The answer is the iBGP split-horizon rule, which prevents a router from advertising routes learned from one iBGP neighbor to another iBGP neighbor within the same autonomous system. This rule exists to avoid routing loops in a full-mesh iBGP topology, since iBGP assumes an AS is a single hop and does not decrement the AS_PATH, meaning a loop could propagate indefinitely without this safeguard. On the Cisco CCNP ENARSI 300-410 exam, this concept frequently appears in troubleshooting scenarios where an engineer expects iBGP to behave like eBGP and is surprised when routes are not forwarded between internal peers, even with reachable next-hops and established sessions. A common trap is assuming BGP will automatically re-advertise all learned routes, but the split-horizon rule is a fundamental iBGP behavior that requires either a full mesh, route reflectors, or confederations to overcome. Memory tip: think of iBGP as a "hear no evil, speak no evil" protocol—once it hears a route from an iBGP peer, it refuses to repeat it to another iBGP peer.
300-410 Network Logging and Syslog Practice Question
This 300-410 practice question tests your understanding of network logging and syslog. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 engineer configures iBGP between two routers in the same AS. The engineer notices that routes learned from one iBGP neighbor are not being advertised to another iBGP neighbor, even though the next-hop is reachable. The engineer verifies that the BGP session is established and that the routes are present in the BGP table. Which is the most likely explanation?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
iBGP split-horizon rule prevents advertising iBGP-learned routes to other iBGP neighbors
By default, iBGP routers do not advertise routes learned from one iBGP neighbor to another iBGP neighbor because of the split-horizon rule. This is a fundamental behavior to prevent routing loops, but it often catches engineers who expect iBGP to behave like eBGP. To overcome this, a route reflector or confederation must be used.
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.
✓
iBGP split-horizon rule prevents advertising iBGP-learned routes to other iBGP neighbors
Why this is correct
Correct. iBGP does not advertise routes learned from one iBGP neighbor to another iBGP neighbor by default.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Next-hop-self is typically used for eBGP or when the next-hop is not reachable via IGP, but it is not the cause of this issue.
✗
The BGP synchronization rule is enabled
Why it's wrong here
Synchronization would prevent routes from being used if not present in IGP, but it does not prevent advertisement between iBGP neighbors.
✗
The routes are not valid because the next-hop is not reachable
Why it's wrong here
The engineer stated the next-hop is reachable, so this is not the issue.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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 300-410 OSPF questions on adjacency and route selection.
Network Logging and Syslog — This question tests Network Logging and Syslog — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: iBGP split-horizon rule prevents advertising iBGP-learned routes to other iBGP neighbors — By default, iBGP routers do not advertise routes learned from one iBGP neighbor to another iBGP neighbor because of the split-horizon rule. This is a fundamental behavior to prevent routing loops, but it often catches engineers who expect iBGP to behave like eBGP. To overcome this, a route reflector or confederation must be used.
What should I do if I get this 300-410 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 300-410 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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