Port 711: Cisco TDP — The graveyard port

Port 711 sits in the well-known port range (0-1023), officially assigned to cisco-tdp (Cisco Tag Distribution Protocol). But this port tells the story of two abandoned protocols that happened to share the same address.

The Official Assignment: Cisco TDP

Cisco's Tag Distribution Protocol was a proprietary system for distributing MPLS labels between routers. In MPLS networks, routers exchange labels—short identifiers that replace long IP addresses—so packets can be switched quickly based on these labels instead of full routing lookups.¹

TDP established sessions over TCP on port 711, ensuring reliable delivery of label mapping information necessary for setting up label switched paths in MPLS networks.²

The problem: TDP was Cisco-only. If you had routers from different vendors, they couldn't speak TDP.

What replaced it: The IETF standardized the Label Distribution Protocol (LDP), defined in RFC 5036.³ LDP does exactly what TDP did, but it's an open standard that works across vendors. Starting with Cisco IOS 12.4, LDP became the default.⁴ TDP is gone.

The Unofficial Use: Apple NetInfo

Port 711 was also used by NetInfo, a directory service built into NeXTSTEP in 1988 and carried forward into early Mac OS X.⁵ NetInfo stored network-wide configuration—users, groups, machine settings—in a distributed database that could be queried across the network.

The problem: NetInfo was a NeXT-era holdover that didn't play well with the rest of the Unix world.

What replaced it: Apple introduced Open Directory in Mac OS X Server 10.2 (2002) and completely removed NetInfo in Mac OS X Leopard 10.5 (2007), replacing it with standard LDAP-based directory services and local property list files.⁶

Why This Port Matters

Port 711 demonstrates something strange about the well-known port registry: two completely unrelated systems—Cisco's MPLS label distribution and Apple's directory services—ended up using the same port for different purposes. IANA assigned it to cisco-tdp, but Apple used it anyway for NetInfo on Mac systems.

This overlap worked because these systems rarely ran on the same machine. A Cisco router running TDP wasn't running Mac OS X, and a Mac running NetInfo wasn't acting as an MPLS router. But it's a reminder that the early Internet's port assignments weren't always perfectly coordinated.

Both protocols are now dead. If you see traffic on port 711 today, it's either legacy equipment that hasn't been upgraded, or something else entirely claiming an abandoned address.

Security Considerations

Since both TDP and NetInfo are deprecated:

• Don't run services on port 711 unless you have a specific reason
• Block port 711 at the firewall if you're not using legacy Cisco or Apple systems
• Check what's listening: On Linux/Mac, use sudo lsof -i :711 or sudo netstat -tuln | grep 711

If something is listening on this port on a modern system, investigate. It shouldn't be there.

Related Ports

• Port 646 - LDP (Label Distribution Protocol), the modern replacement for TDP
• Port 389 - LDAP, the standard directory protocol that replaced NetInfo