Question 1,394 of 2,152

Route Maps and Route Filtering →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the route-map BLOCK_RFC1918 is not applied to the BGP neighbor, so the RFC 1918 routes are being advertised despite the prefix-list configuration. This is the most likely issue because the show ip bgp neighbors advertised-routes output reveals that 10.10.10.0/24 and 10.20.20.0/24 are still being sent, while the route-map and prefix-list are correctly defined to deny those private addresses. The route-map itself is inactive until it is explicitly attached to a neighbor using the neighbor route-map command under the BGP address-family; without that application, the filtering logic never runs. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that BGP outbound filtering requires the route-map to be bound to the neighbor—a common trap is assuming a defined route-map automatically takes effect. A useful memory tip: “Define it, then bind it—or the routes will fly unhindered.”

300-410 Route Maps and Route Filtering Practice Question

This 300-410 practice question tests your understanding of route maps and route filtering. 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 network engineer runs the following command on Router R1:

R1# show ip bgp neighbors 10.1.1.2 advertised-routes

BGP table version is 10, local router ID is 10.1.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete

Network Next Hop Metric LocPrf Weight Path *> 10.10.10.0/24 0.0.0.0 0 32768 i *> 10.20.20.0/24 0.0.0.0 0 32768 i

R1# show route-map

route-map BLOCK_RFC1918, permit, sequence 10 Match clauses:

ip address prefix-list RFC1918

Set clauses: Policy routing matches: 0 packets, 0 bytes route-map BLOCK_RFC1918, deny, sequence 20 Match clauses: Set clauses: Policy routing matches: 0 packets, 0 bytes

R1# show ip prefix-list RFC1918
ip prefix-list RFC1918: 3 entries

seq 5 deny 10.0.0.0/8 le 32 seq

10 deny 172.16.0.0/12 le 32

seq

15 deny 192.168.0.0/16 le 32

seq

20 permit 0.0.0.0/0 le 32

Based on this output, what is the most likely issue?

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.
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.

Question 1hardmultiple choice

Open the full BGP breakdown →

A
The route-map BLOCK_RFC1918 is not applied to the BGP neighbor, so RFC 1918 routes are being advertised.
The prefix-list denies RFC 1918, but the route-map is not applied to the neighbor, so routes are advertised.
B
The prefix-list RFC1918 is misconfigured and permits all routes.
Why wrong: The prefix-list denies RFC 1918 and permits others.
C
The route-map is applied but the match clause is incorrect.
Why wrong: The match clause references the prefix-list correctly.
D
The BGP session is not established.
Why wrong: Routes are being advertised, so session is up.

Full breakdown with real-world context →

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 BLOCK_RFC1918 is not applied to the BGP neighbor, so RFC 1918 routes are being advertised.

The route-map BLOCK_RFC1918 is applied to filter routes, but the advertised routes include 10.10.10.0/24 and 10.20.20.0/24, which are RFC 1918 addresses. The prefix-list RFC1918 should deny these, but the route-map is not being applied to the BGP neighbor, so routes are not filtered. The correct answer is that the route-map is not applied to the BGP neighbor.

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.

✓
The route-map BLOCK_RFC1918 is not applied to the BGP neighbor, so RFC 1918 routes are being advertised.
Why this is correct
The prefix-list denies RFC 1918, but the route-map is not applied to the neighbor, so routes are advertised.
Clue confirmation
The clue words "best", "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The prefix-list RFC1918 is misconfigured and permits all routes.
Why it's wrong here
The prefix-list denies RFC 1918 and permits others.
✗
The route-map is applied but the match clause is incorrect.
Why it's wrong here
The match clause references the prefix-list correctly.
✗
The BGP session is not established.
Why it's wrong here
Routes are being advertised, so session is up.

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 300-410 OSPF questions on adjacency and route selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

Route Maps and Route Filtering — This question tests Route Maps and Route Filtering — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The route-map BLOCK_RFC1918 is not applied to the BGP neighbor, so RFC 1918 routes are being advertised. — The route-map BLOCK_RFC1918 is applied to filter routes, but the advertised routes include 10.10.10.0/24 and 10.20.20.0/24, which are RFC 1918 addresses. The prefix-list RFC1918 should deny these, but the route-map is not being applied to the BGP neighbor, so routes are not filtered. The correct answer is that the route-map is not applied to the BGP neighbor.

What should I do if I get this 300-410 question wrong?

Are there clue words in this question I should notice?

Yes — watch for: "best", "most likely". 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.

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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Same concept, more angles

1 more ways this is tested on 300-410

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. A network engineer runs the following command on Router R1: R1# show ip bgp neighbors 192.168.1.2 advertised-routes BGP table version is 14, local router ID is 10.1.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 10.10.10.0/24 192.168.1.2 0 0 65001 i *> 10.20.20.0/24 192.168.1.2 0 0 65001 i Total number of prefixes 2 Based on this output, what is a likely issue?

hard

✓ A.The neighbor 192.168.1.2 is not receiving all routes because a route-map is filtering them.
B.The BGP session is down because there are no prefixes advertised.
C.The router is not receiving updates from its peers.
D.The prefix-list is misconfigured and blocking all routes.

Why A: The output shows only two prefixes advertised to the neighbor, but the network likely has more routes. The route-map applied to the neighbor may be filtering routes. The correct answer identifies that a route-map is likely filtering out other prefixes.

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