The answer is that the neighbor is missing the 'send-community extended' command under the address-family ipv4 unicast. This is because BGP-SR label advertisement relies on the BGP Prefix-SID attribute, which is encoded as an extended community; without this command, the PE router cannot transmit the label information to the CE, even if a route-policy like SET-LABEL is correctly applied. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding that Segment Routing for BGP extends the traditional BGP label distribution mechanism, and the extended community is the mandatory carrier for the label. A common trap is assuming the route-policy alone suffices, but the 'send-community extended' is the prerequisite for the label to be advertised. Memory tip: think "Prefix-SID rides the extended community—no community, no ride."
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
Refer to the exhibit.
router bgp 65000
neighbor 10.0.0.1 remote-as 65001
neighbor 10.0.0.1 update-source Loopback0
!
address-family vpnv4
neighbor 10.0.0.1 activate
neighbor 10.0.0.1 send-community extended
exit-address-family
!
route-policy SET-LABEL
if destination in (10.1.1.0/24) then
set label-index 100
endif
end-policy
Refer to the exhibit. An engineer is configuring Segment Routing for BGP (BGP-SR) on a PE router to assign labels to prefixes learned from a CE. The route-policy SET-LABEL is applied to the neighbor under the address-family ipv4 unicast. However, the CE prefix 10.1.1.0/24 is not receiving the label. What is the most likely reason?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Refer to the exhibit.
router bgp 65000
neighbor 10.0.0.1 remote-as 65001
neighbor 10.0.0.1 update-source Loopback0
!
address-family vpnv4
neighbor 10.0.0.1 activate
neighbor 10.0.0.1 send-community extended
exit-address-family
!
route-policy SET-LABEL
if destination in (10.1.1.0/24) then
set label-index 100
endif
end-policy
A
The route-policy syntax is incorrect; 'destination' should be 'ip prefix-list'.
Why wrong: The 'destination' condition in route-policy is valid and can match a prefix directly.
B
The update-source should be the interface facing the CE, not Loopback0.
Why wrong: The update-source Loopback0 is appropriate for BGP sessions between PEs, but for a CE-facing session, the direct interface should be used; however, this would not prevent label assignment, only session establishment.
C
The neighbor is missing 'send-community extended' under address-family ipv4 unicast.
BGP-SR uses the prefix-SID attribute carried in extended communities; without this, labels are not advertised.
D
The 'set label-index' command should be 'set label' for BGP-SR.
Why wrong: In BGP-SR, 'set label-index' is the correct command to assign a label index for a prefix-SID.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The neighbor is missing 'send-community extended' under address-family ipv4 unicast.
For BGP-SR (Segment Routing for BGP) to advertise labels for prefixes learned from a CE, the neighbor must be configured with 'send-community extended' under the address-family. This is because BGP-SR uses the BGP Prefix-SID attribute, which is carried as an extended community. Without this command, the PE will not send the label information to the CE, even if the route-policy is correctly applied.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The route-policy syntax is incorrect; 'destination' should be 'ip prefix-list'.
Why it's wrong here
The 'destination' condition in route-policy is valid and can match a prefix directly.
✗
The update-source should be the interface facing the CE, not Loopback0.
Why it's wrong here
The update-source Loopback0 is appropriate for BGP sessions between PEs, but for a CE-facing session, the direct interface should be used; however, this would not prevent label assignment, only session establishment.
✓
The neighbor is missing 'send-community extended' under address-family ipv4 unicast.
Why this is correct
BGP-SR uses the prefix-SID attribute carried in extended communities; without this, labels are not advertised.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The 'set label-index' command should be 'set label' for BGP-SR.
Why it's wrong here
In BGP-SR, 'set label-index' is the correct command to assign a label index for a prefix-SID.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the requirement for 'send-community extended' in BGP-SR scenarios, as candidates may focus on route-policy syntax or interface settings and overlook the mandatory community advertisement needed to carry the label attribute.
Trap categories for this question
Command / output trap
In BGP-SR, 'set label-index' is the correct command to assign a label index for a prefix-SID.
Detailed technical explanation
How to think about this question
BGP-SR leverages the BGP Prefix-SID attribute (RFC 8669) to advertise labels for prefixes. This attribute is encoded as a BGP extended community (Type 0x0005 or 0x8005). Without 'send-community extended', the BGP update will omit all extended communities, including the Prefix-SID, so the CE never receives the label binding. In real-world deployments, this is a common misconfiguration when enabling SR on inter-AS or CE-PE links.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The neighbor is missing 'send-community extended' under address-family ipv4 unicast. — For BGP-SR (Segment Routing for BGP) to advertise labels for prefixes learned from a CE, the neighbor must be configured with 'send-community extended' under the address-family. This is because BGP-SR uses the BGP Prefix-SID attribute, which is carried as an extended community. Without this command, the PE will not send the label information to the CE, even if the route-policy is correctly applied.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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