Virtual circuits and locally significant DLCIs
Frame Relay is a legacy WAN technology that carries frames over virtual circuits. A Data Link Connection Identifier (DLCI) selects the circuit on a particular interface. The same virtual circuit can have a different DLCI at each end because the identifiers usually have local significance.
Local Management Interface (LMI) messages can report circuit status between a router and the attached Frame Relay switch. A circuit reported active is evidence about the data-link service. It does not verify that an IP mapping or routing configuration at the endpoints is correct.
Try it in a lab
- In a legacy WAN lab, inspect a virtual circuit with different DLCIs at its two endpoints. Record each router’s local DLCI and circuit status.
- Send IPv4 traffic across the circuit. If it fails while the circuit is active, compare the next-hop mapping with the local DLCI before treating it as a WAN outage.