SAFI 4 carries a prefix and label binding
BGP labeled unicast uses MP-BGP to advertise an IP prefix with an MPLS label binding. The AFI identifies IPv4 or IPv6 and SAFI 4 identifies labeled unicast. The receiver associates the label with reaching that prefix through the advertised BGP next hop.
An advertisement can be used to stitch labeled transport across routing domains. If a speaker changes the next hop to itself and advertises a label for that prefix, it must provide the corresponding forwarding treatment for packets received with that label.
Transport reachability and VPN routes serve different roles
A labeled-unicast route can provide transport reachability to a remote provider edge. An MPLS VPN route carries customer reachability and VPN information in a different address family. A packet can carry a transport label above an inner VPN label.
A BGP session can be established for ordinary unicast while the labeled-unicast family is unavailable. To diagnose missing labeled routes, verify the negotiated family, routing policy, next-hop resolution and the installed label forwarding entry.
Try it in a lab
- On MPLS-capable lab routers, enable IPv4 labeled unicast between two BGP peers and advertise a loopback. Verify that the receiver sees AFI 1, SAFI 4 and a label binding for that prefix.
- Confirm that the advertised next hop resolves and inspect the resulting forwarding entry. With test traffic, compare the imposed label to the value advertised by the downstream speaker.
- Remove labeled-unicast activation from one peer while retaining ordinary IPv4 unicast. After any session renegotiation, confirm the labeled route is gone while an independently advertised ordinary IPv4 route is still learned.