Circuit turn-up, PE-CE, and VLAN issues

Data-center and edge handoffs have their own failure modes:

• Interface down/down — layer 1: no carrier. A new circuit often sits here until the telco turns it up (you may need to contact the carrier; the line can be in a "wait/down" state pending turn-up, or a cancelled line needs to be re-provisioned). Confirm the demarc and LOA/CFA.
• Interface up/down — layer 2 is up but the protocol isn't; check encapsulation, VLAN, and the far end.
• PE-CE mismatches — on an MPLS L3VPN, the provider edge (PE) and customer edge (CE) must agree on the routing (BGP/OSPF/static), the VRF, the VLAN, and the contracted bandwidth/services. A PE-CE BGP neighbor that won't come up, or a VLAN/bandwidth mismatch, breaks the service even though the link looks up.

With SubnetSlinger: SSH to read interface and BGP-neighbor state, Ping/Path ping across the link to prove reach, and SNMP to confirm the device's view. Log findings in Incidents to hand the carrier exactly what's wrong.