STP Questions 5

Correct answer for number 5 question is D…
because if we don’t enter portfast on host port whenever user PC trun off or turn down TCN BPDU send…
Cisco Switch Book say:
An end-user workstation is usually connected to a switch port in the access layer. If the
workstation is powered off and then turned on, the switch will sense that the port link status
has gone down and back up. The port will not be in a usable state until STP cycles
from the Blocking state to the Forwarding state. With the default STP timers, this transition
takes at least 30 seconds (15 seconds for Listening to Learning, and 15 seconds for
Learning to Forwarding). Therefore, the workstation cannot transmit or receive any useful
data until the Forwarding state finally is reached on the port.
And
To see what effect this has on the STP topology and the network, consider the following
sequence of events:
1. The PC on Catalyst port 2/12 is turned off. The switch detects the link status going
down.
2. Catalyst C begins sending TCN BPDUs toward the root, over its root port (1/1).
3. The root sends a TCN acknowledgment back to Catalyst C and then sends a Configuration
BPDU with the TCN bit set to all downstream switches. This is done to inform
every switch of a topology change somewhere in the network.
4. The TCN flag is received from the root, and both Catalysts B and C shorten their
bridge table aging times. This causes recently idle entries to be flushed, leaving only
the actively transmitting stations in the table. The aging time stays short for the duration
of the Forward Delay and Max Age timers.

@yunas,

No the correct answer is C as shown above. What you are describing is not a broadcast storm. A broadcast storm is when a broadcast (like an ARP request) is forwarded out multiple ports and gets re-forwarded because of loops. The re-forwarding causes the amount of traffic from that one ARP traffic to grow and grow into a “storm”.

Hello, I don`t understand the Q11 – f as correct. From the output – there are depicted 2 ports, one root port and one blocked port. I thought, that only on designated ports, BPDU are sent out. There are no designated ports, thus no BPDU will be sent out.

Q8.

about that command, the official certification guide says:

”when you need to prevent BPDUs from being sent or processed on one or more switchport , you can use BPDU filtering to effectively disable STP on those ports”

so, i think B is the correct choise, isn’t it?

@Q11

F is correct because the hello time value configured in the root bridge switch determines the hello time for all nonroot switches because they just relay the Configuration BPDU as they are received from the root

marco; yes, thanks… but there is still no interface in the output, where the BPDU will be sent, no designated port… Thus guess just not complete output in the picture.

@Q11
let me know if i have understood..
timer ‘hello time’ (listed in root bridge section of the output) refers to Configuration BPDUs sent by root bridge, every 10 seconds, from its designated ports.
interfaces listed below refer to the local nonroot bridge interfaces status (one blk and the other root)

bye

@Q11

i forgot another detail: Once root bridge is elected, Configuration BPDUs are only sent by the root, the other switches (nonroot):
- process,
- relay,
- and change the sender ID

At marco Q 8:

STP is disabled, however the thumb rule is that if it receives a BPDU then PortFast is disabled and the BPDUs are processed normally.

For Q 11:

Not very sure for this one, but i guess you would have to assume they are directly connected and therefore the root would be pointing straight to the designated port from the other switch

@ davrojas on Q8:

do you have any links, documents where the thumb rule is specified?

thks

Q8
“Switch(config)# spanning-tree portfast bpdufilter default” is global configuration that disables all portfast and bpdufliter on all interfaces, so the ports appear as normal so…

Q8

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_55_se/commmand/reference/cli3.html#wp1946892

spanning-tree portfast (global configuration)

Use the spanning-tree portfast global configuration command to globally enable bridge protocol data unit (BPDU) filtering on Port Fast-enabled interfaces, the BPDU guard feature on Port Fast-enabled interfaces, or the Port Fast feature on all nontrunking interfaces. The BPDU filtering feature prevents the switch interface from sending or receiving BPDUs. The BPDU guard feature puts Port Fast-enabled interfaces that receive BPDUs in an error-disabled state. Use the no form of this command to return to the default settings.

spanning-tree portfast {bpdufilter default | bpduguard default | default}

no spanning-tree portfast {bpdufilter default | bpduguard default | default}
Syntax Description

bpdufilter default

Globally enable BPDU filtering on Port Fast-enabled interfaces and prevent the switch interface connected to end stations from sending or receiving BPDUs.

from this I think B is correct.

Answers for Q3 & Q4 seem to contradict each other regarding when a TCN is generated. Could someone please clarify this for me?

#spanning-tree portfast bpdufilter default

here a link with a good explaination:
http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.1E/native/configuration/guide/stp_enha.html#wp1042489

STP operates on all switch ports, BPDUs are sent on all switch ports, even ports where PortFast has been enabled. However in special cases when we need to prevent BPDUs from being sent on one o more switch port we can use bpdufilter to disable STP on those ports. However if a portfast switch port receive a BPDU that port looses portfast status (portfast is disabled) and bpdufilter is disabled on thet port. So that port will act as a normal STP port.

so A is the answer. sorry for the previous !

When enabled globally, BPDU filtering has these attributes:
- It affects all operational PortFast ports on switches that do not have BPDU filtering configured on the individual ports.

- If BPDUs are seen, the port loses its PortFast status, BPDU filtering is disabled, and the STP sends and receives BPDUs on the port as it would with any other STP port on the switch.

Upon startup, the port transmits ten BPDUs. If this port receives any BPDUs during that time, PortFast and PortFast BPDU filtering are disabled.

So Answer A is correct on question 8

@ Anonymous: The answers don’t really contradict each other.

In RTSP a TCN will be sent both:
* when a non-edge port moves to forwarding;
* when a portfast enabled (edge) port receives a BPDU.

The “only” word used in question 4 is being used to differentiate “only non-edge” as opposed to both edge and nonedge ports or in other words, all ports.

@ Q11:
Yep, I agree, it looks like question 11 has only one correct answer.
Since the switch doesn’t have any designated ports and it’s running IEEE STP (PVST+ in this case) it should not send out any BPDUs on any of its two current ports, because one is a root port (pointing towards the root) and that’s the one where it receives the BPDUs and the other port is blocked, hence it doesn’t send any BPDUs, it only listens to the ones it receives on that link.
It looks like the author of the question didn’t think this through very well. But that wouldn’t be a surprise; I found in the cert exam for CCNA, as well, a couple of questions which didn’t have any right answer.

Q.8
Marco, i agree that the document is right, but it is especific for global configuration command. I think this is the mad. Becouse the document goes on to say:

PortFast BPDU filtering can also be configured on a per-port basis. When PortFast BPDU filtering is explicitly configured on a port, it does not send any BPDUs and drops all BPDUs it receives.
——————————————————————————–
Caution Explicate configuring PortFast BPDU filtering on a port that is not connected to a host can result in bridging loops as the port will ignore any BPDU it receives and go to forwarding.

——————————————————————————–

Question 7
Shouldnt the answer for 7 be B&C.
This is from the cisco site.
TCNs are triggered by a port that is transitioning to or from the forwarding state. After the TCN, even if the particular destination MAC address has aged out, flooding should not happen for long in most cases since the address will be relearned. The issue might arise when TCNs are occurring repeatedly with short intervals. The switches will constantly be fast-aging their forwarding tables so flooding will be nearly constant.

Q3:

according to cisco, the answers must be A & D

I am doubtful about Q3. Edge port does not send any TCN .

Hamza – It wont transition to fowarding but will go through listening and learning stages. Its just that STP gets enabled on that port and then it goes through whole cycle to decide the port status. But yes, even I think option B is wrong. Please someone correct me if I am missing something here.

the answer Q5 is correct

Hi Guys,

Seeing the answer right in front your eyes before making selection is annoying. So i grabs all of the questions on this site and put them into VCE file to make my life easier. Be sure to check back CertPrepare site regularly for updates.

Here is the file: http://www.4shared.com/file/C5hmKPbb/CCNP_642-813_CertPrepare-by_Hu.html

Best of luck !!!

Question 3

the answer must be A and D

as edge port facilitate to RSTP a quick conversion to forwarding state and TCN BPDU sent out in only one condition when non0edge port change its state to forwarding state.

the cisco site states as below:-

Edge Ports

The edge port concept is already well known to Cisco spanning tree users, as it basically corresponds to the PortFast feature. All ports directly connected to end stations cannot create bridging loops in the network. Therefore, the edge port directly transitions to the forwarding state, and skips the listening and learning stages. Neither edge ports or PortFast enabled ports generate topology changes when the link toggles. An edge port that receives a BPDU immediately loses edge port status and becomes a normal spanning tree port. At this point, there is a user-configured value and an operational value for the edge port state. The Cisco implementation maintains that the PortFast keyword be used for edge port configuration. This makes the transition to RSTP simpler.

Q8 is wrong. Letter B is correct, of course.

Q8 is yet another case where the CCNP SWITCH Official Certification Guide is totally misleading, and where the Cisco exam is impossible to pass without a cheat-site like this.

A is correct according to marco’s link: http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/ios/12.1E/native/configuration/guide/stp_enha.html#wp1042489. (As I understand it, B would be correct IF the bpdufiltering was configured on a per-port basis.)

Quoting the above link:

“When configured ***globally***, PortFast BPDU filtering applies to all operational PortFast ports. [...] If an operational PortFast port receives a BPDU, it immediately loses its operational PortFast status. In that case, PortFast BPDU filtering is disabled on this port and STP resumes sending BPDUs on this port.

[...]

When PortFast BPDU filtering is ***explicitly*** configured on a port, it does not send any BPDUs and drops all BPDUs it receives.”

The answer for Q8 is correct.
Enabling BPDU filtering globally will stop sending BPDUs after few initial BPDUs but it wont ignore any incomming BPDUs. Upon receiving BPDUs the will loose its portfast status and become a normal STP port. Enabling BPDU filtering on the interface will stop sending BPDUs after few initial BPDUs and ignore any incomming BPDUs.

thaks

Q8 is correct…

there is a difference between configuring bpdu filtering on global configuration mode and the interface configuration mode…
explanation of cisconut is correct

We Use BPDU Filtering to Disable STP on a Port.
I works by ignoring the any BPDU in any Direction(In/Out).
Documents says that ” It’s a

enable BPDU filtering only under controlled circumstances

We Use BPDU Filtering to Disable STP on a Port.
It works by ignoring any BPDU in any Direction(In/Out).
Documents say that ”Enable BPDU filtering only under controlled circumstances in which
you are absolutely sure that a switch port will have a single host connected and that a loop
will be impossible.”
I Think Letter B Is Correct.

There is an typo in Q10. The answers are A,B,C,D instead of A,B,D,D.
Thanks

Hi for Q3
the answer is A and D

Neither Edgeport or porfast generates TCN

The edge port concept is already well known to Cisco spanning tree users, as it basically corresponds to the PortFast feature. All ports directly connected to end stations cannot create bridging loops in the network. Therefore, the edge port directly transitions to the forwarding state, and skips the listening and learning stages.

Neither edge ports or PortFast enabled ports generate topology changes when the link toggles. =======A

An edge port that receives a BPDU immediately loses edge port status and becomes a normal spanning tree port.===========D

At this point, there is a user-configured value and an operational value for the edge port state. The Cisco implementation maintains that the PortFast keyword be used for edge port configuration. This makes the transition to RSTP simpler.

Q4 makes the situation more clear that only non -edged ports can generate TCN upon toggling or moving or changing their rstp state

for q6 spanning-tree portfast bpdu guard
and spanning-tree portfast bpdu filter default both in global config mode
thanks

No documentation is completely clear about BPDU Filter using portfast, neither TCN in some cases. I see a little ambiguous the CCNP SWITCH exam is such sense.

Everybody with different theories (Cisco Pess, Cisco Web Site, Blogs) but none completely justified.

In my exam I will put that I understand.

In case of IEEE 802.1 w , the hello message that is sent over every 2 seconds contains BPDU in itself and when an adjacent L2 device does not receive 3 hello’s in a row, it is believed that the device not responding is dead and thats how RSTP achieves faster convergence by inculcationg TC BPDU in hello messages.

New BPDU Handling

BPDU are Sent Every Hello-Time

BPDU are sent every hello-time, and not simply relayed anymore. With 802.1D, a non-root bridge only generates BPDUs when it receives one on the root port. In fact, a bridge relays BPDUs more than it actually generates them. This is not the case with 802.1w. A bridge now sends a BPDU with its current information every seconds (2 by default), even if it does not receive any from the root bridge.

Faster Aging of Information

On a given port, if hellos are not received three consecutive times, protocol information can be immediately aged out (or if max_age expires). Because of the previously mentioned protocol modification, BPDUs are now used as a keep-alive mechanism between bridges. A bridge considers that it loses connectivity to its direct neighbor root or designated bridge if it misses three BPDUs in a row. This fast aging of the information allows quick failure detection. If a bridge fails to receive BPDUs from a neighbor, it is certain that the connection to that neighbor is lost. This is opposed to 802.1D where the problem might have been anywhere on the path to the root.

reference http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

Q3

RSTP detects a topology change only when a nonedge port transitions to the Forwarding
state. So answer B seems not correct, because we have EDGE port which is already in the Forwarding state. However, after receiving BPDU port looses its edge state and become normal STP port (answer D). In other words it becomes NONEDGE port. The nonedge port immediately is moved to the Forwarding state. (Answer A). But when a nonedge port transitions to the Forwarding state the switch generates a Topology Change Notification (TCN) BPDU because of RSTP detects a topology change in such a case (Answer B).

So I see three correct answers: A,B,D.

@certprepare
Question 3
—
Topology Change Detection

In RSTP, only non-edge ports that move to the forwarding state cause a topology change. This means that a loss of connectivity is not considered as a topology change any more, contrary to 802.1D (that is, a port that moves to blocking no longer generates a TC). When a RSTP bridge detects a topology change, these occur:

It starts the TC While timer with a value equal to twice the hello-time for all its non-edge designated ports and its root port, if necessary.

It flushes the MAC addresses associated with all these ports.

Note: As long as the TC While timer runs on a port, the BPDUs sent out of that port have the TC bit set. BPDUs are also sent on the root port while the timer is active.
—
Source : http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml
—
i See That The Answer Is A, D
Please Correct Me If Im Wrong
Thanks In Advance.

@Certprepare
Q3
Please Change The Answer i Believed Ur’s Was Right… & Even Dumps Same Answer… But The Right Answer Is
A,D

Neither edge ports or PortFast enabled ports generate topology changes when the link toggles. An edge port that receives a BPDU immediately loses edge port status and becomes a normal spanning tree port.

Source Previous Link.
Thanks For The Great Effort u Doing.

Question 3
Answers are B and D.

Answer is not A because the edge port would already be in forwarding state, it therefore doesn’t switch to forwarding state when it receives a bdpu.

An edge port doesn’t generate TCNs, but after the edge port receives the bdpu, it changes to a regular stp port(answer D), and when it changes states as a regular stp port, it will generate TCNs.

“One other benefit of PortFast is that
Topology Change Notification (TCN) BPDUs are not sent when a switch port in PortFast
mode goes up or down. This simplifies the TCN transmission on a large network when
end-user workstations are coming up or shutting down.” That is, a TCN is not sent when a user connects or disconnetcs. But it is sent when the port moves through STP states after receiving a BPDU. The point is the BPDU, not the going up or down of the port.
Answers are B and D

Comment above is for Q3

WHAT ABOUT QUESTION 7?

IS THAT CORRECT?

CCNP , OFFICIAL CERT. GUIDE. – PAGE 200( last edition , page 198 for 1º edition).

“By definition , the port cannot form a loop as it connects to one host, so it can be placed immediately in the fowarding state”

SO THE RIGHT ANSWERS WIL BE

“A” AND “D”

Sorry I read the question again…

and its right the aswers B AND D are correct

Q3, Q8, Q10 on test today

any one willing to share pass4sure… email me escort_brighton@yahoo.com.

Q3 & Q4 in test today.

just a addition to @Pazuzu
The non-edge port will not immediately transfer to the FORWARDING state,if the STP algorithm was designed in this way then definitely it would initiate a LOOP .The Non-Edge should pass the states in case of STP:LSN,LRN,FWD and in case of RSTP:LRN,FWD.

Q 11. in explanation was mention forward delay was 15 seconds, then it becomes 30 seconds. Plz expalin about Q11 quite confusing..

Im a bit confused with Q1

It says that the answer is A, however the explanation says that CAT1 is not receiving a BPDU where as it shows that CATA is receiving BPDU which is 1