300-410 Practice Question: In BGP, what is the effect of using a route-map…

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

  Adding to existing communities requires the additive keyword; without it, the specified community replaces the current attribute entirely. It is tempting because set community is commonly used to tag routes, and additive would be correct when existing community values must be preserved alongside new ones.

• ✓

  The existing communities are overwritten by the specified community.

  Why this is correct

  Without the additive keyword, the set community command replaces the entire community attribute, discarding any communities already attached to the prefix. This satisfies the stem's scenario by overwriting existing values with only those specified, rather than appending them.

• ✗

  The community attribute is not modified; the set command is ignored.

  Why it's wrong here

  The set command without additive overwrites existing communities with the specified value; it is not ignored. It is tempting because additive is what preserves existing values, so omitting it appears to nullify the command, but the actual behaviour is replacement, not suppression.

• ✗

  The route is denied if it already has communities.

  Why it's wrong here

  A set community without additive replaces the attribute; it never denies the route. Denial requires a route-map match clause with a deny action. The option is tempting because communities are often filtered, so a deny outcome feels plausible, but that filtering occurs elsewhere, not through the set command itself.

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