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Route Maps and Route Filtering →hardMultiple ChoiceObjective-mapped

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 administrator is troubleshooting BGP path selection for a dual-homed enterprise. Router R1 (AS 65001) has the following configuration: route-map SET-LOCAL-PREF permit 10 match ip address prefix-list PREFER set local-preference 200. Router R2 (AS 65002) shows: 'show bgp ipv4 unicast 10.1.1.0' has two paths: one via R1 with local pref 200, and one via R3 with local pref 100. However, 'show ip route 10.1.1.0' on R2 shows the path via R3 is installed. What is the root cause?

A
The route-map SET-LOCAL-PREF is applied outbound on R1 instead of inbound on R2, so R2 does not receive the modified local preference.
Why wrong: Incorrect because the show bgp output on R2 shows local preference 200 for the path via R1, which can only be set by an inbound route-map on R2. An outbound route-map on R1 would not affect R2's local preference value, as local preference is not transitive in eBGP.
B
The prefix-list PREFER is missing a permit statement for 10.1.1.0/24.
Why wrong: Incorrect because if the prefix-list PREFER did not match 10.1.1.0/24, the local preference would not be set (default 100), but the output shows 200, proving the route did match.
C
The route via R3 has a lower MED value, overriding local preference.
Why wrong: Incorrect because local preference is compared before MED in BGP path selection. Even if the route via R3 has a lower MED, the route via R1 with higher local preference (200) would be preferred unless the path is invalid.
D
The route via R1 is not synchronized with IGP, causing it to be considered invalid.
Correct because a BGP route that is not synchronized with the IGP (or has an unreachable next-hop) is considered invalid and is not installed in the routing table. This causes R2 to use the path via R3 despite its lower local preference.

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 via R1 is not synchronized with IGP, causing it to be considered invalid.

The show bgp output confirms that R2 receives a local preference of 200 for the path via R1, indicating that the route-map was applied inbound on R2 correctly. However, the path is not installed in the routing table because BGP considers it invalid. The most plausible reason from the options is that the route via R1 is not synchronized with the IGP (e.g., the next-hop is not reachable via an IGP route, or BGP synchronization is enabled and the route is not present in the IGP). This invalidity causes BGP to prefer the next best path, which is the one via R3 with local preference 100.

Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.

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 SET-LOCAL-PREF is applied outbound on R1 instead of inbound on R2, so R2 does not receive the modified local preference.
Why it's wrong here
Incorrect because the show bgp output on R2 shows local preference 200 for the path via R1, which can only be set by an inbound route-map on R2. An outbound route-map on R1 would not affect R2's local preference value, as local preference is not transitive in eBGP.
✗
The prefix-list PREFER is missing a permit statement for 10.1.1.0/24.
Why it's wrong here
Incorrect because if the prefix-list PREFER did not match 10.1.1.0/24, the local preference would not be set (default 100), but the output shows 200, proving the route did match.
✗
The route via R3 has a lower MED value, overriding local preference.
Why it's wrong here
Incorrect because local preference is compared before MED in BGP path selection. Even if the route via R3 has a lower MED, the route via R1 with higher local preference (200) would be preferred unless the path is invalid.
✓
The route via R1 is not synchronized with IGP, causing it to be considered invalid.
Why this is correct
Correct because a BGP route that is not synchronized with the IGP (or has an unreachable next-hop) is considered invalid and is not installed in the routing table. This causes R2 to use the path via R3 despite its lower local preference.
Related concept
Standard ACLs match source addresses.

Common exam traps

Common exam trap: ACLs stop at the first match

ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.

Trap categories for this question

Command / output trap
Incorrect because the show bgp output on R2 shows local preference 200 for the path via R1, which can only be set by an inbound route-map on R2. An outbound route-map on R1 would not affect R2's local preference value, as local preference is not transitive in eBGP.

Detailed technical explanation

How to think about this question

ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.

KKey Concepts to Remember

Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.

TExam Day Tips

Check inbound versus outbound direction.
Read the ACL from top to bottom.
Look for a broader permit or deny above the intended line.

Key takeaway

ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.

Real-world example

How this comes up in practice

A security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.

What to study next

Got this wrong? Here's your next step.

Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.

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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 — Standard ACLs match source addresses..

What is the correct answer to this question?

The correct answer is: The route via R1 is not synchronized with IGP, causing it to be considered invalid. — The show bgp output confirms that R2 receives a local preference of 200 for the path via R1, indicating that the route-map was applied inbound on R2 correctly. However, the path is not installed in the routing table because BGP considers it invalid. The most plausible reason from the options is that the route via R1 is not synchronized with the IGP (e.g., the next-hop is not reachable via an IGP route, or BGP synchronization is enabled and the route is not present in the IGP). This invalidity causes BGP to prefer the next best path, which is the one via R3 with local preference 100.

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

What is the key concept behind this question?

Standard ACLs match source addresses.

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