Intermittent Fiber Connectivity: Interface Errors or Cable Fault?
Identify interface and cable issues (collisions, errors, mismatch duplex, and/or speed)
Compare physical interface and cabling types
Answer
Correct answer: A — SiteA's reliability of 166/255 shows its interface errors are incrementing, degrading the intermittent traffic to SiteB.
[sampleXML/xmlfile-49_1.png] Refer to the exhibit. Shortly after SiteA was connected to SiteB over a new single-mode fiber path, users at SiteA report intermittent connectivity issues with applications hosted at SiteB. What is the cause of the intermittent connectivity issue?
Community Insight
The question tests whether you can read reliability and load counters to confirm a fault, and the trap is picking the physically 'plausible' cable or SFP fault instead of what the exhibit actually evidences.
This CCNA 200-301 troubleshooting item examines a new single-mode fiber link between SiteA and SiteB where SiteA users see intermittent connectivity, using interface counters (reliability, load, SFP type) to isolate the fault. The exhibit proves SiteA's interface reliability has dropped to 166/255 while load stays at 1/255 and the optics/cabling match, so incrementing interface errors is the documented cause.
Most learners choose D (wrong cable type) or C (wrong SFP at SiteA), reasoning that a bad fiber run is the real cause; however, the output shows single-mode cabling with matching long-reach (LR) optics on both ends, so the only item the exhibit supports is A, incrementing interface errors (reliability below 255/255).
Community Discussion (45 comments)
RudeGrimmy
👍 1
Selected: B
In Cisco IOS, interface reliability is measured on a scale out of 255, where 255/255 represents 100% perfect, error-free operational reliability. A drop down to 166/255 (roughly 65% reliability) indicates that the link is suffering from severe packet loss, frame errors, or extreme congestion bottlenecks.Looking closer at the transmission metrics explains why:The input rate at SiteA is 264,797,000 bits/sec (~265 Mbps).The output rate at SiteB matches this exact heavy stream (264,797,000 bits/sec).While the physical interface capacity is a 10Gbps link, a low reliability score alongside constant massive throughput bursts indicates that high traffic volumes and utilization on the path are overwhelming buffers, dropping packets, and driving up latency—causing the intermittent application drops.
JRossJ
👍 1
Selected: A
A is the right answer, not D. Exhibit shows that this is single mode fiber and "media type is SFP-LR".
d106915
👍 1
Selected: A
A: reliability 166/255
Fakecon
👍 1
Selected: C
I will pick C here
Portcullis
👍 3
Selected: A
This question is incredibly stupid. If we refer to the exhibit as per the instructions, A is the only logical answer. If we use our common sense and again, refer to the question, A is not a CAUSE. Although I disagree with answer A and would of chosen answer D, I have seen some official answers and when the question asks you to refer to the exhibit, always use information from the exhibit rather than extrapolating an answer. This exam often test your ability to interpret the given data, not your broader technical judgment...
1Abel1
👍 3
Selected: D
According to the output of interface0/1/0 , option A shows the problem that site A is experiencing (reliability 166/255). But the question is asking for the "CAUSE of the problem". As we know, cable related issues can cause intermittent connectivity. Although, the output also shows that the right cable was used, but, it's worth checking the cable which is likely the CAUSE instead of focusing on the output of the cause.
befa13c
👍 3
Selected: D
The most likely cause is an incorrect fiber cable type. Although both sides report using SFP-LR (which is meant for single-mode fiber), the issue began after installing a new fiber path. Site A shows degraded reliability (166/255) while Site B is fine (255/255), indicating a one-sided physical issue. This is consistent with using multi-mode fiber with single-mode optics, which can result in intermittent connectivity without fully dropping the link. Therefore, the wrong cable type is the root cause — not interface errors (which are symptoms).
ClaudeBalls
👍 1
Selected: D
What is the cause...? A just confirms an issue, not the cause. C SFPs are the same LR which you use on SM Fiber D wrong cable will connect sites but cause issues.
jojolabubu
👍 2
Selected: D
Will go for D. The question is unusually long as it insists on the single mode finer path. Using a multi mode cable would result in some packet loss and probably this degraded reliability in the stats.
Simrankoor
👍 2
Selected: A
Value of 166/255: This value translates to approximately 65% reliability. This means that the interface is experiencing some issues that affect its ability to deliver or receive packets reliably
Omooba_Adeposi
👍 1
D is AI's response. Using the appropriate SFP modules as is the case here does not explicitly imply that the right fibre cable (single-mode or multi-mode) has been used for this point to point connection.
askar430
👍 3
Selected: D
Error increments is the result of the reason, the question ask about the reason not the result or what happen , the answer D
Sublime_Cheese
👍 1
one cause will effect the interface errors and connectivity issues, therefore interface errors is not the cause. A is wrong.
caminf
👍 2
A is not correct because it shows the effect of the problem and NOT the connectivity issue cause (that is what the question requires) B is not correct because, according to the TxLoad and RxLoad values (1/255) there is a very low usage. C is not correct because the used SFP are equals both side. The correct answer is D. Probably the sites distance is 5km but are connected with an OS1 single-modal type that support 10G on maximum distance of 2km.
zahari_zaen
👍 2
physical issue
lxxxxxxxxx
👍 1
Can someone explain how its A? Thats the outcome isnt it?
zbeugene7
👍 3
Looks like a case of bad English to me, apparently operations moved out of the US.... A cause and effect are two different things, incrementing errors is an "effect" caused by some underlying issue, it can't be a "cause" by itself, incrementing errors is an equivalent to connectivity issues, it's nonsensical to define it as a cause for technician reporting it
Hope_12
👍 3
Selected: B
A is result not the cause which is the one asked in the question.
Rydaz
👍 3
txload and rxload are low... so it can't be high usage right?
VictorCisco
👍 6
Error increase is not the cause of the issue is the consequence/result of the issue!! A wrong cabel could be an issue...
gc999
👍 1
I think A is the outcome, not the cause. So answer is B
shutie
👍 3
I had had an confusing idea why A is correct, not B because of what the question says UNTIL I found rx/rxload indicatior show 1/255, which refers to very low amount of traffic you are sending/receiving. Therefore it's safely said that In/Output rate is not problematic at all.
iMo7ed
👍 2
Selected: A
A is correct
Elidor
👍 5
Selected: A
The question is written in a confusing manner. They ask what is the cause, not the PROBABLE cause. We can't know for sure through the exhibit that B is really the cause. Therefore, answer is A.
RougePotatoe
👍 15
Why aren't people reading the question? It asks what is causing the issue not can you confirm the issue. Yes the reliability is down BUT WHY IS IT DOWN? That is the question not if there is an issue with reliability. We know there is a problem with reliability the question stated it and is confirmed by the reliability counter.
splashy
👍 6
Selected: A
166/255 reliability is going down = errors increasing txload & rxload 1/255 which means low sent/transfer and received traffic = so definitely not high usage
ptfish
👍 1
I think B is a trap answer. Not sure if this is the reason, maybe the quality of the SFP connector or something else. But in the show interface command we can clearly see that there are some issues with reliability counter (reliability 166/255).
sasquatchshrimp
👍 1
Selected: B
The question is asking the cause, and the cause can very well be high volume, the other options are either potential facts or not related. I am going with B.
iGlitch
👍 3
Selected: A
A, 166/255
LordScorpius
👍 1
Selected: B
Again, not what is the problem but, what is the cause: B.
askar430
👍 1
txload and rxload are low... so it can't be high usage right? 1/255 it mean less than 1% usage
LordScorpius
👍 1
However, I really, really wanna say "Force-up" on an incompatible SFP. In the field, THAT would be the cause but, we can't know it here.
zbeugene7
👍 1
Interface errors could be the cause for whatever unknown reason, , because of problems with the interface, it’s not saying counter is the cause
jahinchains
👍 1
Selected: B
reliability does show us that there is a input output discrepancy due to high volume of frames reliability 255/255: When the input and output errors increase, they affect the reliability counter. This indicates how likely it is that a packet can be delivered or received succesfully. Reliability is calculated like this: reliability = number of packets / number of total frames. The value of 255 is the highest value meaning that the interface is very reliable at the moment. The calculation above is done every 5 minutes.
ZUMY
👍 4
A is correct Reliability 255/255 = no input/out put errors in the interface Reliability 166/255 = yes. some problem with input/output in the interface
jahinchains
👍 3
and the question is what causes it? B!
Vinarino
👍 1
"Shortly after Site-A was connected to Site-B..." What happened and why? WHY a failure occurs is sought (and the answer) = 1000 users click on the new link (to SiteB to play with the app there) simultaneously = NO BANDWIDTH / utilization / the pipe is clogged.
chrisp31
👍 3
A'' may be the indication that there is a problem, but the question asks what caused it. Im confused.
Kane002
👍 2
Reliability is high, so A is wrong. There isn't much traffic, rxload and txload are both 1/255, so B is wrong. The SFP-LR is the right device for a single mode fiber, so C is wrong. Ergo the cable must actually be incorrect, D.
Kane002
👍 1
Nevermind, A actually has a 166/255, A is correct.
Scooter96
👍 3
If you have Reliability 255/255 it means there are no input or output errors on the interface. If you have interface errors, the Reliability factor will decrease.
Zara2stra
👍 33
reliability 255/255: When the input and output errors increase, they affect the reliability counter. This indicates how likely it is that a packet can be delivered or received succesfully. Reliability is calculated like this: reliability = number of packets / number of total frames. The value of 255 is the highest value meaning that the interface is very reliable at the moment. The calculation above is done every 5 minutes. So answer A is correct.
LordScorpius
👍 3
Yes but, this definition is for "Reliability" which could be composed of unknown factors, none of which are specifically stating, "Interface Errors". Could be. What is the CAUSE? The only indication is throughput is blowing up the link. High volume of frames.
JamesDean_YouIdiots
👍 2
answer is B. This output doesn't even show the interface errors..
shanem
👍 7
I might be wrong but doesn't it clearly show both tx and rx 1/255? Site A shows reliability 166/255. That means it isn't 100% reliable, which means A is correct.
Comments & Corrections
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Expert Analysis
Why the Answer Is Correct
SiteA's interface counters report reliability 166/255 instead of the healthy 255/255 seen at SiteB, and the reliability metric on Cisco IOS is derived from successful frame delivery versus received frames, so a value below 255 means input/output errors (CRC, giants, input errors) are accumulating on that link. Because the load counters remain 1/255, the errors are not traffic-driven, and the exhibit therefore documents incrementing interface errors as the condition producing the intermittent loss to SiteB. Option A is the only statement directly evidenced by the output.Why the Other Options Are Wrong
Option B is eliminated by txload and rxload of 1/255, which represent minimal utilization, so high usage causing high latency is contradicted by the counters. Option C fails because both ends of the link show the same long-reach single-mode optic, so an incorrect SFP media type at SiteA is not present in the exhibit. Option D, a wrong cable type, is a plausible real-world physical-layer cause but the exhibit confirms a single-mode fiber path paired with LR optics, so the output does not support it; as one commenter put it, "Error increase is not the cause of the issue is the consequence/result of the issue!!"Community Comment Notes
The comments split on exam semantics. Several voters, including Elidor and splashy, argue that since the exhibit is what the question tells you to use, A is the only defensible choice, with splashy noting "166/255 reliability is going down = errors increasing txload & rxload 1/255 which means low sent/transfer and received traffic = so definitely not high usage". Portcullis acknowledges preferring D personally but warns to "always use information from the exhibit", while RougePotatoe and VictorCisco object that errors are a symptom, not a root cause, and befa13c and 1Abel1 defend D by pointing to one-sided degradation consistent with a bad fiber run. On exam day, the exhibit-based, verifiable observation (A) is the safer selection.Exam Strategy
When the exhibit shows counters, answer with what the counters prove rather than a speculative root cause you cannot verify. Use reliability 255/255 versus 166/255 to spot errors, and use load values to quickly rule out congestion-based options like high usage.
Frequently Asked Questions
Why isn't a wrong fiber cable type the correct answer when errors suggest a physical fault?
The exhibit already confirms a single-mode fiber path with matching long-reach optics at both sites, so the cable/media options are disproven by the output; only the reliability counter shows an actual fault.
What does reliability 166/255 on a Cisco interface mean compared to 255/255?
Reliability is a 255-based scale where 255/255 means error-free delivery; a value like 166/255 indicates frames are being dropped or errored, typically from incrementing input or CRC errors.
Related Analysis
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