The answer is that using a route-map with a set community command but without the additive keyword overwrites the existing community attributes on the route. This happens because BGP treats the community attribute as a single, replaceable value unless you explicitly instruct it to preserve the existing list. Without additive, the router discards any previous community strings—whether standard, extended, or large—and applies only the new community you specify. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of route-map manipulation and is a common trap: many candidates mistakenly assume communities are always appended. The exam often presents a scenario where a route already has a community like 100:100, and a route-map sets 200:200 without additive; the result is that 100:100 is lost entirely. A reliable memory tip is to think of "additive" as "add to it"—without it, you are "overwriting it."
300-410 Route Maps and Route Filtering Practice Question
This 300-410 practice question tests your understanding of route maps and route filtering. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
In BGP, what is the effect of using a route-map with a set community command but without the additive keyword?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The existing communities are overwritten by the specified community.
When set community is used without the additive keyword, it replaces any existing community attributes on the route with the specified community. With the additive keyword, the specified community is added to the existing communities.
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 specified community is added to the existing communities.
Why it's wrong here
Incorrect. That is the behavior with the additive keyword.
✓
The existing communities are overwritten by the specified community.
Why this is correct
Correct. Without additive, set community replaces the community attribute.
Related concept
Read the scenario before looking for a memorised answer.
✗
The community attribute is not modified; the set command is ignored.
Why it's wrong here
Incorrect. The set command is applied; it replaces the community.
✗
The route is denied if it already has communities.
Why it's wrong here
Incorrect. The route is not denied; the community is simply replaced.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Trap categories for this question
Keyword trap
Incorrect. That is the behavior with the additive keyword.
Command / output trap
Incorrect. The set command is applied; it replaces the community.
Detailed technical explanation
How to think about this question
This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.
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.
Use explanations to understand the rule behind the answer.
TExam Day Tips
→Underline the problem statement mentally.
→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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
Route Maps and Route Filtering — This question tests Route Maps and Route Filtering — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The existing communities are overwritten by the specified community. — When set community is used without the additive keyword, it replaces any existing community attributes on the route with the specified community. With the additive keyword, the specified community is added to the existing communities.
What should I do if I get this 300-410 question wrong?
Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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