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.”
300-410 Route Summarization Practice Question
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?
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.
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: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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
Static NAT maps one inside address to one outside address.
✗
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: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
Route Summarization — This question tests Route Summarization — Static NAT maps one inside address to one outside address..
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 the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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