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300-410 Practice Question: An MPLS network with IPv6 over MPLS (6PE) is…
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?
⚠ Common exam trap
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.
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
✓
PE1 is missing the network 2001:DB8:2::/64 command under router bgp for IPv6 unicast address family.
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.
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.
- ✗
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.
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.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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