350-501 Services Practice Question
An SP is designing a MPLS L3VPN service with BGP as the PE-CE routing protocol. They want to ensure that the CE router does not become a transit router between two sites. Which BGP feature should be configured on the PE to prevent the CE from advertising routes received from one site to another?
⚠ Common exam trap
Cisco often tests the confusion between SoO and as-override, as both deal with BGP updates in MPLS L3VPN, but as-override modifies the AS_PATH to allow route acceptance, while SoO prevents route redistribution to avoid transit routing.
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
✓
Site of Origin (SoO)
The Site of Origin (SoO) BGP community is the correct feature because it prevents routing loops in MPLS L3VPN environments where the same CE router connects to multiple sites. When configured on the PE, SoO tags routes received from a CE with a unique identifier for that site; if the PE receives a route with the same SoO value from another site, it discards the route, ensuring the CE does not become a transit router between sites.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Site of Origin (SoO)
Why this is correct
SoO marks routes so that a CE will ignore routes with its own SoO, preventing transit.
- ✗
allowas-in
Why it's wrong here
allowas-in allows the CE to receive routes with its own AS, which could cause loops.
- ✗
disable-connected-check
Why it's wrong here
disable-connected-check allows eBGP peering over loopback without IGP route, unrelated.
- ✗
as-override
Why it's wrong here
as-override replaces the CE's AS number with a dummy, used when ASN duplication occurs, not for transit prevention.
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