The answer is that PE1 is missing the `network 2001:DB8:2::/64` command under the IPv6 unicast address family in BGP. This is the root cause because, in a 6PE deployment, the provider edge router must explicitly inject the locally connected IPv6 prefix into BGP using the network command; simply having the IPv6 address on the interface and redistributing BGP into OSPF does not advertise the prefix into the BGP IPv6 table. The show output confirms that MPLS forwarding to PE1’s loopback works (label 16), yet the IPv6 route is absent from PE2’s BGP table, isolating the issue to the missing BGP network statement. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of 6PE prefix advertisement mechanics, where a common trap is assuming OSPF redistribution or interface configuration alone suffices for BGP injection. Remember the memory tip: “6PE needs a BGP network—OSPF redistribution won’t carry the IPv6 prefix across the MPLS core.”
300-410 IPv6 Tunneling Techniques Practice Question
This 300-410 practice question tests your understanding of ipv6 tunneling techniques. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 MPLS network with IPv6 over MPLS (6PE) is experiencing loss of IPv6 routes from a remote provider edge (PE) router. Router PE1 has the following relevant configuration: interface GigabitEthernet0/0 ipv6 address 2001:DB8:1::1/64 mpls ip interface Loopback0 ip address 192.0.2.1 255.255.255.255 router ospf 1 router-id 192.0.2.1 redistribute bgp 65000 subnets. Router PE2 shows: PE2# show bgp ipv6 unicast 2001:DB8:2::/64 % Network not in table. PE2# show mpls forwarding-table 192.0.2.1 Label: 16, Interface: GigabitEthernet0/1. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
PE1 is missing the network 2001:DB8:2::/64 command under router bgp for IPv6 unicast address family.
The correct answer is A because the output shows that PE2 has an MPLS label (16) for PE1's loopback (192.0.2.1) and can forward labeled traffic, but the IPv6 route 2001:DB8:2::/64 is missing from the BGP table. This indicates that PE1 is not advertising the IPv6 prefix into BGP. The missing `network 2001:DB8:2::/64` command under the IPv6 unicast address family on PE1 prevents the prefix from being injected into BGP, even though the interface is configured with the IPv6 address and OSPF redistribution is in place.
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.
✓
PE1 is missing the network 2001:DB8:2::/64 command under router bgp for IPv6 unicast address family.
Why this is correct
Without this, the IPv6 prefix is not injected into BGP, so PE2 never learns it.
Related concept
Read the scenario before looking for a memorised answer.
✗
The MPLS label distribution between PE1 and PE2 is failing due to LDP mismatch.
Why it's wrong here
The show mpls forwarding-table shows a label for the loopback, indicating LDP is working.
✗
OSPF is not redistributing the IPv6 prefix correctly.
Why it's wrong here
OSPF redistribution is not needed for 6PE; BGP carries the IPv6 routes.
✗
The IPv6 address family is not enabled under router bgp on PE1.
Why it's wrong here
While necessary, the missing network statement is the specific cause of the missing route.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between interface configuration and BGP advertisement, where candidates assume that having an IPv6 address on an interface automatically makes it reachable via BGP in a 6PE design.
Trap categories for this question
Command / output trap
The show mpls forwarding-table shows a label for the loopback, indicating LDP is working.
Detailed technical explanation
How to think about this question
In 6PE, the IPv6 prefix is advertised via BGP with a label (using the IPv6 SAFI 128), while the MPLS core uses LDP to distribute labels for the IPv4 loopbacks. The PE router must explicitly inject the IPv6 prefix into BGP using the `network` command under the IPv6 unicast address family; simply having the IPv6 address on an interface does not automatically advertise it. A common real-world scenario is when a network engineer configures the interface IPv6 address and enables OSPF but forgets to add the network statement under BGP, leading to missing IPv6 routes on remote PEs.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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: PE1 is missing the network 2001:DB8:2::/64 command under router bgp for IPv6 unicast address family. — The correct answer is A because the output shows that PE2 has an MPLS label (16) for PE1's loopback (192.0.2.1) and can forward labeled traffic, but the IPv6 route 2001:DB8:2::/64 is missing from the BGP table. This indicates that PE1 is not advertising the IPv6 prefix into BGP. The missing `network 2001:DB8:2::/64` command under the IPv6 unicast address family on PE1 prevents the prefix from being injected into BGP, even though the interface is configured with the IPv6 address and OSPF redistribution is in place.
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 →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.