The answer is the Site of Origin (SoO) BGP extended community. SoO prevents a CE router from becoming a transit router in an MPLS L3VPN by tagging each route with a unique identifier for its originating site; when the CE receives a route carrying its own SoO value, it discards the advertisement, effectively blocking inter-site transit through the CE. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of loop prevention in multi-homed PE-CE designs, where misconfiguration can cause the CE to inadvertently forward traffic between VPN sites. A common trap is confusing SoO with allowas-in or as-override, which solve different problems (AS-path duplication), not transit prevention. Remember the memory tip: SoO stands for “Site of Origin” – think “Same Origin, Shut Out” – if the CE sees its own tag, it drops the route, keeping it from acting as a transit hop.
350-501 Services Practice Question
This 350-501 practice question tests your understanding of services. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
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)
Option D is correct because disabling route propagation (e.g., using 'neighbor x.x.x.x next-hop-self' or 'no bgpdefault route-advertise' but the key is to prevent the CE from learning other CE routes; typically on the PE, you set 'neighbor CE next-hop-self' and also ensure that the CE only receives routes from its own site. However, the exact feature is to use 'neighbor CE soft-reconfiguration inbound' plus filters, but specifically, 'disable-optimal-route-splitting' is not standard. Actually, the standard is to use 'neighbor CE prefix-list' to filter outgoing routes, but among options, 'disable-route-propagation' is a Cisco feature that prevents routes from being sent back to the same BGP AS. Alternatively, 'allowas-in' would allow duplicates. The correct answer is to not allow routes from other sites to be sent to the CE; typically you use 'neighbor CE route-map' to filter. Option D is correct because 'neighbor x.x.x.x disable-connected-check' is unrelated. Wait, checking options: A: allowas-in, B: as-override, C: soo, D: disable-connected-check. Actually, SoO (Site of Origin) is used to prevent loops in multi-homed BGP, but the question asks to prevent the CE from becoming a transit. With SoO, the PE marks routes with a SoO extended community, and the CE can be prevented from advertising routes back if it sees its own SoO. But SoO is not the primary method; best practice is to set next-hop-self on the PE and not advertise other site routes. However, among given options, SoO is the correct one. Let's rethink: Option C (SoO) is used to prevent routing loops by adding a unique SoO value per site; when the CE receives a route with its own SoO, it drops it. Thus it prevents a CE from learning routes from other sites. So Option C is correct. Explanation: SoO prevents the CE from accepting routes that originated from its own site, thus it cannot become a transit.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 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.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
Services — This question tests Services — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Site of Origin (SoO) — Option D is correct because disabling route propagation (e.g., using 'neighbor x.x.x.x next-hop-self' or 'no bgp default route-advertise' but the key is to prevent the CE from learning other CE routes; typically on the PE, you set 'neighbor CE next-hop-self' and also ensure that the CE only receives routes from its own site. However, the exact feature is to use 'neighbor CE soft-reconfiguration inbound' plus filters, but specifically, 'disable-optimal-route-splitting' is not standard. Actually, the standard is to use 'neighbor CE prefix-list' to filter outgoing routes, but among options, 'disable-route-propagation' is a Cisco feature that prevents routes from being sent back to the same BGP AS. Alternatively, 'allowas-in' would allow duplicates. The correct answer is to not allow routes from other sites to be sent to the CE; typically you use 'neighbor CE route-map' to filter. Option D is correct because 'neighbor x.x.x.x disable-connected-check' is unrelated. Wait, checking options: A: allowas-in, B: as-override, C: soo, D: disable-connected-check. Actually, SoO (Site of Origin) is used to prevent loops in multi-homed BGP, but the question asks to prevent the CE from becoming a transit. With SoO, the PE marks routes with a SoO extended community, and the CE can be prevented from advertising routes back if it sees its own SoO. But SoO is not the primary method; best practice is to set next-hop-self on the PE and not advertise other site routes. However, among given options, SoO is the correct one. Let's rethink: Option C (SoO) is used to prevent routing loops by adding a unique SoO value per site; when the CE receives a route with its own SoO, it drops it. Thus it prevents a CE from learning routes from other sites. So Option C is correct. Explanation: SoO prevents the CE from accepting routes that originated from its own site, thus it cannot become a transit.
What should I do if I get this 350-501 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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