350-501 Architecture Practice Question
Exhibit
Refer to the exhibit. PE1 configuration: router bgp 65000 neighbor 10.0.0.2 remote-as 65000 neighbor 10.0.0.2 update-source Loopback0 ! address-family vpnv4 neighbor 10.0.0.2 activate neighbor 10.0.0.2 send-community extended exit-address-family ! ip vrf CUSTOMER_A rd 65000:1 route-target export 65000:100 route-target import 65000:100 ! interface GigabitEthernet0/0/0/0 ip vrf forwarding CUSTOMER_A ip address 192.168.1.1 255.255.255.0 ! PE2 configuration: router bgp 65000 neighbor 10.0.0.1 remote-as 65000 neighbor 10.0.0.1 update-source Loopback0 ! address-family vpnv4 neighbor 10.0.0.1 activate neighbor 10.0.0.1 send-community extended exit-address-family ! ip vrf CUSTOMER_A rd 65000:1 route-target export 65000:100 route-target import 65000:100 ! interface GigabitEthernet0/0/0/0 ip vrf forwarding CUSTOMER_A ip address 192.168.2.1 255.255.255.0 !
A customer reports that CE routers attached to PE1 and PE2 in the same VRF cannot ping each other. Based on the exhibit, what is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between control-plane issues (BGP VPNv4, RT, RD) and data-plane issues (LDP, MPLS forwarding), leading candidates to focus on route advertisement problems when the actual fault is at the MPLS transport layer.
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
✓
LDP is not enabled on the core interfaces between PE1 and PE2
B is correct because LDP must be enabled on the core interfaces between PE1 and PE2 to establish LDP sessions, which are required to exchange MPLS labels for the transport LSP. Without LDP, the MPLS forwarding path between the PEs is broken, preventing CE-to-CE ping even if BGP VPNv4 routes are correctly advertised.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Missing address-family ipv4 for BGP neighbor
Why it's wrong here
The VPNv4 address-family is activated; the ipv4 unicast is not required for VPNv4.
- ✓
LDP is not enabled on the core interfaces between PE1 and PE2
Why this is correct
LDP is required to distribute labels for BGP next-hop reachability; without it, MPLS forwarding fails.
- ✗
Mismatched route distinguisher (RD) values on PE1 and PE2
Why it's wrong here
Both have RD 65000:1, so they match.
- ✗
Route target (RT) import/export mismatch
Why it's wrong here
Both have same RT 65000:100 for export and import.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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