350-601 Network Practice Question
An engineer is designing a spine-leaf fabric using eBGP for the underlay. The spine switches are in AS 65000 and each leaf switch uses a different private AS from 65001 to 65010. For optimal convergence and to avoid BGP path hunting, which BGP feature should be configured on the spines?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
allowas-in 1
In a BGP underlay with a common spine AS (65000) and unique leaf AS, each leaf may receive routes from multiple spines. Without 'allowas-in', when a spine receives a route from a leaf that already contains the spine's own AS in the AS_PATH (because another spine previously advertised it), the spine would reject the update due to AS loop detection. Configuring 'allowas-in 1' on spines allows them to accept routes containing their own AS once, enabling them to consider all available paths and avoid path hunting. Options B, C, and D are incorrect: 'disable-peer-as-check' would ignore AS checks altogether, potentially causing loops; 'multipath-relax' allows multipath with different AS_PATHs but doesn't address the path-hunting issue; 'maxas-limit' limits the maximum AS number but is not relevant to this scenario.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
allowas-in 1
Why this is correct
Allowas-in 1 permits the spine to accept updates containing its own AS once, which is needed when leaf uses spine AS as part of AS path.
- ✗
disable-peer-as-check
Why it's wrong here
This disables AS path validation but is insufficient for loop prevention.
- ✗
bestpath as-path multipath-relax
Why it's wrong here
For load balancing, not for AS path issue.
- ✗
maxas-limit 10
Why it's wrong here
Limits AS path length but does not solve receipt of own AS.
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