- A
The summary route changes the return path for traffic to the management station, and the return interface has an ACL with implicit deny blocking the SSH reply.
The summary can cause asymmetric routing, and the return path ACL blocks the traffic.
- B
The ACL does not permit SSH traffic (port 22).
Why wrong: The ACL permits all IP traffic from the source range.
- C
The summary route is not installed, so traffic is dropped.
Why wrong: The summary is installed.
- D
EIGRP is not advertising the loopback route.
Why wrong: The loopback is reachable.
Question 1,861 of 2,152
Route Summarization ACL Asymmetric Routing
This 300-410 practice question tests your understanding of route summarization. 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.
An ACL implicit deny is blocking management traffic due to route summarization. Router R1 has:
access-list 100 permit ip 10.0.0.0 0.0.3.255 any
!
interface GigabitEthernet0/0 ip access-group 100 in
!
router eigrp 100 network 10.0.0.0
! R1 also has:
interface GigabitEthernet0/1 ip address 192.168.1.1 255.255.255.0 ip summary-address eigrp 100 10.0.0.0 255.255.252.0
! A management station at 10.0.1.100 cannot SSH to R1's loopback 0 (10.0.0.1). What is the root cause?
Quick Answer
The root cause is that the summary route changes the return path for management traffic, causing asymmetric routing where the SSH reply from R1’s loopback is sent out a different interface that has an ACL with an implicit deny. While the inbound ACL on GigabitEthernet0/0 correctly permits the management station’s source IP (10.0.1.100), the EIGRP summary route 10.0.0.0/22 advertised out GigabitEthernet0/1 alters the routing table, forcing return traffic destined for 10.0.1.100 to exit via that interface instead of the one where the original SSH request arrived. On the CCNP ENARSI 300-410 exam, this scenario tests your understanding of how route summarization can break stateful flows by creating asymmetric paths, a common trap where candidates focus only on the ACL permit statement and miss the routing impact. Remember the key insight: an ACL is always evaluated per interface, so a change in the return path can bypass the permit and hit an implicit deny. Memory tip: “Summary shifts the path, ACL blocks the back.”
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 summary route changes the return path for traffic to the management station, and the return interface has an ACL with implicit deny blocking the SSH reply.
The ACL permits traffic from 10.0.0.0/22 (10.0.0.0-10.0.3.255) to any, but the summary route is also 10.0.0.0/22. However, the ACL is applied inbound on GigabitEthernet0/0. The management station's traffic comes from 10.0.1.100, which is within the permitted range, so the ACL should allow it. But the summary route may cause the return traffic to be sent via a different interface, and if that interface has an ACL with implicit deny, the SSH session fails. The root cause is that the summary route changes the path, and the return path has an ACL blocking traffic.
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 summary route changes the return path for traffic to the management station, and the return interface has an ACL with implicit deny blocking the SSH reply.
Why this is correct
The summary can cause asymmetric routing, and the return path ACL blocks the traffic.
Related concept
Standard ACLs match source addresses.
- ✗
The ACL does not permit SSH traffic (port 22).
Why it's wrong here
The ACL permits all IP traffic from the source range.
- ✗
The summary route is not installed, so traffic is dropped.
Why it's wrong here
The summary is installed.
- ✗
EIGRP is not advertising the loopback route.
Why it's wrong here
The loopback is reachable.
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.
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
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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 Summarization — This question tests Route Summarization — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: The summary route changes the return path for traffic to the management station, and the return interface has an ACL with implicit deny blocking the SSH reply. — The ACL permits traffic from 10.0.0.0/22 (10.0.0.0-10.0.3.255) to any, but the summary route is also 10.0.0.0/22. However, the ACL is applied inbound on GigabitEthernet0/0. The management station's traffic comes from 10.0.1.100, which is within the permitted range, so the ACL should allow it. But the summary route may cause the return traffic to be sent via a different interface, and if that interface has an ACL with implicit deny, the SSH session fails. The root cause is that the summary route changes the path, and the return path has an ACL blocking traffic.
What should I do if I get this 300-410 question wrong?
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.
What is the key concept behind this question?
Standard ACLs match source addresses.
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Last reviewed: Jun 18, 2026
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