H12-821_V1.0 · Question #765
Route aggregation in BGP will cause the AS_Path attribute to be lost, which may cause the risk of loops.
The correct answer is A. TRUE. A (TRUE) is correct because when BGP performs route aggregation, the resulting summary route's AS_PATH attribute defaults to containing only the aggregating router's AS number, discarding the AS_PATH information of all contributing (more-specific) routes. Since AS_PATH is BGP's…
Question
Options
- ATRUE
- BFALSE
How the community answered
(47 responses)- A83% (39)
- B17% (8)
Explanation
A (TRUE) is correct because when BGP performs route aggregation, the resulting summary route's AS_PATH attribute defaults to containing only the aggregating router's AS number, discarding the AS_PATH information of all contributing (more-specific) routes. Since AS_PATH is BGP's primary loop-prevention mechanism - routers reject routes already containing their own AS number - losing this history means a downstream AS could re-advertise the aggregate back toward its origin, creating a routing loop.
B (FALSE) is wrong because the risk is real and well-documented; this is not a theoretical concern. The as-set option on the aggregate-address command exists precisely to mitigate this by preserving the AS_PATH information of contributing routes in the aggregate.
Memory tip: Think of AS_PATH as a "breadcrumb trail" through the internet. Aggregation can erase those breadcrumbs - without them, BGP has no way to know it's walking in circles. The keyword as-set is your way of saying "keep the crumbs."
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