Question 1,739 of 2,152

MPLS Operations →hardMultiple ChoiceObjective-mapped

VRF Route Leaking Asymmetric — Route Target Mismatch | Cisco CCNP ENARSI 300-410 Explained

This 300-410 practice question tests your understanding of mpls operations. 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 VRF route leaking configuration causes unexpected reachability. Router R1 has VRF RED and VRF BLUE. Configuration: 'ip vrf RED rd 100:1 route-target export 100:1 route-target import 100:2' and 'ip vrf BLUE rd 100:2 route-target export 100:2 route-target import 100:1'. Router R2 is a route reflector with 'address-family ipv4 vrf RED' and 'address-family ipv4 vrf BLUE'. A host in VRF RED can ping a host in VRF BLUE, but not vice versa. What is the root cause?

A
The route reflector lacks 'address-family ipv4 vrf' configuration for both VRFs, causing incomplete route propagation.
Why wrong: It is configured as per scenario.
B
The VRF BLUE has a static route pointing to a next-hop that is not reachable from VRF RED, causing asymmetric reachability.
If VRF BLUE imports routes from RED but uses a static route for the return path that is not leaked, return traffic fails.
C
The route-target import/export values are reversed; they should be identical for both VRFs.
Why wrong: The configuration is correct for bidirectional leaking.
D
MPLS LDP is not enabled on the interfaces between routers, preventing label switching.
Why wrong: LDP is not directly related to VRF route leaking.

Full breakdown with real-world context →

Quick Answer

The root cause is a next-hop unreachable issue in VRF BLUE, not a route-target mismatch. While the route-target import and export statements appear symmetric—VRF RED exports 100:1 and imports 100:2, while VRF BLUE exports 100:2 and imports 100:1—this configuration actually allows bidirectional route leaking, so the communities themselves are correctly matched. The asymmetry arises because VRF BLUE has a static route pointing to a next-hop that is not reachable from VRF RED, causing the return traffic to fail. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that VRF route leaking asymmetric behavior is often due to recursive routing failures or missing VRF interfaces in the route reflector’s global table, not just the route-target values. A common trap is assuming that if one direction works, the route-target configuration must be correct; in reality, always verify next-hop reachability in both VRFs. Memory tip: “Leak both ways, but check the next-hop’s gaze.”

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 VRF BLUE has a static route pointing to a next-hop that is not reachable from VRF RED, causing asymmetric reachability.

The correct answer is B. For a host in VRF RED to ping a host in VRF BLUE, the route from RED to BLUE must work, and the return route from BLUE to RED must also work. In this scenario, VRF BLUE has a static route pointing to a next-hop that is not reachable from VRF RED. When the BLUE host tries to respond to the ping, it uses that static route to reach the RED host's subnet. Since the next-hop is unreachable (e.g., missing interface, wrong IP, or not present in the global or VRF routing table), the return traffic fails. This causes asymmetric reachability: RED can reach BLUE (via some other path, possibly the route-leaking mechanism), but BLUE cannot reach RED. The route reflector configuration is not the issue because the route-target import/export values are correctly matched for bidirectional leaking. Therefore, the static route misconfiguration is the root cause.

Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

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 reflector lacks 'address-family ipv4 vrf' configuration for both VRFs, causing incomplete route propagation.
Why it's wrong here
It is configured as per scenario.
✓
The VRF BLUE has a static route pointing to a next-hop that is not reachable from VRF RED, causing asymmetric reachability.
Why this is correct
If VRF BLUE imports routes from RED but uses a static route for the return path that is not leaked, return traffic fails.
Related concept
CIDR notation defines the prefix length.
✗
The route-target import/export values are reversed; they should be identical for both VRFs.
Why it's wrong here
The configuration is correct for bidirectional leaking.
✗
MPLS LDP is not enabled on the interfaces between routers, preventing label switching.
Why it's wrong here
LDP is not directly related to VRF route leaking.

Common exam traps

Common exam trap: usable hosts are not the same as total addresses

Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.

Trap categories for this question

Scenario analysis trap
It is configured as per scenario.

Detailed technical explanation

How to think about this question

Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.

KKey Concepts to Remember

CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.

TExam Day Tips

Write the block size before choosing the subnet.
Check whether the question asks for hosts, subnets or a specific address range.
Do not confuse /24, /25, /26 and /27 host counts.

Key takeaway

Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.

Real-world example

How this comes up in practice

A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.

Visual reference

Quick reference

Asymmetric Encryption Algorithm Comparison

Algorithm	Key Exchange	Signatures	Equivalent Security Key	Notes
RSA-3072	Yes	Yes	128-bit	Widely deployed; slow for bulk data
ECDSA P-256	No	Yes	128-bit	Fast signatures; standard TLS certs
ECDH / ECDHE	Yes	No	128-bit	Perfect forward secrecy in TLS 1.3
DH / DHE	Yes	No	128-bit (3072-bit key)	Replaced by ECDHE in modern TLS
Ed25519	No	Yes	~128-bit	SSH keys, modern PKI

What to study next

Got this wrong? Here's your next step.

Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this 300-410 question test?

MPLS Operations — This question tests MPLS Operations — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The VRF BLUE has a static route pointing to a next-hop that is not reachable from VRF RED, causing asymmetric reachability. — The correct answer is B. For a host in VRF RED to ping a host in VRF BLUE, the route from RED to BLUE must work, and the return route from BLUE to RED must also work. In this scenario, VRF BLUE has a static route pointing to a next-hop that is not reachable from VRF RED. When the BLUE host tries to respond to the ping, it uses that static route to reach the RED host's subnet. Since the next-hop is unreachable (e.g., missing interface, wrong IP, or not present in the global or VRF routing table), the return traffic fails. This causes asymmetric reachability: RED can reach BLUE (via some other path, possibly the route-leaking mechanism), but BLUE cannot reach RED. The route reflector configuration is not the issue because the route-target import/export values are correctly matched for bidirectional leaking. Therefore, the static route misconfiguration is the root cause.

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

What is the key concept behind this question?

CIDR notation defines the prefix length.

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