lecture 4.0 spanning tree protocols - uniroma2.it · 802.1d spanning tree protocol standard 802.1d...

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Giuseppe Bianchi

Lecture 4.0Lecture 4.0

Spanning Tree ProtocolsSpanning Tree Protocols

Giuseppe Bianchi

MutipleMutiple pathspaths ((loopsloops) ) �� broadcastbroadcast stormstorm! !

LAN1 LAN2

LAN3

LAN4 LAN5

1

1DUPLICATION!!

2

2

2

2

2

3

3

3

3

3

DUPLICATION!!

DUPLICATION!!

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…… butbut multiple multiple pathspaths essentialessential

forfor reliabilityreliability!!

Giuseppe Bianchi

Canalina metallica forata

Prese RJ45

Cablaggio orizzontale in rame

Armadio di

pianoPrese RJ45

Stanza Stanza Stanza

StanzaStanzaStanza

Armadio di

piano

Tubo in PVC – Cablaggio verticale in Fibra Ottica

Canalina metallica - Cablaggio verticale di backup in rame

Canalina in PVC �

TypicalTypical buildingbuilding

(collapsed backbone – star center – redundant)

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MeshMesh network MUST network MUST transformtransform

intointo treetree

LAN1 LAN2

LAN3

LAN4 LAN5

Many possible solutions

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LogicalLogical viewview

1 2

54

3

1 2

54

3

LAN = circle

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Giuseppe Bianchi

802.1D 802.1D SpanningSpanning TreeTree ProtocolProtocol

Standard 802.1D - 1990, 1993, 1998

(STP no more included in 802.1D-2004)

Giuseppe Bianchi

SpanningSpanning treetree keykey conceptsconcepts

�Tree topology

�Root bridge

�Root port

�Designated bridge/port

�Topology creation

�BPDUs and distributed operation

�Reconfiguration

�Port states & timers

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RootRoot bridgebridge

B1 B2

B3 B4

B5 B6 B7

C

B

A

D

FE

J

G

H I

�Root bridge choice -practical rules:

�Bridge in the “center” of the network�In average, shorter

network paths

�High reliability and high performance bridge�With higher rate ports

(e.g. gigabit)

�Must NOT becomeperformance bottleneck

Example: B4 is better choice than B1…

Giuseppe Bianchi

RootRoot bridge: bridge: electionelection rulerule

� Default rule:

� Root bridge = lowest Bridge Identifier�Bridge Identifier = Bridge MAC address

� Configuration:

� Priority field prior to MAC address�By changing priority field, sysadmin may freely choose the root

among his switches

nn:nn:nn:nn:nn:nn0xN000

Bridge Priority(2 B)

Bridge MAC address(6 B)

default0x800032768

Given by vendorNot modificable

(but vendors are smart ☺ - e.g. Cisco)

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RootRoot portport

B1 B2

B3 B4

B5 B6 B7

C

B

A

D

FE

J

G

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�Port used to sendframes to the root

�Port from which frames are received from root

�Exactly one root port

�Except root bridge

Giuseppe Bianchi

RootRoot portport: : electionelection rulerule /1/1� Root port

� port on the least cost path to the root

� Port cost

� sum of cost of links to reach the root

� Link cost

� function of link speed

LINK COST:

� 1000/link_speed_Mbps� 1993 802.1D specification

� Specific Table� 1998 802.1D specification

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Giuseppe Bianchi

RootRoot portport: : electionelection rulerule /2/2

� What if equal cost ports?

� Tie-break rule:

1) Root port = towards lowest bridge identifier (includingpriority)

2) If still tie (2+ links to same bridge): root port = towardslowest (remote) port number (including priority)

� By playing with bridge priorities and port priorities, a sysadmin may design his own tree

� Among the minimum cost trees conditioned to the specificchoice for the root bridge

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WhyWhy designateddesignated portport??

B1 B2

B3 B4

B5 B6 B7

C

B

A

D

FE

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�Root bridge and root portsgenerate a treeof bridges

�But loops are still there (e.g. LAN D)

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DesignatedDesignated bridge/bridge/portport

B1 B2

B3 B4

B5 B6 B7

C

B

A

D

FE

J

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�ONE bridge/ portper LAN

�Port from which frames are received

�All the otherbridge ports on the LAN are blocked

�except root ports

ROOT and DESIGNATED ports �� FORWARDING state

Other ports �� BLOCKING state

Giuseppe Bianchi

DesignatedDesignated bridge/bridge/portport: :

electionelection rulerule

�Designated = port for which the

path root cost is lower

�If tie, lowest bridge identifier

�If tie again, lowest port identifier

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Giuseppe Bianchi

ResultResult: : spanningspanning treetree

B1 B2

B3 B4

B5 B6 B7

C

B

A

D

FE

J

G

H I

B1

B2B4

CB A

DH I

B6 B7

G

B3 B5

F J

E

Giuseppe Bianchi

Domanda esame 13/4/2004Domanda esame 13/4/2004

12-12-12-00-00-20

12-12-12-00-00-10

12-12-12-00-00-50

12-12-12-00-00-70

12-12-12-00-00-30

12-12-12-00-00-F0

12-12-12-00-00-80

12-12-12-00-00-40

12-12-12-00-00-60

12-12-12-00-00-90

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Giuseppe Bianchi

Soluzione esame 13/4/2004Soluzione esame 13/4/2004

12-12-12-00-00-20

12-12-12-00-00-10

12-12-12-00-00-50

12-12-12-00-00-70

12-12-12-00-00-30

12-12-12-00-00-F0

12-12-12-00-00-80

12-12-12-00-00-40

12-12-12-00-00-60

12-12-12-00-00-90

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�Tree topology

�Root bridge

�Root port




�Reconfiguration


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BPDUBPDU� Bridge Protocol Data Unit (BPDU)

� Exchanged among bridges

� Multicast address 01-80-c2-00-00-00

� 802.3 encapsulation with SAP=0x42 (Spanning Tree Protocol – STP)

� Two BPDUs�Configuration BPDU

�Topology Change Notification BPDU

� Configuration BPDU - contains

� Root identifier (according to the bridge belief)

� Cumulative path cost from the root to the tx port

� Bridge identifier

� Transmit Port identifier

� … etc …

� Topology Change Notification BPDU: empty

Giuseppe Bianchi

ConfigurationConfiguration BPDU BPDU structurestructure

01-80-c2-00-00-00

(multicast address)Unicast bridge addr len 0x42

Configuration BPDU

Or topology change notifFCS0x42 0x03

0x000x0000

nn:nn:nn:nn:nn:nn

Protocol Identifier

Root Path Cost

Protocol Version Identifier

0xN0 BPDU Type (0x00 = Configuration BPDU; 0x80 = Topology Change Notification BPDU)

flags TC – 0 – 0 – 0 – 0 – 0 – 0 – TCA (TCA= Topology Change ACK; TC = Topology Change)

0xN000xxxxxxxx (4 B)

nn:nn:nn:nn:nn:nn0xN000

Root priority + Root Identifier

Bridge priority + Bridge Identifier

0xP0 pp Port priority + Port Identifier

xxxx (2B)xxxx (2B)xxxx (2B)xxxx (2B)

Message Age

Max Age

Hello Time

Forward DelayTOTAL: 35 bytes payload

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Giuseppe Bianchi

IEEE 802.3 EthernetDestination: 01:80:c2:00:00:00 (Spanning-tree-(for-bridges)_00)Source: 00:09:7c:4d:37:c1 (Cisco_4d:37:c1)Length: 38Trailer: 0000000000000000

Logical-Link ControlDSAP: Spanning Tree BPDU (0x42)IG Bit: IndividualSSAP: Spanning Tree BPDU (0x42)CR Bit: CommandControl field: U, func = UI (0x03)

000. 00.. = Unnumbered Information.... ..11 = Unnumbered frame

Spanning Tree ProtocolProtocol Identifier: Spanning Tree Protocol (0x0000)Protocol Version Identifier: Spanning Tree (0)BPDU Type: Configuration (0x00)BPDU flags: 0x00

0... .... = Topology Change Acknowledgment: No.... ...0 = Topology Change: No

Root Identifier: 0 / 00:09:12:e8:02:61Root Path Cost: 8Bridge Identifier: 32768 / 00:09:7c:4d:37:c2Port identifier: 0x800dMessage Age: 2Max Age: 20Hello Time: 2Forward Delay: 15

0000 01 80 c2 00 00 00 00 09 7c 4d 37 c1 00 26 42 42 ........|M7..&BB0010 03 00 00 00 00 00 00 00 00 09 12 e8 02 61 00 00 .............a..0020 00 08 80 00 00 09 7c 4d 37 c2 80 0d 02 00 14 00 ......|M7.......0030 02 00 0f 00 00 00 00 00 00 00 00 00 ............

Giuseppe Bianchi

SpanningSpanning treetree creationcreation

elementaryelementary case case (and STP (and STP failurefailure -- courtesycourtesy of of ciscocisco))

12_spt_cisco.swf

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Giuseppe Bianchi

DetailedDetailed bridge bridge operationoperation /1/1

Bridge # 3333

1

2

3

4

BPDU root: 3333

BPDU root: 3333

BPDU root: 3333

BPDU root: 3333

Upon bootstrap, start sending BDPU claiming to be the root

1 BPDU each “hello time” (default = 2s); At least one “hold time” (fixed to 1s) between subsequent BPDUs

BPDU root path cost = 0 (bridge acts as root bridge, initially)

Giuseppe Bianchi


BPDU root: 3333

Bridge # 3333

1

2

3

4

BPDU root: 3333

BPDU root: 3333

BPDU root: 3333

Incoming BPDU root ID > actual root ID

1. ignore on remaining ports

2. react immediately on reception port

BPDU root: 5555

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Giuseppe Bianchi


Bridge # 3333

1

2

3

4

Incoming BPDU root ID < actual root ID:

1. elect reception port = Root port

2. no not send BPDU on reception port

3. forward BPDU on remaining ports (outgoing BPDU cost = received BPDU cost + output link cost)

BPDU root: 2222, cost=20



BPDU root: 2222, cost=30Root port

Giuseppe Bianchi


Incoming BPDU with same root ID but lower cost:

1. elect new Root port

2. Forward BPDU (increase cost) on other ports

3. Elect designated ports (obvious for #1 and #3 – since they do not receive BPDUs

(#2 designated as same BPDU cost but Bridge ID 3333 < received 4444)

Bridge # 3333

1

2

3

4

BPDU root: 2222, cost=20, BID=4444BPDU root: 2222, cost=10

Rootport




Designated

Designated

Designated

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Giuseppe Bianchi

DetailedDetailed bridge bridge operationoperation /4bis/4bis

Bridge # 3333

1

2

3

4

BPDU root=2222, cost=20

BPDU root=2222,cost=10BPDU root=2222, cost=20

BPDU root=2222, cost=20

This is how we end up:

BPDUs received through port 4 are forwarded to all the other ports

Giuseppe Bianchi


Bridge # 3333

1

2

3

4

Incoming BPDU from port 1 with same root: tie on BPDU root cost:

1. Port 1 becomes root port (lower BID)

2. Port 4 goes into BLOCKED STATE (incoming cost=10; outgoing cost=20)

3. Ports 3 and 4 remain designated

BPDU, BID=3333, cost=20

Root port

BPDU, BID=6666,cost=10BPDU, BID=3333, cost=20

BPDU, BID=5555, cost=10

X

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Giuseppe Bianchi


((conclusionconclusion))

Bridge # 3333

1

2

3

4

Root port = 1

Designated ports = 2, 3

(2 has been designated by comparison with incoming BPDU: will not receive BPDUs (peer port blocked)

(3 has never received BPDU �� designated by default)

Blocked = 4 (a blocked port does NOT send BPDUs but will continue receiving BPDUs!!)



Root port



X

Giuseppe Bianchi


�Tree topology

�Root bridge

�Root port




�Reconfiguration


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PortPort statesstates

� Two possibilities in stabilized topology operation:

� Blocked port

� Forwarding port�Root port or designated port

� Upon riconfiguration, two additional transient states:

� Listening

� Learning

� Finally, a port can be disabled

Giuseppe Bianchi

WhyWhy slow slow BB��FF transitiontransition neededneeded??

�Too fast topologyvariations mightcreate

�duplicated packets

�misordered delivery

�(both forbidden byIEEE specs)!

A B

X

B not in

forwarding DB

BLOCK

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Giuseppe Bianchi

WhyWhy slow slow BB��FF transitiontransition neededneeded??

�Too fast topologyvariations mightcreate

�duplicated packets

�misordered delivery

�(both forbidden byIEEE specs)!

A BFast reconfig

�� duplicate!

FWD

Giuseppe Bianchi

PortPort statesstates

� Blocking

� Port discards incoming frames and do not transmit frames

� Receives BPDUs but does not forward them

� Transition to listening state when no BPDU arrive by max-age expiration

� Listening

� Port discards incoming frames and do not transmit frames

� Receives BPDUs and forwards them

� Transition to learning state after a forward delay time

� Learning

� Discard frames, but learns and prepares forwarding DB

� Transition to forwarding state after a forward delay time

� Forwarding

� Port (root or designated) activated

Blocking �� forwarding = max-age + 2 x Forward_delay

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PortPort state state transitionstransitions

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ExampleExample: : addingadding a a linklink /1/1

1 2

3

45

76

A new link between

B1 and B5 is activated.

Corresponding ports

go into listening state

(= get involved in BPDU

exchange)

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1 2

3

45

76

Listening

stateB5 in listening state

Receives BDPU.

Sets new root port #1

(previous root port = #2)#1

#2

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1 2

3

45

76

Listening

stateB5 forwards

the BPDU on its

designated ports

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1 2

3

45

76

Listening

stateB3 and B4 receive mutual

BPDUs

Tie situation: B3 designated

B4 BLOCKS port!

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1 2

3

45

76

Listening

stateNew topology found!

Unfortunately network

will remain cut until

port of B1 on link 1-5

will enter forwarding

state (2 x forward delay)

Not a problem if timers were short, but…

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Giuseppe Bianchi

TimersTimers

In seconds

Coded in the BPDU in 1/256 of a second

Convenient hex notation: 0x0400 = 4 seconds

Why so long?

Fwd delay must account for

1) Propagation of BPDUs along the whole

network diameter (each bridge introduces

as much as 1s delay)

2) Lifetime in the network of a frame forwarded

by previous active topology

Approx 2 x maximum network crossing-time

Giuseppe Bianchi

TopologyTopology changechange: : whywhy??

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Giuseppe Bianchi

TopologyTopology changechange: : whywhy??

Giuseppe Bianchi

TopologyTopology ChangeChange NotificationNotification

BPDUBPDU

� To notify upstreams a detected change of topology

� Port switched to forwarding state

� Port switched to blocking state (from listening of learning states)

� Forwarded upstreams by intermediate bridges, up to the Root

� No parameters carried by the TCN BPDU

� Just 4 bytes

� Acknowledged by designated bridge via TCA flag

� And so on until root

� Root transmits TC in subsequent BPDU

0x000x0000 Protocol Identifier

Protocol Version Identifier

0x80 0x80 = Topology Change Notification BPDU)

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TCA managementTCA management

Giuseppe Bianchi

TopologyTopology changechange flagflag

root

bridge

TCN TCA TC

Topology Change Time = max-age + 2 forward-delay

Extremely important!!!!

Tells the switches to USE forward delay as age of the forwarding database entries

instead of ageing time

Allows fast clean of obsolete entries

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Giuseppe Bianchi

TopologyTopology changechange operationoperation

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RapidRapid SpanningSpanning TreeTree ProtocolProtocol

RSTPRSTP

Standard 802.1w–2001

Now included in standard 802.1D-2004, clause 17

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Giuseppe Bianchi

RSTPRSTP

� Design goal

� Drop reconfiguration time from o(50 seconds) to o(few ms)

� Major enhancements

� Rely on hardware techniques to detect link/port failure

� Brand new BPDU handling during reconfiguration

� New Port roles, port states and state transitions�Faster transition (up to order of ms)

�Entries explicitly removed from forwarding DB

� STP compliant

� If STP BPDU detected, rolls back to STP operation

� Conformance to 802 standard

� From 2004, RSTP has become THE spanning tree standard�A 2004+ switch MUST support RSTP

� STP will be abandoned in time

Giuseppe Bianchi

PortPort statesstates & & rolesroles((unclearunclear separationseparation betweenbetween state and state and rolerole in 802.1D)in 802.1D)

YesYesForwardingForwarding

YesYesLearningLearning

NoYesDiscardingListening

NoNoDiscardingBlocking

NoNoDiscardingDisabled

Is Port Learning

MAC Addresses?

Is Port Included in

Active Topology?

RSTP

(802.1w) Port State

STP

(802.1D) Port State

Backup PortReplace BLOCKED portAlternate Port

Same as STPDesignated Port

Same as STPRoot Port

commentRSTP (802.1w)

Port Role

Port notation:

Role State

(e.g.

-root forwarding

-Designated discarding

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Giuseppe Bianchi

New New PortPort rolesroles

Alternate path available in the network

Redundant connectivity to the same LAN

Giuseppe Bianchi

EdgeEdge portsports

Edge ports – unlike 802.1D, their failure does not trigger RSTP!

Edge port is external to STP operation (cannot create loops)

Hence can be immediately set to forwarding state

Concept NOT available in 802.1D (but vendors had included it in their products – e.g. Cisco portfast

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Giuseppe Bianchi

New BPDU handlingNew BPDU handling

�STP:

�Generated by root

�Forwarded by bridge

�RSTP

�Generated by bridge» Not necessarily in response to a

BPDU received on root port

�Contain specific information on tx port» 6 flags added

�Usage:

�Lack of reception of 3 consecutive BPDU = assume connectivity lost

» BPDUs = keep-alive messages among adjacent bridges!

Giuseppe Bianchi


1 2

3

45

76

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1 2

3

45

76

No more listening state

Ports placed in designated discarding state

BPDU exchanged with

“proposal bit” set

B5 realizes new root path:SYNC operation!1. Blocks all his ports!

2. Sends to B1 BPDU with agreement flag

3. B1 will transition to forwarding!

IDEA: close all possible paths (loops)

before telling B1 to activate port

Designated

discarding

state

Giuseppe Bianchi


1 2

3

45

76

SYNC procedure

propagates (fast convergence)

…

B4 port on 3-4 link will

be blocked, as before

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Giuseppe Bianchi

TopologyTopology ChangesChanges / 1/ 1

1 2

3 4

5 6

7

Initial state

And failure

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1 2

3 4

5 6

7

B5 and B7 flush

entries associated to

failed ports

B7 selects alternate port

B7 sends TCN

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1 2

3 4

5 6

7

B6 flush entries

(but keeps entries

for downstream port)

B6 sends TCN on

port towards B3

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1 2

3 4

5 6

7

B3 flush entries

(but keeps entries


B3 sends TCN on

both ports!!

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Giuseppe Bianchi


1 2

3 4

5 6

7

B1 (root) flush entries

(but keeps entries


B1 sends TCN

downstream!!

Giuseppe Bianchi

TopologyTopology changechange –– conclusionconclusion(one single iteration necessary)(one single iteration necessary)

lecture 4.0 spanning tree protocols - uniroma2.it · 802.1d spanning tree protocol standard 802.1d...

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