Quick Answer
The answer is to use iBGP with route reflectors. This approach minimizes routing table size on core routers by eliminating the need for a full iBGP mesh, as route reflectors allow a single router to propagate prefixes to all clients without each router needing a direct session to every other router. In the context of the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this question tests your understanding of BGP scaling techniques within a service provider core carrying both IPv4 and IPv6 traffic, where maintaining full reachability without overwhelming router resources is critical. A common trap is confusing eBGP with iBGP for internal route propagation, or assuming OSPF can handle external BGP routes efficiently. Remember the memory tip: “Reflect to protect your table”—route reflectors reflect routes to reduce session count, protecting the core from a bloated routing table.
This 350-501 practice question tests your understanding of architecture. 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.
A service provider is designing its core network to carry both IPv4 and IPv6 traffic. Which BGP approach best minimizes routing table size on core routers while maintaining full reachability?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Use iBGP with route reflectors
Reduces BGP sessions and maintains full reachability efficiently.
Use eBGP between all core routers
Why wrong: eBGP full mesh creates high session count and administrative overhead.
Use static routing for all prefixes
Why wrong: Static routing does not scale for full internet routing tables.
Rely on OSPF for external routes
Why wrong: OSPF is an IGP and not designed to carry external BGP routes.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Use iBGP with route reflectors
Option C is correct because using an iBGP full mesh with route reflectors reduces the number of BGP sessions and allows efficient prefix propagation. Option A is wrong because eBGP between all core routers creates a full mesh with high session count and administrative overhead. Option B is wrong because static routing cannot scale for the full internet table. Option D is wrong because OSPF is not designed to carry external BGP routes efficiently.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Use iBGP with route reflectors
Use eBGP between all core routers
Why it's wrong here
eBGP full mesh creates high session count and administrative overhead.
Use static routing for all prefixes
Why it's wrong here
Static routing does not scale for full internet routing tables.
Rely on OSPF for external routes
Common exam traps
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
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.
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
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
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.
Architecture — study guide chapter
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FAQ
Architecture — This question tests Architecture — OSPF neighbours must agree on key parameters..
The correct answer is: Use iBGP with route reflectors — Option C is correct because using an iBGP full mesh with route reflectors reduces the number of BGP sessions and allows efficient prefix propagation. Option A is wrong because eBGP between all core routers creates a full mesh with high session count and administrative overhead. Option B is wrong because static routing cannot scale for the full internet table. Option D is wrong because OSPF is not designed to carry external BGP routes efficiently.
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.
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 24, 2026
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