Question 286 of 2,152

SPAN, RSPAN, and ERSPAN →hardMultiple ChoiceObjective-mapped

ERSPAN ACL Blocking Encapsulated Packets

This 300-410 practice question tests your understanding of span, rspan, and erspan. 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.

Router R1 is configured as an ERSPAN source to monitor traffic on interface GigabitEthernet0/0/0. The destination is a collector at 10.10.10.10. R1's configuration: monitor session 1 type erspan-source source interface Gi0/0/0 both destination erspan-id 3 ip address 10.10.10.10 origin ip address 192.168.1.1. The router has an ACL applied to the interface Gi0/0/0 that denies all IP traffic from source 192.168.1.1. The collector receives no packets. What is the root cause?

A
The ACL on Gi0/0/0 blocks the original traffic, so there is no traffic to mirror.
Why wrong: The symptom is that the collector receives no packets, but the original traffic may still be present; the ACL might block it, but the question is about the mirroring process.
B
The ACL on Gi0/0/0 blocks the encapsulated ERSPAN packets because the source IP 192.168.1.1 is denied.
If the encapsulated packets are sent out Gi0/0/0, the ACL will block them because the source IP is 192.168.1.1, which is denied.
C
The ERSPAN session is missing the 'no shutdown' command.
Why wrong: The configuration shows no 'shutdown' command, so it is enabled.
D
The destination IP 10.10.10.10 is not reachable.
Why wrong: The scenario does not indicate unreachability.

Full breakdown with real-world context →

Quick Answer

The answer is that the ACL on GigabitEthernet0/0/0 blocks the encapsulated ERSPAN packets because the source IP 192.168.1.1 is denied. ERSPAN works by taking a copy of the monitored traffic, then wrapping it in a new IP header using the origin IP address as the source; this newly encapsulated packet is then routed toward the collector. The critical technical concept here is that the ACL applied to the source interface filters the original traffic entering that interface, but if the router uses the same interface to forward the encapsulated packets outbound, the ACL will also inspect and potentially drop those packets. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of how ERSPAN encapsulation interacts with interface ACLs—a common trap is assuming the ACL only affects the mirrored traffic, not the generated ERSPAN headers. Remember the memory tip: “ACLs see the packet as it leaves, not just as it arrives; if the origin IP is denied, the mirror is denied.”

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 ACL on Gi0/0/0 blocks the encapsulated ERSPAN packets because the source IP 192.168.1.1 is denied.

The ACL on Gi0/0/0 denies traffic from source 192.168.1.1. The ERSPAN-encapsulated packets have a source IP of 192.168.1.1 (from the 'origin ip address' command). If the router forwards these encapsulated packets out of the same interface Gi0/0/0, the ACL will block them because the source IP is denied. This prevents the mirrored traffic from reaching the collector. Therefore, the correct answer is B.

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 ACL on Gi0/0/0 blocks the original traffic, so there is no traffic to mirror.
Why it's wrong here
The symptom is that the collector receives no packets, but the original traffic may still be present; the ACL might block it, but the question is about the mirroring process.
✓
The ACL on Gi0/0/0 blocks the encapsulated ERSPAN packets because the source IP 192.168.1.1 is denied.
Why this is correct
If the encapsulated packets are sent out Gi0/0/0, the ACL will block them because the source IP is 192.168.1.1, which is denied.
Related concept
Standard ACLs match source addresses.
✗
The ERSPAN session is missing the 'no shutdown' command.
Why it's wrong here
The configuration shows no 'shutdown' command, so it is enabled.
✗
The destination IP 10.10.10.10 is not reachable.
Why it's wrong here
The scenario does not indicate unreachability.

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
The configuration shows no 'shutdown' command, so it is enabled.
Scenario analysis trap
The scenario does not indicate unreachability.

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.

Visual reference

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?

SPAN, RSPAN, and ERSPAN — This question tests SPAN, RSPAN, and ERSPAN — Standard ACLs match source addresses..

What is the correct answer to this question?

The correct answer is: The ACL on Gi0/0/0 blocks the encapsulated ERSPAN packets because the source IP 192.168.1.1 is denied. — The ACL on Gi0/0/0 denies traffic from source 192.168.1.1. The ERSPAN-encapsulated packets have a source IP of 192.168.1.1 (from the 'origin ip address' command). If the router forwards these encapsulated packets out of the same interface Gi0/0/0, the ACL will block them because the source IP is denied. This prevents the mirrored traffic from reaching the collector. Therefore, the correct answer is B.

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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