The correct answer is that the tunnel will successfully encapsulate both IPv4 and IPv6 payloads over IPv4. This is because the `tunnel mode gre ip` command establishes a generic routing encapsulation (GRE) tunnel over an IPv4 transport network, and GRE is a protocol-agnostic encapsulation method capable of carrying any Layer 3 protocol, including both IPv4 and IPv6. The IPv6 address configured on the tunnel interface allows IPv6 traffic to be routed into the tunnel, where it is encapsulated in IPv4 packets and forwarded to the tunnel destination. On the Cisco CCNP ENARSI 300-410 exam, this question tests your understanding of GRE tunnel mode and features, specifically how GRE differs from IP-in-IP tunnels, which only support IPv4 payloads. A common trap is assuming that `tunnel mode gre ip` restricts the payload to IPv4 only, but the key is that GRE itself is protocol-independent. Memory tip: GRE stands for Generic Routing Encapsulation—think “Generic” means any protocol goes, while “ip” only specifies the transport.
300-410 IPv6 Tunneling Techniques Practice Question
This 300-410 practice question tests your understanding of ipv6 tunneling techniques. 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.
Examine this configuration on Router R4:
```
interface Tunnel0
ip address 10.0.0.1 255.255.255.252
ipv6 address 2001:DB8:6::1/64
tunnel source GigabitEthernet0/0
tunnel destination 172.16.1.2
tunnel mode gre ip
```
What will be the effect?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The tunnel will successfully encapsulate both IPv4 and IPv6 payloads over IPv4.
The 'tunnel mode gre ip' command creates a generic routing encapsulation (GRE) tunnel over IPv4. GRE is protocol-agnostic and can encapsulate any Layer 3 protocol, including IPv4 and IPv6. The IPv6 address configured on the tunnel interface allows IPv6 traffic to be routed into the tunnel, where it is encapsulated in IPv4 packets and sent to the tunnel destination. Therefore, the tunnel will successfully carry both IPv4 and IPv6 payloads.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 tunnel will only carry IPv4 traffic because the mode is 'gre ip'.
Why it's wrong here
GRE can carry multiple protocols; 'gre ip' means transport is IPv4, but payload can be IPv4, IPv6, etc.
✓
The tunnel will successfully encapsulate both IPv4 and IPv6 payloads over IPv4.
Why this is correct
GRE allows multiprotocol payload; the IPv4 and IPv6 addresses on the tunnel interface are valid.
Related concept
Read the scenario before looking for a memorised answer.
✗
The tunnel will fail because the tunnel source is an interface, not an IP address.
Why it's wrong here
Using an interface as tunnel source is valid and recommended.
✗
The tunnel mode should be 'ipv6ip' to carry IPv6 traffic.
Why it's wrong here
GRE can carry IPv6; ipv6ip is only for IPv6 over IPv4 without GRE.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that 'tunnel mode gre ip' only supports IPv4 traffic, when in fact GRE is protocol-agnostic and can carry IPv6, IPX, or other Layer 3 protocols as payload.
Detailed technical explanation
How to think about this question
GRE (RFC 2784) uses a protocol type field in its header to identify the encapsulated payload protocol, allowing it to carry IPv4 (type 0x0800), IPv6 (type 0x86DD), or other protocols. When the tunnel interface has both an IPv4 and IPv6 address, the router will route packets into the tunnel based on the destination address, and GRE will encapsulate them regardless of the payload type. In real-world scenarios, GRE tunnels are commonly used to connect IPv6 islands over an IPv4-only core network, providing a simple and effective transition mechanism without requiring native IPv6 support from the intermediate network.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
IPv6 Tunneling Techniques — This question tests IPv6 Tunneling Techniques — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The tunnel will successfully encapsulate both IPv4 and IPv6 payloads over IPv4. — The 'tunnel mode gre ip' command creates a generic routing encapsulation (GRE) tunnel over IPv4. GRE is protocol-agnostic and can encapsulate any Layer 3 protocol, including IPv4 and IPv6. The IPv6 address configured on the tunnel interface allows IPv6 traffic to be routed into the tunnel, where it is encapsulated in IPv4 packets and sent to the tunnel destination. Therefore, the tunnel will successfully carry both IPv4 and IPv6 payloads.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Consider this configuration on Router R2:
```
interface Tunnel0
ipv6 address 2001:DB8:3::1/64
tunnel source GigabitEthernet0/0
tunnel mode gre ipv6
tunnel destination 2001:DB8:4::2
```
Which statement is true?
medium
A.This tunnel can only carry IPv6 traffic.
B.The tunnel source and destination must be IPv4 addresses.
✓ C.This tunnel will encapsulate packets using GRE over an IPv6 transport.
D.The tunnel mode should be 'ipv6ip' for IPv6 transport.
Why C: The configuration shows a tunnel with `tunnel mode gre ipv6`, which explicitly sets the tunnel to use GRE encapsulation over an IPv6 transport. The tunnel source and destination are IPv6 addresses, and GRE can carry multiple protocols (including IPv4, IPv6, and others) over the IPv6 transport. Therefore, option C is correct because the tunnel encapsulates packets using GRE over an IPv6 transport.
Variation 2. Given this configuration on Router R6:
```
interface Tunnel0
ipv6 address 2001:DB8:8::1/64
tunnel source 2001:DB8:9::1
tunnel destination 2001:DB8:10::2
tunnel mode gre ipv6
tunnel path-mtu-discovery
```
What is the effect?
medium
A.The tunnel will not work because path-mtu-discovery is not supported with GRE.
✓ B.The tunnel will dynamically adjust the MTU based on the path MTU discovery.
C.The tunnel mode should be 'ipv6ip' for IPv6 transport.
D.The tunnel source and destination must be IPv4 addresses.
Why B: The `tunnel path-mtu-discovery` command enables Path MTU Discovery (PMTUD) for the GRE tunnel, which dynamically determines the maximum MTU along the path to the tunnel destination and adjusts the tunnel interface's MTU accordingly. This prevents fragmentation issues by allowing the tunnel to use the smallest MTU in the path. Option B correctly identifies this behavior.
Variation 3. Which authentication method is supported by default for GRE tunnels in Cisco IOS-XE?
medium
A.MD5 authentication
B.SHA-256 authentication
✓ C.No authentication by default
D.Plain-text password authentication
Why C: GRE tunnels in Cisco IOS-XE do not include any built-in authentication mechanism by default. The GRE protocol (RFC 2784) defines a simple encapsulation method without authentication or encryption; any security features must be added externally, such as using IPsec to protect the tunnel traffic. Therefore, the correct answer is that no authentication is enabled by default.
Last reviewed: Jun 24, 2026
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