Cisco's pre-standard Tag Distribution Protocol, the ancestor of MPLS LDP on 646.
TDP is Cisco's proprietary label distribution protocol from before the IETF standardised LDP. Cisco's MPLS FAQ states plainly that "LDP uses TCP port 646, and TDP uses TCP port 711", and that the port only opens on an interface once mpls ip is configured; TDP hellos are sent to UDP 711. Routers use the TCP session to advertise label bindings to their TDP peers. Anything still speaking TDP instead of LDP is old IOS running a legacy MPLS core.
Do not expose
This is an MPLS control-plane session between trusted routers; reachable from outside the core it lets an attacker attempt to inject label bindings.
$ nmap -p 711 TARGETSeen 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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