&5"it @ridge riority (ield
a((ect the priority. $here(ore, priority can "e incremented
only in steps o( 0/&, onto +hich are added the =An!m"er. >or e-ample, (or =A 11D I( the priority is le(t at
de(a!lt, the 1&5"it riority (ield +ill hold 327& 8 11 E 3277/.
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#STP an- 4)',!5 STP Compatibilit
§ )S$ can operate +ith 02.1' S$. %o+ever, 02.1+Fs
(ast5convergence "ene(its are lost +hen interacting +ith
02.1' "ridges.
§ ach port maintains a varia"le that de(ines the protocol to
r!n on the corresponding segment. I( the port receives
@'s that do not correspond to its c!rrent operating mode(or t+o times the hello time, it s+itches to the other S$
mode.
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5e$a6lt STP Con$ig6ration on Cisco Switch
§ S$8
§ @ridge priority 32,7& (or each =A
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Chapter 317© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco !"lic
Spanning Tree Port/ast
§ @ypass 02.1' S$ listening and learning states "loc;ing
state (or+arding state4
§ orts connected to end stations§ revents '%C timeo!ts
§ *ay create "ridging loops i( ena"led on tr!n; port
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Con$ig6ring Port/ast on %ccess Ports
§ se the spanning-tree portfast inter(ace command
to ena"le the ort>ast (eat!re.
§ S+itchG con$ig6re terminal§ nter con(ig!ration commands, one per line. nd +ith
C$=H.§ S+itchcon(ig4G inter$ace /ast1thernet 3+'7§ S+itchcon(ig5i(4G spanning8tree port$ast§ J#arningD port(ast sho!ld only "e ena"led on ports
connected to a single§ host. Connecting h!"s, concentrators, s+itches, "ridges,
etc... to this§ inter(ace +hen port(ast is ena"led, can ca!se temporary
"ridging loops.§ se +ith CA$I?
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Con$ig6ring Port/ast 9loball
§ se the spanning-tree portfast de(a!lt glo"al
con(ig!ration mode command to ena"le the ort>ast
(eat!re on all nontr!n;ing inter(aces.
§ S+itchcon(ig4G spanning8tree port$ast -e$a6lt
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Chapter 320© 2007 – 2010, Cisco Systems, Inc. All rights reserved. Cisco !"lic
Con$ig6ring Port/ast on Tr6n( Ports
§ se the spanning-tree portfast trunk inter(ace
command to ena"le the ort>ast (eat!re on a tr!n; port.
§ S+itchcon(ig4G spanning8tree port$ast tr6n(
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Con$ig6ring %ccess Port *acro
§ se the switchport host macro command on an
inter(ace connecting to an end station.
§ S+itchcon(ig5i(4G switchport host
§ s+itchport mode +ill "e set to access
§ spanning5tree port(ast +ill "e ena"led
§ channel gro!p +ill "e disa"led
§ S+itchcon(ig5i(4G en-
§
S+itchG
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Implementing P2#ST
1. na"le )S$8 glo"ally. )S$8 sho!ld "e con(ig!red on all
s+itches in the "roadcast domain.
2. 'esignate and con(ig!re a s+itch to "e the root "ridge.3. 'esignate and con(ig!re a s+itch to "e the secondary "ac;!p4 root
"ridge.
. ns!re load sharing on !plin;s !sing priority and cost parameters.
. eri(y the con(ig!ration.
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2eri$ing P2#ST
§ $he o!tp!t "elo+ ill!strates ho+ to veri(y the )S$
con(ig!ration (or =A2 on a nonroot s+itch in a topology.
§ S+itchG sho+ spanning5tree vlan 2
§ =A0002
§ Spanning tree ena"led protocol rstp§ )oot I' riority 327&
§ Address 000".(c".dac0
§ Cost 3
§ ort 7 >astthernet0H74
§ %ello $ime 2 sec *a- Age 20 sec >or+ard 'elay
1 sec
§ @ridge I' riority 32770 priority 327& sys5id5e-t 24
§ Address 0013.(1c.e1c0
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*6ltipleSpanning Tree
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*ST *otivation
§ A"oveD 2 lin;s – 1000 =As – 2 *S$ instances.
§ ach s+itch maintains only t+o spanning trees, red!cing
the need (or s+itch reso!rces.§ Concept e-tenda"le to 0/& =AsD =A load "alancing.
§ *S$ converges (aster than )S$8 and is "ac;+ard
compati"le +ith 02.1' S$ and 02.1+.
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*ST Instances
§ 2 distinct S$ topologies re!ire 2 *S$ instances 00 per instance here4.
§ =oad5"alancing +or;s "eca!se hal( o( the =As (ollo+ each separate
instance.
§ S+itch !tili9ation is lo+ "eca!se it only has to handle t+o instances.
§ *S$ is the "est sol!tion (or this scenario.
§ ConsiderationsD *S$ is more comple- than 02.1' and 02.1+, so it re!ires
additional training. Interaction +ith legacy "ridges can "e challenging.
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*ST #egions
§ ach s+itch that r!ns *S$ in the net+or; has a single *S$ con(ig!ration that consists o(
three attri"!tesD
K An alphan!meric con(ig!ration name 32 "ytes4
K A con(ig!ration revision n!m"er 2 "ytes4
K A 0/&5element ta"le that associates each o( the potential 0/& =As s!pported on the chassis
to a given instance
§ $he port on @1 is at the "o!ndary o( )egion A, +hereas the ports on @2 and @3 are
internal to )egion @.
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*ST se o$ 1;ten-e- Sstem I5
§ *S$ carries the instance n!m"er in the 125"it -tended
System I' (ield o( the @ridge I'.
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*ST Con$ig6ration
§ na"le *S$ on s+itch.
Switch(config)# spanning-tree mode mst
§ nter *S$ con(ig!ration s!"mode.
Switch(config)# spanning-tree mst configuration
§
'isplay c!rrent *S$ con(ig!ration.Switch(config-mst)# show current
§ ame *S$ instance.
Switch(config-mst)# name name
§ Set the 1&5"it *S$ revision n!m"er. It is not incremented
a!tomatically +hen yo! commit a ne+ *S$ con(ig!ration.
Switch(config-mst)# revision revision_number
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*ST Con$ig6ration "cont&
§ *ap =As to *S$ instance.
S+itchcon(ig5mst4G instance instance_number vlan vlan_range
§ 'isplay ne+ *S$ con(ig!ration to "e applied.
S+itchcon(ig5mst4G show pen-ing
§
Apply con(ig!ration and e-it *S$ con(ig!ration s!"mode.S+itchcon(ig5mst4G e;it
§ Assign root "ridge (or *S$ instance. $his synta- ma;es the
s+itch root primary or secondary only active i( primary
(ails4. It sets primary priority to 27& and secondary to2&72.
S+itchcon(ig4G spanning8tree mst instance_number root primary L secondary
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*ST Con$ig6ration 1;ample
SwitchA(config)# spanning-tree mode mst
SwitchA(config)# spanning-tree mst configuration
SwitchA(config-mst)# name XYZ
SwitchA(config-mst)# revision 1
SwitchA(config-mst)# instance 1 vlan 11, 21, 31
SwitchA(config-mst)# instance 2 vlan 12, 22, 32
SwitchA(config)# spanning-tree mst 1 root primary
SwitchB(config)# spanning-tree mode mst
SwitchB(config)# spanning-tree mst configuration
SwitchB(config-mst)# name XYZ
SwitchB(config-mst)# revision 1
SwitchB(config-mst)# instance 1 vlan 11, 21, 31
SwitchB(config-mst)# instance 2 vlan 12, 22, 32
SwitchB(config)# spanning-tree mst 2 root primary
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2eri$ing *ST Con$ig6ration 1;ample "!&
§ Switch# configure terminal
§ Enter configuration commands, one per line.
End with CN!".
§ Switch(config)# spanning-tree mode mst
§
Switch(config)# spanning-tree mstconfiguration
§ Switch(config-mst)# show current
§ Current $S configuration
§
Name %&§ 'eision
§ *nstance +lans mapped
§ --------
--------------------------------------------
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2eri$ing *ST Con$ig6ration 1;ample "'&
§ Switch# show spanning-tree mst
§
###### $S lans mapped0 1-§ Bridge address .e21.32 priorit4 56732 (56732 s4sid )
§ 'oot this switch for CS and *S
§ Configured hello time 6, forward dela4 1, ma8 age 6, ma8 hops 6
§ *nterface 'ole Sts Cost /rio.N9r 4pe
§ ---------------- ---- --- --------- -------- -------
§ :a5"6 ;esg :<; 6 62.16 Shr
§ :a5"56 ;esg :<; 6 62.3 /6p
§ :a5"6 Bac= B!> 6 62.7 /6p
§ ###### $S lans mapped0 -6
§ Bridge address .e21.32 priorit4 5673 (56732 s4sid )
§ 'oot this switch for $S
§ *nterface 'ole Sts Cost /rio.N9r 4pe
§ ---------------- ---- --- --------- -------- -------
§ :a5"6 ;esg :<; 6 62.16 Shr
§ :a5"56 ;esg :<; 6 62.3 /6p
§ :a5"6 Bac= B!> 6 62.7 /6p
§ ###### $S6 lans mapped0 5-
§ Bridge address .e21.32 priorit4 5677 (56732 s4sid 6)
§ 'oot this switch for $S6
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2eri$ing *ST Con$ig6ration 1;ample "3&
§ Switch# show spanning-tree mst 1
§
###### $S lans mapped0 -6§ Bridge address .e21.32 priorit4 5673 (56732 s4sid )
§ 'oot this switch for $S
§ *nterface 'ole Sts Cost /rio.N9r 4pe
§ ---------------- ---- --- ------ -------- -----------------
§ :a5"6 ;esg :<; 6 62.16 Shr
§ :a5"56 ;esg :<; 6 62.3 /6p
§ :a5"6 Bac= B!> 6 62.7 /6p
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2eri$ing *ST Con$ig6ration 1;ample "<&
§ Switch# show spanning-tree mst interface ast!thernet 3"2#
§ :astEthernet5"6 of $S is designated forwarding
§ Edge port0 no (default) port guard 0 none (default)
§ !in= t4pe0 shared (auto) 9pdu filter0 disa9le (default)
§ Boundar4 0 internal 9pdu guard 0 disa9le (default)
§ Bpdus sent 2, receied 2
§ *nstance 'ole Sts Cost /rio.N9r +lans mapped
§ -------- ---- --- ------- -------- -------------------------
§ ;esg :<; 6 62.16 1-
§ ;esg :<; 6 62.16 -6
§
6 ;esg :<; 6 62.16 5-
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2eri$ing *ST Con$ig6ration 1;ample "=&
§ S+itchG show spanning8tree mst ! -etail
§ GGGGGG *S$01 vlans mappedD 152
§ @ridge address 000/.e.&0 priority 327&/
327& sysid 14
§
)oot this s+itch (or *S$01§ >astthernet3H2 o( *S$01 is designated (or+arding
§ ort in(o port id 12.12 priority 12 cost 2000000
§ 'esignated root address 000/.e.&0 priority 327&/
cost 0§ 'esignated "ridge address 000/.e.&0 priority 327&/
port id 12.12
§ $imersD message e-pires in 0 sec, (or+ard delay 0, (or+ard
transitions 1
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n-erstan-ing
Spanning Tree1nhancements
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Spanning Tree 1nhancements
§ BP5 g6ar-D revents accidental connection o( s+itching
devices to ort>ast5ena"led ports. Connecting s+itches to
ort>ast5ena"led ports can ca!se =ayer 2 loops or topologychanges.
§ BP5 $iltering: )estricts the s+itch (rom sending
!nnecessary @'s o!t access ports.
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BP5 96ar-
§ @' M!ard p!ts an inter(ace con(ig!red (or S$ ort>ast
in the err5disa"le state !pon receipt o( a @'. @'
g!ard disa"les inter(aces as a preventive step to avoid
potential "ridging loops.
§ @' g!ard sh!ts do+n ort>ast5con(ig!red inter(aces thatreceive @'s, rather than p!tting them into the S$
"loc;ing state the de(a!lt "ehavior4. In a valid
con(ig!ration, ort>ast5con(ig!red inter(aces sho!ld not
receive @'s. )eception o( a @' "y a ort>ast5
con(ig!red inter(ace signals an invalid con(ig!ration, s!ch
as connection o( an !na!thori9ed device.
§
@' g!ard provides a sec!re response to invalid
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BP5 96ar- Con$ig6ration
§ $o ena"le @' g!ard glo"ally, !se the commandD
spanning-tree portfast $pduguard default
§ $o ena"le @' g!ard on a port, !se the commandD
spanning-tree $pduguard ena$le
§ @' g!ard logs messages to the consoleD6 $a4 6 105056 ?S/AN'EE-6-
'@/':AS0'eceied B/; on /ort:ast ena9le port.
;isa9ling 6"
6 $a4 6 105056 ?/AD/-1-/':'$S/0/ort 6"left 9ridge port 6"
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BP5 96ar- Con$ig6ration 1;ample
§ Switch(config)# spanning-tree portfast edge $pduguard default
§ Switch(config)# end § Switch# show spanning-tree summary totals
§ 'oot 9ridge for0 none.
§ /ort:ast B/; Duard is ena9led
§ Etherchannel misconfiguration guard is ena9led
§ plin=:ast is disa9led§ Bac=9one:ast is disa9led
§ ;efault pathcost method used is short
§ Name Bloc=ing !istening !earning :orwarding S/ Actie
§ ------------ -------- --------- -------- ---------- ---------
§ 5 +!ANs 53 53
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BP5 /iltering
§ @' (iltering prevents a Cisco s+itch (rom sending
@'s on ort>ast5ena"led inter(aces, preventing
!nnecessary @'s (rom "eing transmitted to host
devices.
§ @' g!ard has no e((ect on an inter(ace i( @' (iltering
is ena"led.
§ #hen ena"led glo"ally, @' (iltering has these attri"!tesD
K It a((ects all operational ort>ast ports on s+itches that do not have
@' (iltering con(ig!red on the individ!al ports.K I( @'s are seen, the port loses its ort>ast stat!s, @' (iltering is
disa"led, and S$ sends and receives @'s on the port as it +o!ld
+ith any other S$ port on the s+itch.
K pon start!p, the port transmits ten @'s. I( this port receives any
@'s d!ring that time, ort>ast and ort>ast @' (iltering are
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BP5 /iltering Con$ig6ration
§ $o ena"le @' (iltering glo"ally, !se the commandD
spanning-tree portfast $pdufilter default
§ $o ena"le @' g!ard on a port, !se the commandD
spanning-tree $pdufilter ena$le
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2eri$ing BP5 /iltering Con$ig6ration "!&
§ ort>ast @' (iltering stat!sD
§ S+itchG sho+ spanning5tree s!mmary
§ S+itch is in pvst mode
§ )oot "ridge (orD none
§ -tended system I' is ena"led
§ ort(ast 'e(a!lt is disa"led
§ ort>ast @' M!ard 'e(a!lt is disa"led
§ ort(ast @' >ilter 'e(a!lt is disa"led
§ =oopg!ard 'e(a!lt is disa"led
§ therChannel miscon(ig g!ard is ena"led
§ plin;>ast is disa"led
§ @ac;"one>ast is disa"led§ Con(ig!red athcost method !sed is short
§ ame @loc;ing =istening =earning >or+arding S$ Active
§ 5555555 5555 55555555 5555555 55555555 555555555
§ =A0001 2 0 0 &
§ 5555555 5555 55555555 5555555 55555555 555555555
§ 1 vlan 2 0 0 &
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2eri$ing BP5 /iltering Con$ig6ration "'&
§ eri(ying ort>ast @' (iltering on a speci(ic portD
§ S+itchG show spanning8tree inter$ace $ast1thernet <+<
-etail
§ ort 1/& >astthernetH4 o( =A0010 is (or+arding
§ort path cost 1000, ort priority 1&0, ort Identi(ier
1&0.1/&.
§'esignated root has priority 327&, address
00d0.00".10a
§'esignated "ridge has priority 327&, address
00d0.00".10a
§'esignated port id is 1&0.1/&, designated path cost 0
§$imersDmessa e a e 0 (or+ard dela 0 hold 0
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#oot 96ar-
§ )oot g!ard is !se(!l in avoiding =ayer 2 loops d!ring
net+or; anomalies. $he )oot g!ard (eat!re (orces aninter(ace to "ecome a designated port to prevent
s!rro!nding s+itches (rom "ecoming root "ridges.
§ )oot g!ard5ena"led ports are (orced to "e designated ports.
I( the "ridge receives s!perior S$ @'s on a )ootg!ard5ena"led port, the port moves to a root5inconsistent
S$ state, +hich is e((ectively e!ivalent to the S$
listening state, and the s+itch does not (or+ard tra((ic o!t o(
that port. As a res!lt, this (eat!re en(orces the position o(the root "ridge.
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#oot 96ar- *otivation
§ S+itches A and @ comprise the core o( the net+or;. S+itch A is the root "ridge.
§ S+itch C is an access layer s+itch. #hen S+itch ' is connected to S+itch C, it
"egins to participate in S$. I( the priority o( S+itch ' is 0 or any val!e lo+er
than that o( the c!rrent root "ridge, S+itch ' "ecomes the root "ridge.
§ %aving S+itch ' as the root ca!ses the Miga"it thernet lin; connecting the t+o
core s+itches to "loc;, th!s ca!sing all the data to (lo+ via a 1005*"ps lin;
across the access layer. $his is o"vio!sly a terri"le o!tcome.
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#oot 96ar-Operation
§ A(ter the root g!ard (eat!re is ena"led on a port, the s+itch
does not ena"le that port to "ecome an S$ root port.
§
Cisco s+itches log the (ollo+ing message +hen a rootg!ard–ena"led port receives a s!perior @'D
?S/AN'EE-6-'DA';B!C>0 /ort " tried to 9ecome
non-designated in +!AN 77.
$oed to root-inconsistent state.
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#oot 96ar-Operation
§ $he c!rrent design recommendation is to ena"le root g!ard on all access ports so that a
root "ridge is not esta"lished thro!gh these ports.
§ In this con(ig!ration, S+itch C "loc;s the port connecting to S+itch ' +hen it receives a
s!perior @'. $he port transitions to the root5inconsistent S$ state. o tra((ic passesthro!gh the port +hile it is in root5inconsistent state.
§ #hen S+itch ' stops sending s!perior @'s, the port !n"loc;s again and goes thro!gh
reg!lar S$ transition o( listening and learning, and event!ally to the (or+arding state.
)ecovery is a!tomaticN no intervention is re!ired.
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#oot 96ar- Con$ig6ration
Switch(config)# interface ast!thernet %"&
Switch(config-if)# spanning-tree guard rootSwitch(config-if)# end
Switch# show running-config interface ast!thernet %"&
Building configuration...
Current configuration0 37 94tes
interface :astEthernet1"2
switchport mode access
spanning-tree guard root
end
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2eri$ing #oot 96ar- Con$ig6ration
Switch# show spanning-tree inconsistentports
Name *nterface *nconsistenc4-------------------- ---------------------- ------------------
+!AN :astEthernet5" /ort 4pe *nconsistent
+!AN :astEthernet5"6 /ort 4pe *nconsistent
+!AN6 :astEthernet5" /ort 4pe *nconsistent
+!AN6 :astEthernet5"6 /ort 4pe *nconsistent
Num9er of inconsistent ports (segments) in the s4stem 0
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0oop 96ar-
§ $he =oop M!ard S$ (eat!re improves the sta"ility o( =ayer 2 net+or;s "y preventing "ridging loops.
§ In S$, s+itches rely on contin!o!s reception or transmission o( @'s, depending on the port role. A
designated port transmits @'s +hereas a nondesignated port receives @'s.
§ @ridging loops occ!r +hen a port erroneo!sly transitions to (or+arding state "eca!se it has stopped
receiving @'s.
§ orts +ith loop g!ard ena"led do an additional chec; "e(ore transitioning to (or+arding state. I( a
nondesignated port stops receiving @'s, the s+itch places the port into the S$ loop-inconsistent
"loc;ing state.
§ I( a s+itch receives a @' on a port in the loop5inconsistent S$ state, the port transitions thro!gh
S$ states according to the received @'. As a res!lt, recovery is a!tomatic, and no man!al
intervention is necessary.
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0oop 96ar- *essages
§ #hen the =oop M!ard (eat!re places a port into the loop5
inconsistent "loc;ing state, the s+itch logs the (ollo+ing
messageDS/AN'EE-6-!/DA';B!C>0 No B/;s were receied on port
5"6 in lan 5.
$oed to loop-inconsistent state.
§ A(ter recovery, the s+itch logs the (ollo+ing messageD
S/AN'EE-6-!/DA';NB!C>0 port 5"6 restored in lan 5.
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0oop 96ar- Operation
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0oop 96ar- Con$ig6ration Consi-erations
§ Con(ig!re =oop M!ard on a per5port "asis,
altho!gh the (eat!re "loc;s inconsistent ports on aper5=A "asisN (or e-ample, on a tr!n; port, i(
@'s are not received (or only one partic!lar
=A, the s+itch "loc;s only that =A that is,
moves the port (or that =A to the loop5
inconsistent S$ state4. In the case o( an
therChannel inter(ace, the channel stat!s goes
into the inconsistent state (or all the ports
"elonging to the channel gro!p (or the partic!lar
=A not receiving @'s.
§ na"le =oop M!ard on all nondesignated ports.
=oop g!ard sho!ld "e ena"led on root and
alternate ports (or all possi"le com"inations o(
active topologies.
§ =oop M!ard is disa"led "y de(a!lt on Cisco
s+itches.
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0oop 96ar- Con$ig6ration
§ se the (ollo+ing inter(ace5level con(ig!ration command to
ena"le =oop M!ardDSwitch(config-if)# spanning-tree guard loop
§ I( =oop M!ard is ena"led glo"ally, the s+itch ena"les =oop
M!ard only on ports considered to "e point5to5point lin;s
(!ll5d!ple- lin;s4.§ $he glo"al con(ig!ration can "e overridden on a per5port
"asis. $o ena"le =oop M!ard glo"ally, !se the (ollo+ing
glo"al con(ig!ration commandD
Switch(config)# spanning-tree loopguard default
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2eri$ing 0oop 96ar- Con$ig6ration
§ $o veri(y =oop M!ard stat!s on an inter(ace, iss!e the
(ollo+ing DSwitch(config-if)# spanning-tree guard loop
§ I( =oop M!ard is ena"led glo"ally, the s+itch ena"les =oop
M!ard only on ports considered to "e point5to5point lin;s
(!ll5d!ple- lin;s4. $he glo"al con(ig!ration can "eoverridden on a per5port "asis. $o ena"le =oop M!ard
glo"ally, !se the (ollo+ing glo"al con(ig!ration commandD
Switch(config)# spanning-tree loopguard default
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2eri$ing 0oop 96ar- Con$ig6ration
§ $o veri(y =oop M!ard stat!s on an inter(ace, iss!e the
command show spanning-tree interfaceinterface-id detail.
§ S+itchG show spanning8tree inter$ace /ast1thernet 3+<'
-etail
§ort 170 >astthernet3H24 o( =A0001 is "loc;ing
§ort path cost 1/, ort priority 12, ort Identi(ier
12.170.
§
'esignated root has priority 1/3, address000/.e.&0
§'esignated "ridge has priority 1/3, address
000/.e.&0
§
'esignated port id is 12.1&0, designated path cost 0
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ni-irectional 0in( 5etection "505&
§ $he lin; "et+een S+itches @ and C "ecomes !nidirectional. S+itch @ can
receive tra((ic (rom S+itch C, "!t S+itch C cannot receive tra((ic (rom S+itch @.
§ ?n the segment "et+een S+itches @ and C, S+itch @ is the designated "ridge
sending the root @'s and S+itch C e-pects to receive the @'s.§ S+itch C +aits !ntil the ma-5age timer 20 seconds4 e-pires "e(ore it ta;es
action. #hen this timer e-pires, S+itch C moves thro!gh the listening and
learning states and then to the (or+arding state. At this moment, "oth S+itch @
and S+itch C are (or+arding to each other and there is no "loc;ing port in the
net+or;.
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505 *o-es
§ Normal *o-e –'=' detects !nidirectional lin;s d!e to
misconnected inter(aces on (i"er5optic connections. '='changes the '='5ena"led port to an !ndetermined state i(
it stops receiving '=' messages (rom its directly
connected neigh"or.
§ %ggressive *o-e – re(erred4 #hen a port stopsreceiving '=' pac;ets, '=' tries to reesta"lish the
connection +ith the neigh"or. A(ter eight (ailed retries, the
port state changes to the err5disa"le state. Aggressive
mode '=' detects !nidirectional lin;s d!e to one5+aytra((ic on (i"er5optic and t+isted5pair lin;s and d!e to
misconnected inter(aces on (i"er5optic lin;s.
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505 Con$ig6ration
§ '=' is disa"led on all inter(aces "y de(a!lt.
§ $he udld glo"al con(ig!ration command a((ects (i"er5optic inter(aces only.K udld ena$le ena"les '=' normal mode on all (i"er inter(aces.
K udld aggressive ena"les '=' aggressive mode on all (i"er inter(aces.
§ $he udld port inter(ace con(ig!ration command can "e !sed (or t+isted5pair
and (i"er inter(aces.
K $o ena"le '=' in normal mode, !se the udld port command. $o ena"le '=' inaggressive mode, !se the udld port aggressive.
K se the no udld port command on (i"er5optic ports to ret!rn :control< o( '=' to
the udld ena$le glo"al con(ig!ration command or to disa"le '=' on non(i"er5optic
ports.
K se the udld port aggressive command on (i"er5optic ports to override the
setting o( the udld ena$le or udld aggressive glo"al con(ig!ration command.se the no (orm on (i"er5optic ports to remove this setting and to ret!rn control o(
'=' ena"ling to the udld glo"al con(ig!ration command or to disa"le '=' on
non(i"er5optic ports.
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505 Con$ig6ration an- 2eri$ication
Switch(config)# interface giga$it!thernet %"1
Switch(config-if)# udld port aggressive
Switch# show udld giga$it!thernet %"1
*nterface Di1"
---/ort ena9le administratie configuration
setting0 Ena9led " in aggressie mode
/ort ena9le operational state0 Ena9led " in
aggressie modeCurrent 9idirectional state0 Bidirectional
Current operational state0 Adertisement -
Single neigh9or detected
$essage interal0 1
9
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Clic; icon to add ta"le
0oop 96ar- vers6s %ggressive *o-e505
=oop M!ard Aggressive *ode '='
Con(ig!ration er port er port
Action gran!larity er =A er port
A!to5recovery Bes Bes, +ith err5disa"le timeo!t(eat!re
rotection against S$(ail!res ca!sed "y!nidirectional lin;s
Bes, +hen ena"led onall root ports andalternate ports inred!ndant topology
Bes, +hen ena"led on alllin;s in red!ndant topology
rotection against S$(ail!res ca!sed "y
pro"lem in so(t+are indesignated "ridge notsending @'s
Bes o
rotection againstmis+iring
o Bes
/l 0i (
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/le; 0in(s
§ >le- =in;s is a =ayer 2 availa"ility (eat!re
that provides an alternative sol!tion to S$and allo+s !sers to t!rn o(( S$ and still
provide "asic lin; red!ndancy.
§ >le- =in;s can coe-ist +ith spanning tree on
the distri"!tion layer s+itchesN ho+ever, the
distri"!tion layer s+itches are !na+are o(
the >le- =in;s (eat!re.
§ >le- =in;s ena"les a convergence time o(
less than 0 milliseconds. In addition, this
convergence time remains consistent
regardless o( the n!m"er o( =As or *AC
addresses con(ig!red on s+itch !plin; ports.
§ >le- =in;s is "ased on de(ining an
activeHstand"y lin; pair on a common accesss+itch. >le- =in;s are a pair o( =ayer 2
inter(aces, either s+itchports or port
channels, that are con(ig!red to act as
"ac;!p to other =ayer 2 inter(aces.
/l 0i ( C $i ti C i- ti
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/le; 0in(s Con$ig6ration Consi-erations
§ A >le- =in; is con(ig!red on one =ayer 2 inter(ace the
active lin;4 "y assigning another =ayer 2 inter(ace as the
>le- =in; or "ac;!p lin;. #hen one o( the lin;s is !p and
(or+arding tra((ic, the other lin; is in stand"y mode, ready to
"egin (or+arding tra((ic i( the other lin; sh!ts do+n. At anygiven time, only one o( the inter(aces is in the lin; !p state
and (or+arding tra((ic. I( the primary lin; sh!ts do+n, the
stand"y lin; starts (or+arding tra((ic. #hen the active lin;
comes "ac; !p, it goes into stand"y mode and does not(or+ard tra((ic.
§ >le- =in;s are s!pported only on =ayer 2 ports and port
channels, not on =As or on =ayer 3 ports.
/l 0i ( C $i ti - 2 i$i ti
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/le; 0in(s Con$ig6ration an- 2eri$ication
§ >le-=in;s are con(ig!red at the inter(ace level +ith the
command switchport $ackup interface.§ %ere +e con(ig!re an inter(ace +ith a "ac;!p inter(ace and
veri(y the con(ig!ration.
§
S+itchcon(ig4G inter$ace $astethernet!+)+!§ S+itchcon(ig5i(4G switchport bac(6p inter$ace
$astethernet!+)+'
§ S+itchcon(ig5i(4G en-
§ S+itchG show inter$ace switchport bac(6p§ S+itch @ac;!p Inter(ace airsD
§ Active Inter(ace @ac;!p Inter(ace State
§ 55555555555555555 555555555555555555 555555555555555555555
§ >astthernet1H0H1 >astthernet1H0H2 Active H@ac;!
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STP Best
Practices an-Tro6bleshooting
S it hi 5 i B t P ti
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Switching 5esign Best Practices
§ se =ayer 3 connectivity
at the distri"!tion and core
layers.
§ se )S$8 or *S$. 'o
not disa"le S$ at theaccess layer. Isolate
di((erent S$ domains in a
m!ltivendor environment.
§ se =oop M!ard on =ayer
2 ports "et+een
distri"!tion s+itches and
on !plin; ports (rom
P t ti l STP P bl
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Potential STP Problems
§ '!ple- mismatch
§ nidirectional lin; (ail!re
§ >rame corr!ption
§ )eso!rce errors
§
ort>ast con(ig!ration error
5 l *i t h
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56ple; *ismatch
§ oint5to5point lin;.
§ ?ne side o( the lin; is man!ally con(ig!red as (!ll d!ple-.
§ ?ther side is !sing the de(a!lt con(ig!ration (or a!to5negotiation.
i-i ti l 0i ( / il
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ni-irectional 0in( /ail6re
§ >re!ent ca!se o( "ridge loops.
§ ndetected (ail!re on a (i"er lin; or a pro"lem +ith a
transceiver.
/ C ti
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/rame Corr6ption
§ I( an inter(ace is e-periencing a high rate o( physical errors,
the res!lt may "e lost @'s, +hich may lead to aninter(ace in the "loc;ing state moving to the (or+arding
state.
§ ncommon scenario d!e to conservative de(a!lt S$
parameters.§ >rame corr!ption is generally a res!lt o( a d!ple- mismatch,
"ad ca"le, or incorrect ca"le length.
#eso6rce 1rrors
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#eso6rce 1rrors
§ S$ is per(ormed "y the C so(t+are5"ased4. $his
means that i( the C o( the "ridge is over5!tili9ed (or anyreason, it might lac; the reso!rces to send o!t @'s.
§ S$ is generally not a processor5intensive application and
has priority over other processesN there(ore, a reso!rce
pro"lem is !nli;ely to arise.§ -ercise ca!tion +hen m!ltiple =As in S$8 or
)S$8 mode e-ist. Cons!lt the prod!ct doc!mentation (or
the recommended n!m"er o( =As and S$ instances on
any speci(ic s+itch to avoid e-ha!sting reso!rces.
Port/ast Con$ig6ration 1rror
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Port/ast Con$ig6ration 1rror
§ S+itch A has ort p1 in the (or+arding state and ort p2 con(ig!red (or ort>ast.
'evice @ is a h!". ort p2 goes to (or+arding and creates a loop "et+een p1
and p2 as soon as the second ca"le pl!gs in to S+itch A. $he loop ceases as
soon as p1 or p2 receives a @' that transitions one o( these t+o ports into"loc;ing mode.
§ $he pro"lem +ith this type o( transient loop condition is that i( the looping tra((ic
is intensive, the "ridge might have tro!"le s!ccess(!lly sending the @' that
stops the loop. @' g!ard prevents this type o( event (rom occ!rring.
Tro6bleshooting *etho-olog
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Tro6bleshooting *etho-olog
§ $ro!"leshooting S$ iss!es can "e di((ic!lt i( logical
tro!"leshooting proced!res are not deployed in advance.?ccasionally, re"ooting o( the s+itches might resolve the
pro"lem temporarily, "!t +itho!t determining the !nderlying
ca!se o( the pro"lem, the pro"lem is li;ely to ret!rn. $he
(ollo+ing steps provide a general overvie+ o( amethodology (or tro!"leshooting S$D
§ Step 1. 'evelop a plan.
§ Step 2. Isolate the ca!se and correct an S$ pro"lem.
§ Step 3. 'oc!ment (indings.
Chapter 3 S6mmar "!&
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Chapter 3 S6mmar "!&
§ Spanning $ree rotocol is a (!ndamental protocol to
prevent =ayer 2 loops and at the same time providered!ndancy in the net+or;. $his chapter covered the "asic
operation and con(ig!ration o( )S$ and *S$.
nhancements no+ ena"le S$ to converge more !ic;ly
and r!n more e((iciently.K )S$ provides (aster convergence than 02.1' +hen topology
changes occ!r.
K )S$ ena"les several additional port roles to increase the overall
mechanismFs e((iciency.
K show spanning-tree is the main (amily o( commands !sed toveri(y )S$ operations.
K *S$ red!ces the enc!m"rance o( )S$8 "y allo+ing a single
instance o( spanning tree to r!n (or m!ltiple =As.
Chapter 3 S6mmar "'&
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Chapter 3 S6mmar "'&
§ $he Cisco S$ enhancements provide ro"!stness and
resiliency to the protocol. $hese enhancements add
availa"ility to the m!ltilayer s+itched net+or;. $hese
enhancements not only isolate "ridging loops "!t also
prevent "ridging loops (rom occ!rring. $o protect S$
operations, several (eat!res are availa"le that control the
+ay @'s are sent and receivedD
K @' g!ard protects the operation o( S$ on ort>ast5con(ig!red
ports.K @' (iltering prevents @'s (rom "eing sent and ignores received
@'s +hile leaving the port in (or+arding state.
K )oot g!ard prevents root s+itch "eing elected via @'s received on
a root5g!ard con(ig!red port.
Chapter 3 0abs
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§ 0ab 38!Spanning Tree Protocol "STP& 5e$a6lt Behavior
§ 0ab 38'*o-i$ing 5e$a6lt Spanning Tree Behavior§ 0ab 383Per820%N Spanning Tree Behavior
§ 0ab 38<*6ltiple Spanning Tree
Chapter 3 0abs
#eso6rces
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#eso6rces
§ Cisco Spanning $ree rotocol Con(ig!ration M!ideD
+++.cisco.comHenHSHdocsHs+itchesHlanHcatalyst3&0Hso(t+areHreleaseH12.2O2OseHcommandHre(erenceH3&0cr.html
§ Con(ig!ring *S$ Con(ig!ration M!ideD
+++.cisco.comHenHSHdocsHs+itchesHlanHcatalyst3&0Hso(t+areHreleaseH12.2O2OseHcon(ig!rationHg!ideHs+stp.html
§ Cisco ?ptional Spanning5$ree >eat!res Con(ig!ration
M!ideD
+++.cisco.comHenHSHdocsHs+itchesHlanHcatalyst3&0Hso(t+areHreleaseH12.2O2OseHcon(ig!rationHg!ideHs+mstp.html
+++.cisco.comHenHSHdocsHs+itchesHlanHcatalyst3&0Hso(t+areHreleaseH12
.2O2OseHcon(ig!rationHg!ideHs+stpopt.html
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