Spanning Tree Operations. Topology Changes & Topology Change BPDUs

Transcription

1 Spanning Tree Operations Topology Changes & Topology Change BPDUs 1

2 Rationale: There are a lot of misconceptions concerning Topology Changes and Topology Change BPDUs, these misconceptions are predicated upon the knowledge we have failed to gather from the IEEE 802.1D Bridging Standard. The reason for this document, which will also be included in an update for Everything You Ever Wanted to Know about Spanning Tree, but were Afraid to Ask., is to clarify these misconceptions and to define this area of operation in the Spanning Tree Algorithm. Review: The 802.1D standard provides us with the capability to provide a certain level of redundancy within our Local Area Networks by allowing us to provide multiple paths between the segments that make up our Bridged Local Area Network. It provides us with the capability to identify multiple paths ( data loops ) and will ensure that there is always only one active link between segments. During the initialization process all Bridge ports will block user traffic and will only process BPDUs during the LISTENING, LEARNING states. Let me provide a basic description of these states derived from the 802.1D Standard from IEEE. The Listening State A Port in this state is preparing to participate in frame relay ( this is the term that the standard uses to reference user traffic ), Frame Relay is temporarily disabled in order to prevent data loops, which may occur in a Bridged Local Area Network during the lifetime of this state as the active topology of the Bridged Local Area Network changes. Learning is disabled since changes in active topology may lead to the information acquired being incorrect when the active topology becomes stable. The Learning State A Port in this State is preparing to participate in frame relay. Frame relay is disabled in order to prevent temporary loops, which may occur in a Bridged Local Area Network during the lifetime of this State as the active topology of the Bridged Local Area Network changes. Learning is enabled to allow information (station location data) to be acquired prior to frame relay in order to reduce the number of frames unnecessarily relayed These states are entered during the Spanning Tree initialization process, and the ports will proceed to the FORWARDING state or the BLOCKING state dependent upon the results of the Spanning Tree Algorithm. At this point in time we can consider the active topology stable, and the network bridges will respond to different changes in the topology in different manners. This is the area that we will be trying to clarify with this document, it is the algorithms constant monitoring of the active topology to ensure changes are propagated to the Bridge for handling. Introduction: The Spanning Tree Algorithm is continually monitoring the active topology and if any bridge detects a change in the topology of the network it will ensure that the is informed. The value of a Bridged network is the ability of the bridge to forward/filter traffic based on addresses that it maintains in it s Filtering Data Base. In normal stable operations, user station location found in the Data base need only change as a consequence of the physical relocation of stations. It is therefore, desirable to employ a long aging time for entries in the Filtering Data Base ( 300 seconds ), especially as many end stations transmit frames following power up after relocation which would cause station location information to be relearned. However, when the active topology of a Bridge Local Area Network reconfigures, end stations may appear to move from the point of view of a Bridge in the Network. This is true even if the states of the Ports on 2

3 that Bridge have not changed. It is necessary for station location to be relearned following a change in the active topology, even if only a part of the Bridged Local Area Network has reconfigured. What is a Topology Change A Topology Change indicates the fact that a difference has been detected in the active topology of the network, it can be detected anywhere within the Bridged Local Area Network. This detection has to be forwarded to the. The will then follow a procedure whereby every bridge within the network will be notified of this occurrence. A Topology Change occurs when one of the following events occurs: 1) The receipt of a TOPOLOGY CHANGE BPDU on a Port that is the Designated Port for the LAN to which it is attached 2) When a Bridge Port enters the FORWARDING State following the expiration of the Forwarding Timer for the Port, providing that this Bridge is the Designated Bridge for at least one of the LANs that it is connected to. 3) When a Bridge Port in either the FORWARDING State or the LEARNING State is moved to the BLOCKING State. 4) When the Bridge becomes. These occurrences will also cause the Topology Change count to increment. The Mib instance that reflects this is dot1dstptopologychanges. Elements of a Topology Change Here we would like to highlight the components of the Spanning Tree Algorithm that are involved in the handling of a Topology Change Configuration BPDU Flags The Flags Field in a Configuration BPDU contains two bits which are used during topology change detection and propagation. They are illustrated in the following diagram. Their use will be appropriately defined in the following descriptions Flags X X 7 Topology Change Acknowledgement Topology Change 35 3

4 Topology Change Flag This is the lsb ( bit 1) in the Flags field of a Configuration BPDU, it can only originate from the Bridge, and will be propagated through the Bridged Local Area Network by its retransmission by the Designated Bridges of all attached LANs. The Bridge will use this flag to force all Bridges to lower their SAT Aging Timer from a default value of 300 seconds to the value of the FORWARD Delay, which is a default value of 15 seconds. This will cause the associated Bridges to flush all SAT entries that are older than 15 seconds and to relearn station locations. Topology Change Acknowledgment Flag This is the msb ( bit 8) in the Flags field of a Configuration BPDU, and is implemented by a non Bridge to signal the reception of a TCN BPDU, which will cause that Bridge to stop sending TCN BPDUs. Topology Change BPDU The Topology Change BPDU is implemented to notify the Bridge on the path to the that an extension of the topology has been detected by the transmitting Bridge. A sample BPDU is illustrated below Protocol ID Prot Version # BPDU Type The Protocol ID Field and Protocol Version # field are both all zeroes. The BPDU Type field is set to This is the only data contained within this BPDU. It s only function is to notify the Bridge about the Topology Change. It is addressed to the Bridge Group c and is propagated towards the by each Bridge in it s path. The remainder of the packet contains zero padding. Times, Timers, Time-outs, and Expirations There are a few other components involved in the handling of a Topology Change, and they are described initially below: MAX Age This is the maximum age of received protocol information before it is discarded. By default it is set to a value of 20 seconds. HELLO Time The time interval ( default 2 seconds) between the transmission of Configuration BPDUs by a Bridge that: 1) is attempting to become 2) is already FORWARD Delay The time ( default 15 seconds) spent in moving from one STA state to another. 1) LISTENING BLOCKING 2) LISTENING LEARNING 3) LEARNING FORWARDING This is also the value that will be used to age out dynamic entries while the Topology Change Flag is set in the Configuration BPDUs received from the. 4

5 TOPOLOGY Change Time /Timer In the, this is the time period ( default 35 seconds)for which Configuration BPDUs are transmitted with the Topology Change Flag set following the detection of a Topology change. This value is the sum of: 1) Bridge Forward Delay ( default is 15 seconds) 2) Bridge MAX Age ( default is 20 seconds) Hold Time/Timer This is the minimum time period ( 1 second ) elapsing between the transmission of Configuration BPDUs through a given Bridge Port. The HOLD Time is started after each Configuration BPDU is transmitted, if we activate the Transmit Configuration BPDU function while the HOLD Timer is activated, we will set the Configuration BPDU Pending Flag, and wait for the Hold Timer to expire. This ensures that we will not flood out BPDUs, we are throttling the traffic. Topology Change Detected This bit is set by a Bridge when it has detected a Topology Change condition It is the responsibility of this Bridge to notify the next device on the path to the. This flag will be reset when the Bridge receives an acknowledgment from the Root Path Port. Topology Change Notification Timer Serves to ensure that the Designated Bridge on the LAN to which the Bridge s Port is attached is notified of any detected topology change. This timer is set to 1 second, by default. As long as the Topology Change Detected Flag is set, this bridge will generate Topology Change BPDUs, every second. When an acknowledgment is received the flag is reset and the timer is shut off. 5

6 The Network The following illustration will be used in the remainder of the discussion: Clint Harry Bert Ernie The Current Active Topology 1) Assume for these scenarios that all Path Costs are 100, all Bridge Priority values are 8000, all Port Priority values are 80, and the MAC Address for each bridge is denoted below. 2) FRED d is the. 3) FRED is also the Designated Bridge for LANs and. 4) WILMA d is the Designated Bridge for LAN 5) BARNEY d is Blocking on it s LAN Port. 6) CLINT d is the Designated Bridge for LAN C. 7) HARRY d-1d is Blocking on it s LAN C Port. 8) BERT d is the Designated Bridge for LAN D 9) ERNIE d is Blocking on it s LAN D Port. 10) BUD d and LOU d are not currently connected into our network, but will be used to tie the users on LAN into the Bridged Local Area Network. Topology Change Examples We will step through 4 different examples of a Topology Change; 1) Introducing BUD and LAN into the Bridged Local Area Network. 2) Including LOU as a redundant path from LAN 3) Changing the Path Cost on one of the Blocking Bridges. 4) Causing another Bridge to become. 6

7 Topology Change #1 Page 1 of 5 This scenario will represent the addition of another segment to our Bridged Local Area Network by cabling in a Bridge between LAN C and LAN. In this case the new Bridge will not replace the current Bridge, because it possesses a lower priority Bridge ID. BUD transmits Configuration BPDUs out all of it s Ports = BUD Path Cost =0 BPDU Flags= Clint Harry Bert Ernie a) Upon initialization each Bridge will consider itself until a more qualified Bridge is recognized, ( a Configuration BPDU is received which contains a higher priority Bridge, and/or a lower Path cost, and/or a higher priority Designated Bridge, and/or a higher priority Designated Port). The Bridge with the highest priority Bridge ID ( lowest numerical ID ) will always be considered the Bridge in an 802.1D compliant environment. b) Establishing itself as, BUD sets the Topology Change Flag. (Topology Change Count = +1)This flag will be the source for bit 1 in the Bridge s Configuration BPDU and the Topology Change Timer is activated, this timer controls the period during which the Bridge will generate Configuration BPDUs with the Topology Change bit in the flags field set c) BUD thinks it is, and starts transmitting Configuration BPDUs, out of all of its ports, promoting itself as and with bit 1 of the Flags field set. 7

8 Topology Change #1 Page 2 of 5 We will now see how CLINT reacts to this unexpected Configuration BPDU BUD transmits Configuration BPDUs out onto LAN C = FRED Desig Br= CLINT Path Cost =200 BPDU Flags= Clint Harry Bert Ernie a) CLINT is the Designated Bridge for LAN C, and it does not expect to see any other Bridge s Configuration BPDUs being generated onto LAN C. When CLINT receives a Configuration BPDU from BUD it will invoke The Reply to Configuration BPDU process, because the data contained within the BPDU is inferior to the current active topology information maintained at the Port. b) The Reply to Configuration BPDU process will now be invoked to indicate the Designated Bridge and Designated Port for LAN C. CLINT will send out a Configuration BPDU onto LAN C indicating that FRED is the real and that CLINT is the Designated Bridge for LAN C. c) With the reception of this BPDU from CLINT, BUD realizes that it is no longer to be considered as, and with the Topology Change Detected flag currently set, BUD stops the Topology Change Timer. d) The Transmit Topology Change Notification BPDU process is invoked and the Topology Change Notification Timer is started. This timer will ensure that the Designated Bridge on LAN C is notified of the Topology Change condition. It is set to a value of 2 seconds ( HELLO Timer value). This timer controls the frequency of TCN BPDU generation. 8

9 Topology Change #1 3 of 5 Now we ll see how BUD reacts to this Configuration BPDU deposing him as. CLINT transmits Config BPDUs onto LAN C = FRED Desig Br. = CLINT Path Cost =200 BPDU Flags = BUD transmits Topology Change Notification BPDUs out onto LAN Clint Harry Bert Ernie a) BUD will begin transmitting TCN ( Topology Change Notification) BPDUs out of it s Port, which is it s access to LAN C. The Port on a Bridge is that Port through which it gains access to the Bridge! b) CLINT which is the Designated Bridge, but not the, for LAN C will receive the TCN BPDU ( Topology Change count = +1)and will perform the following operations. CLINT is not the Bridge, so it will: 1. Start the Topology Change Notification Timer. 2. Activate the Transmit Topology Change Notification Process. CLINT will also set the Topology Change Acknowledge Flag bit, which be the source of bit 8 in the Configuration BPDUs Flags field. CLINT will also activate it s Transmit Configuration BPDU process. c) CLINT will transmit a Configuration BPDU to LAN C with the Topology Change Acknowledgment bit ( bit 8 ) in the BPDU s Flag field. d) BUD receives this Configuration BPDU and determining that the Topology Change Acknowledgment flag is set, it will reset it s Topology Change Detected Flag, and stop it s Topology Change Notification Timer. It will not generate any more TCN BPDUs. e) CLINT will transmit a TCN BPDU out of it s Port onto LAN, and wait for an acknowledgment from the Designated Bridge for LAN. The Topology Change Notification is handled by Designated Bridges across each LAN on the way to the. f) WILMA will become involved in the same bilateral communication with CLINT. g) WILMA and FRED who is the will also become involved in the identical handshaking sequence. h) The effect of this process is to eventually notify the of the Topology Change. i) FRED which is the Bridge will set the Topology Change Flag parameter and it will start the Topology Change Timer is started. 9

10 Topology Change #1 Page 4 of 5 What I really said, was; This is the English translation of the previous 3 pages, The following illustration attempts to simplify the process involved to this point. -6- CFG BPDU -4- CFG BPDU -5- TCN BPDU -3- TCN BPDU Clint Harry Bert Ernie -1- TCN BPDU -2- CFG BPDU a) We want to configure the users on LAN into our Bridged Local Area Network, to do so we configured a 2 port Bridge with connections to LAN and to LAN C. b) BUD proclaimed himself as after initialization, ( Topology Change Count = +1 )and when it sent BPDUs out of all of it s ports, CLINT responded with a Configuration BPDU which reflected FRED as the and CLINT as the Designated Bridge for LAN C c) BUD (-1- TCN BPDU) sends a Topology Change Notification BPDU onto LAN C, it will continue to do so until this TCN BPDU is acknowledged. d) CLINT, the Designated Bridge for LAN C, will generate a Config BPDU ( -2- CFG BPDU ) with the Topology Change Acknowledged Flag bit set. The reception of this BPDU will cause BUD to stop sending TCN BPDUs e) CLINT now sends a Topology Change Notification BPDU ( Topology Change Count = +1 ) onto LAN ( -3- TCN BPDU ), which will be receive by WILMA, the Designated Bridge for LAN. f) WILMA responds with a Configuration BPDU ( -4- CFG BPDU ) which will acknowledge it s reception, by setting the Topology Changes Acknowledged bit in the BPDU flags field. g) WILMA now sends a Topology Change Notification BPDU ( Topology Change Count = +1 ) ( -5- TCN BPDU ) onto LAN, which will be received by FRED ( Topology Change Count = +1 ) which is the. h) FRED in turn will respond with a Configuration BPDU, with the Topology Change Acknowledged bit set in the flags field. i) At this point in time the Bridge has been notified of the Topology Change, BUD is now part of the active topology. This the reason for the TCN BPDU, to notify the Bridge of any changes in the active topology. j) FRED, the Bridge has now been notified and it will now perform its Topology Change operation. 10

11 Topology Change #1 Page 5 of 5 The Responds The Bridge has now been notified about the Topology Change, the following illustration and text will reflect the Bridge s actions. Clint Harry FRED transmits Config Bert Ernie BPDUs onto LAN C C D = FRED D Desig Br. = FRED Path Cost =0 BPDU Flags = a) Upon reception of the Topology change Notification BPDU, FRED will; 1) Set the TOPOLOGY CHANGE Flag, which is the source for bit 01 in the Configuration BPDU Flags field. 2) Start the TOPOLOGY CHANGE Timer whose value is set to the sum of MAX Age ( a default value of 20 seconds) and FORWARD Delay ( a default value of 15 seconds). b) FRED will generate Configuration BPDUs every 2 seconds, until the expiration of the TOPOLGY Change Timer, with bit 01 set Topology Change. c) FRED sends it s BPDUs onto LAN, both WILMA and BARNEY will receive the BPDU and will set their aging timers to the value of FORWARD Delay. They will also remove any entries that are older than 15 seconds, this will allow the bridge to relearn any addresses that may be sourced into the Bridged Local Area Network from a new point of reference. d) This BPDU (with bit 01 set in the Flags field) will be forwarded by WILMA onto LAN. e) It will then be forwarded onto LAN C by CLINT and onto LAN D by BERT. f) BUD will then forward this BPDU onto LAN. g) All Bridges will have lowered their Aging Timer from 300 seconds to 15 seconds to ensure proper path recognition for all devices within the Bridged Local Area Network. 11

12 Topology Change #2 Page 1 of 2 In this scenario we will connect the upper ( currently unattached port ) from LOU to LAN D. We will be adding a redundant path to the rest of the network to the users that populate LAN. At this point in time both ports on BUD and LOU are in the FORWARDING state. When we connect LOU to LAN D, we will have a data loop. The following illustrations will supplement the text. A Connection is established here and and the fun begins Path Cost = 0 Path Cost = 0 Path Cost =100 Clint Harry Bert Ernie Path Cost =200 Path Cost =300 a) Prior to this connection, LOU had been receiving BPDUs from LAN only, and both of LOU s ports were in a FORWARDING state. b) Once the connection has been established LOU will also begin receiving BPDUs from LAN D, the protocol data representing the two possible Designated Bridges for LAN are reflected below, LOU will be vying to be the Designated Bridge for LAN, these are the values that LOU will be considering in order to make it s decision. BPDUs for BUD BPDUs for LOU Bridge FRED FRED Desig. Bridge BUD d LOU d Path Cost c) The BPE ( Bridge protocol Entity ) in LOU will determine that BUD is the higher priority Designated Bridge and will proceed to put it s LAN port into BLOCKING ( Topology Change Count = +1) d) LOU has now experienced a TOPOLOGY CHANGE and the Topology Change Detected Flag is set.. e) LOU will begin it s Transmit Topology Change Notification BPDU process and it will set the Topology Change Notification Timer. f) LOU will begin transmitting TCN BPDUs onto it s port on LAN until it receives a Topology Change Acknowledgment from the Designated Bridge for LAN 12

13 Topology Change # 2 Page 2 of 2 The following illustration reflects the path for the TCN BPDUs through the Bridged Local Area Network, and this is the same scenario as was provided in the first example. The thing to remember is the fact that the Bridge must be notified of any change in the active topology. As you can see from these two examples that no traffic in these two scenarios were suspended during the Topology Change Notification Process, none of the Bridges entered the LISTENING or LEARNING state. As we will see in the next two examples -8- CFG BPDU -6- CFG BPDU -7- TCN BPDU -5- TCN BPDU Clint Harry Bert Ernie -4- CFG BPDU -3- TCN BPDU -1- TCN BPDU -2- CFGBPDU a) Each Bridge that receives a TCN BPDU will increment its Topology Change Count by 1. b) The Bridge, FRED, is now aware of the Topology Change and will force all Bridges to change their SAT Aging Time from a default of 300 seconds to the value of the Forwarding Delay, which is a default of 15 seconds. c) Reference the example Topology Change #1, this will reflect the termination of this Topology Change. 13

14 Topology Change # 3 Page 1 of 2 This scenario will reflect the results of changing the Path Cost of a Bridge that is currently in the BLOCKING State, and in the process making this Bridge Port the Designated Port for the LAN that it is connected to. 11 We set the Path Cost for LAN C to a value of 90. Clint Harry Bert Ernie HARRY s LAN C port C C enters the LISTENING D State D 22 CLINT forces it s LAN C port into the BLOCKING State 44 HARRY generates Config BPDUs with a lower Path Cost to FRED 33 A View from HARRY a) When we set the Path Cost through management, we will invoke the Configuration Update procedure and the Port State Selection procedure within the Spanning Tree protocol. b) These procedures will allow us to change the value of the Path Cost associated with HARRY s LAN port from 100 to a new value of 90. This is important to realize, because the important aspect of Designated Bridge determination is the value of the Port on the Bridge. ( The Port is the port that we would have to pass through in order to access the Bridge). This happens to be the Bridge port directly connected to LAN C. c) HARRY now appears to be better qualified to be Designated Bridge for LAN C. d) The Port State Selection procedure is activated and it enables The Forward Delay Timer and forces the Port into the LISTENING State. e) HARRY will then begin to transmit Configuration BPDUs onto LAN C professing a Path Cost of 290. f) Upon receipt of the more qualified Designated Bridge protocol data, CLINT will immediately place it s LAN C port into BLOCKING. ( Topology Change Count = +1 ) g) 15 seconds after entering the LISTENING State, the Forwarding Delay Timer will expire, HARRY s LAN C port will enter the LEARNING State and the Forward Delay Timer is restarted. h) 15 seconds after entering the LEARNING State, the Forward Delay Timer will expire and HARRY s LAN C port will enter the FORWARDING State. ( Topology Change Count = +1 ) i) HARRY is now the Designated Bridge for all users on LAN ; for a 30 second period, users on LAN could not talk to anyone ( except the users on LAN C ) located on any of the other LANs and vice or versa. 14

15 Topology Change # 3 Page 2 of 2 We are now going to review the processes that are occurring on CLINT while HARRY is going through it s process of becoming the Designated Bridge for LAN. CLINT s responsibilities are to ensure that it puts it s LAN port into the BLOCKING State as well as notify the of this Topology Change. -4- CFG BPDU -2- CFG BPDU Clint -1- TCN BPDU Harry -3- TCN BPDU CLINT performs the Topology Change Notification process, to inform the of the Topology Change 22 Bert Ernie 11 After receiving a Config BPDU with superior data CLINT forces this port into BLOCKING A View from CLINT a) When CLINT receives a Configuration BPDU from HARRY he will compare the protocol data he currently has stored in the BPE. Current Protocol held by CLINT Protocol Data within the BPDU Bridge FRED FRED Path Cost Designated Bridge CLINT HARRY b) CLINT immediately places its LAN C port into BLOCKING, and for the next 30 seconds the users on LANs and C can only communicate with each other, and users on LANs,,, and D will only be able to communicate with each other. c) CLINT will follow the procedure to generate a Topology Change Notification BPDU. It will transmit the TCN BPDU onto it s Port, and will continue to do so until it receives a Configuration BPDU with the Topology Change Acknowledge bit set in the Flags field. d) And so it goes, until the Topology Change Notification BPDU is received by the. The will then begin generating Configuration BPDUs with the Topology Change Flag set, which will force each Bridge to lower its SAT timer to a value of Forwarding Delay ( 15 seconds). e) As you can see in this example the network is temporarily segmented into two separate entities for a period of 30 seconds. 15

16 Topology Change #4 1 of 5 The following series of illustrations will take us through a scenario wherein we will change the Bridge Priority value of one of the other Bridges ( BARNEY ) in the Network in order to re-deploy our Bridge. Set the Bridge Priority to a value of Clint Harry Bert Ernie a) We want to make BARNEY the new Bridge, the easiest way to accomplish this is to lower its Bridge Priority value, ( in this scenario all Bridges are Cabletron Bridges and their Bridge Priorities are all set to the default value of 8000), to b) This will cause the Configuration Update Procedure and the Port Selection Procedure to be implemented. BARNEY will now copy his Bridge ID to the Designated parameter and set the Path Cost to 0. c) BARNEY now considers itself as Bridge and will force the Bridge Params to the value of its MAX Age, HELLO Time, and FORWARD Delay parameters. d) A Topology Change has been detected, ( BARNEY has determined that it is ) and the Topology Change flag is set ( Topology Change Count = +1 )and the Topology Change Timer for the Bridge is started. The Topology Change Flag will be set in all the Configuration BPDUs generated by BARNEY for the duration of the Topology Change Timer ( typically = 35 seconds). This will cause all devices when they receive and accept this BPDU to lower their SAT aging Timer to the value of the FORWARDING Delay Parameter. 16

17 Topology Change #4 2 of 5 will now begin to source Configuration BPDUs onto the network, as it now considers itself as the Bridge BARNEY will begin generating This Port will Clint Harry Bert Ernie Configuration transition from BPDUs onto BLOCKING to C LANs and C D LISTENING D to LEARNING to 33 FORWARDING 22 a) BARNEY will start its HELLO) timeer and begin transmitting BPDUs out of all of its Ports professing itself as Bridge, he will be able to transmit onto LAN as it will transition its state from BLOCKING to FORWARDING. b) BARNEY will as a result of determining that it is will activate the Make Forwarding Procedure for its LAN Port. BARNEY will transition from the BLOCKING state to the LISTENING state, with its SAT aging timer set to 15 seconds, ( this will eventually cause BARNEY to flush all of its entries ), after the Forwarding delay times out it will transition to the LEARNING state. During the 15 seconds that is resides in the LEARNING state, BARNEY will not forward any user traffic, it will learn station locations via the Source Address of packets that it encounters. When the Forward Delay Timer times out again, BARNEY will transition to the FORWARDING state. When BARNEY transitions to the FORWARDING state, it will again latch a Topology Change detected, ( Topology Change Count = +1 ) which will reset its timers. c) In order to attempt to clarify this entire process as much as possible, we are examining a small portion of the network at a time. You must realize that all of this activity is progressing in a fairly simultaneous manner. 17

18 Topology Change #4 3 of 5 Here we will see how FRED ( the old Bridge) reacts during this Topology Change scenario. FRED will relinquish its Bridge status with the reception of the first BPDU from BARNEY 44 Clint Harry Bert Ernie a) FRED receives a Configuration BPDU from BARNEY and determines that there is superior protocol information contained therein, he is no longer the Bridge. FRED will update the STA data base to reflect this new protocol information. FRED will stop its HELLO timer as he will no longer source Configuration BPDUs. b) FRED will continue to be the Designated Bridge for LAN. 18

19 Topology Change #4 4 of 5 Now we will follow WILMA s activity during this Topology Change. 66 WILMA will generate a TCN BPDU and transmit it out its Port Clint Harry Bert Ernie C WILMA will move this port C D D to BLOCKING 55 a) WILMA will receive a BPDU from BARNEY on both of its ports, it will detect the fact that there is a data loop and will determine that it should force its LAN B port into BLOCKING, Why does it force its LAN port into BLOCKING and not its LAN port? Call me at X73672 with the right answer and win a prize. b) WILMA will set the Topology Change Detected flag ( Topology Change Count = +1 ) and will transmit a TCN BPDU out of its port. WILMA will continue to do so until it receives a Configuration BPDU with the Topology Change Flag set. c) Notice that all access to LAN and from the lower three LANs has ceased because the access port through WILMA is in a BLOCKING state and the access through BARNEY s port is in the LISTENING/LEARNING state. 19

20 Topology Change #4 5 of 5 When can now look at the remainder of the network as there should not be any further alterations to the spanning tree protocol information. The relationship among the remaining Bridge pairs remains unchanged Clint Harry Bert Ernie a) The Port Status within the remaining redundant Bridge pairs does not change, even though Bridge, path cost and Designated Bridges may have changed, there is no reason to change the state of any other Bridge Ports. b) The last thing that will happen here is when BARNEY s LAN port transitions to FORWARDING it will detect another Topology Change. This will cause BARNEY to continue to generate BPDUs with the Topology Change Flag bit set,for another 35 seconds. This will force all Bridges to maintain their SAT aging value to 15 seconds for the next 35 seconds. The Topology Change count that everyone appears to dwell on may not be an accurate indication of the actual number of changes that a Bridge is seeing. It should be noted that a Bridge will update its Topology Change count each any every time that it sees a Topology Change Detected. A Bridge will increment this count any time it sees any of the following events. (1) The reception of a TCN BPDU on a Port that is the Designated Port for the LAN to which it is attached. (2) Whenever a Bridge Port is put into the FORWARDING state following the expiry of the Forward Delay Timer for the Port, provided that the Bridge is the Designated Bridge for at least one of the LANs to which it is connected. (3) When a Bridge Port in either the FORWARDING or the LEARNING state is put into the BLOCKING state. (4) When the Bridge becomes (5) If the Bridge was selected as prior to the Configuration Update, but is no longer, and the Topology Change Detected Flag is set. Topology Changes should not occur on a regular basis, because if they do, it would indicate that there is some inherent problem within the network. 20