MPLS label distribution between routers — this is carrier and enterprise-core network gear.
LDP, specified in RFC 5036, is how MPLS label switching routers tell each other which labels map to which forwarding equivalence classes. It uses both transports on port 646: UDP 646 carries the periodic Hello discovery messages sent to the "all routers on this subnet" multicast group, and TCP 646 carries the session, advertisement, and notification messages once two routers decide to peer. Anything answering here is a router in an MPLS network — Cisco IOS, Juniper Junos, or a Linux router running FRRouting.
Do not expose
A routing control plane. An unauthenticated LDP peer that can reach the port can influence label distribution, so restrict it to known neighbours and protect the sessions (RFC 5036 provides for TCP MD5 authentication).
$ nc -vz -w 3 TARGET 646Seen this on your network? Beige Box scans your Wi-Fi and shows every open port on every device — and its Ports tool links straight back here.
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Corrections or a missing port? Reply @rimrocksystems.