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300-410 Practice Question: Is troubleshooting IPv6 BGP path selection on…
A network engineer is troubleshooting IPv6 BGP path selection on Router R1. Router R1 is receiving a prefix from two different BGP peers, but it is not selecting the expected best path. Router R1 has the following relevant configuration:
router bgp 65000
address-family ipv6 unicast
neighbor 2001:DB8:1::2 route-map SET_LOCAL_PREF in neighbor 2001:DB8:2::2 route-map SET_MED in
! route-map SET_LOCAL_PREF permit 10 set local-preference 200 ! route-map SET_MED permit 10 set metric 50 ! The output of show bgp ipv6 unicast 2001:DB8:3::/64 on Router R1 indicates that the path from 2001:DB8:1::2 has local preference 200, but the path from 2001:DB8:2::2 is selected. What is the root cause?
⚠ Common exam trap
Candidates may jump to later BGP attributes such as MED or IGP metric to next-hop. However, local preference is evaluated first and dominates those attributes. If the expected local-preference change is not reflected in the BGP table, the route-map direction or application is the likely fault.
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
✓
The route-map SET_LOCAL_PREF is not actually affecting the received prefix (for example, it is applied in the wrong direction or the neighbor is not matched), so the higher local preference is never set.
Local preference is the first BGP path-selection attribute compared after weight, and it is evaluated before AS path, origin, MED, and IGP metric to next-hop. The route-map SET_LOCAL_PREF is configured with 'set local-preference 200' and applied inbound to neighbor 2001:DB8:1::2. If it were taking effect, that path would have local preference 200 and would be preferred over the other path (default local preference 100). Since the path from 2001:DB8:2::2 is selected instead, the local-preference route-map is not actually affecting the received prefix. The most consistent cause is that the route-map is misapplied or otherwise ineffective, so the higher local preference is never set. IGP metric to next-hop (option D) is only considered much later in the decision process, after local preference, and cannot override a higher local preference.
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-map SET_LOCAL_PREF is not actually affecting the received prefix (for example, it is applied in the wrong direction or the neighbor is not matched), so the higher local preference is never set.
Why this is correct
Incorrect. The configuration shows the route-map is applied inbound, so it does affect the received prefix. The path from that neighbor has local pref 200, but it is not selected, indicating another factor.
- ✗
The MED value of 50 is lower than the default, causing it to be preferred over local preference.
Why it's wrong here
Incorrect. MED is compared after local preference; a higher local preference (200) should be preferred over a lower MED.
- ✗
The prefix is not being advertised by the neighbor with the higher local preference route-map.
Why it's wrong here
Incorrect. The show output indicates the prefix is advertised by the neighbor with higher local pref, so this is not the issue.
- ✗
The BGP table has a route from an iBGP peer with a lower IGP metric to the next-hop, overriding the local preference.
Why it's wrong here
Correct. In BGP, if the path from 2001:DB8:2::2 is from an iBGP peer with a lower IGP metric to the next-hop, and the eBGP path with local pref 200 is not selected, it may be because the eBGP path's next-hop is unreachable or the local pref is not effective. However, given the options, D is the most plausible: the iBGP path's lower IGP metric overrides the local preference when the eBGP path has a higher IGP metric or is not valid. Note: In standard BGP path selection, local pref is compared first, so this scenario requires that the local pref is not actually applied or the eBGP path is not considered.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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