The answer is that the EIGRP summary route 10.0.0.0/22 on the hub causes spoke-to-spoke tunnel failure by overriding the more specific routes needed for NHRP redirects. When a spoke attempts to reach a subnet within that summary on another spoke, it forwards traffic to the hub because the summary route is preferred over the individual /32 or /24 routes that would normally trigger NHRP redirect messages. This is a classic DMVPN route summarization pitfall tested on the Cisco CCNP ENARSI 300-410 exam, where candidates must recognize that summarization on the hub can break the on-demand spoke-to-spoke tunnel establishment logic. The common trap is assuming summarization always improves convergence, but here it actually prevents the NHRP redirect mechanism from working. Memory tip: “Summary steals the spotlight” — the summary route grabs traffic that should be redirected, so use a leak-map to let specific routes shine through.
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.
DMVPN spoke-to-spoke tunnel failures are occurring due to route summarization. Hub router R1 has:
interface Tunnel0
ip address 172.16.0.1 255.255.255.0
ip nhrp network-id 1
ip nhrp map multicast dynamic
tunnel source GigabitEthernet0/0
tunnel mode gre multipoint
!
router eigrp 100
network 172.16.0.0
!
Spoke R2 has:
interface Tunnel0
ip address 172.16.0.2 255.255.255.0
ip nhrp network-id 1
ip nhrp nhs 172.16.0.1
tunnel source GigabitEthernet0/0
tunnel mode gre multipoint
!
R1 also has:
interface GigabitEthernet0/0
ip summary-address eigrp 100 10.0.0.0 255.255.252.0
!
Spokes cannot establish direct tunnels to each other for subnets within 10.0.0.0/22. What is the root cause?
The summary route causes spokes to forward traffic to the hub instead of establishing direct tunnels, as the summary is preferred over more specific routes.
Spokes use the summary route to reach the hub, bypassing NHRP redirect.
B
NHRP is not configured correctly on the spokes, so they cannot register.
Why wrong: Spokes are registered with the NHS.
C
EIGRP is not enabled on the tunnel interface, so routes are not exchanged.
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 causes spokes to forward traffic to the hub instead of establishing direct tunnels, as the summary is preferred over more specific routes.
The summary route 10.0.0.0/22 is advertised by R1 to all spokes. When a spoke wants to reach a subnet within that summary on another spoke, it sends traffic to R1 (the summary route) instead of using the NHRP redirect to establish a spoke-to-spoke tunnel. The summary overrides the more specific routes that would trigger NHRP redirects. The fix is to use a leak-map to advertise specific routes or disable summarization.
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 summary route causes spokes to forward traffic to the hub instead of establishing direct tunnels, as the summary is preferred over more specific routes.
Why this is correct
Spokes use the summary route to reach the hub, bypassing NHRP redirect.
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
Command / output trap
EIGRP is enabled via network command.
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.
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 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.
Route Summarization — This question tests Route Summarization — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The summary route causes spokes to forward traffic to the hub instead of establishing direct tunnels, as the summary is preferred over more specific routes. — The summary route 10.0.0.0/22 is advertised by R1 to all spokes. When a spoke wants to reach a subnet within that summary on another spoke, it sends traffic to R1 (the summary route) instead of using the NHRP redirect to establish a spoke-to-spoke tunnel. The summary overrides the more specific routes that would trigger NHRP redirects. The fix is to use a leak-map to advertise specific routes or disable summarization.
What should I do if I get this 300-410 question wrong?
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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