# WHATPORT(646)

## NAME

port 646 — LDP (MPLS Label Distribution Protocol) — MPLS label distribution between routers — this is carrier and enterprise-core network gear.

Protocol: TCP/UDP  
IANA registry

## DESCRIPTION

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.

## EXPOSURE

**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).

## CHECK

```sh
nc -vz -w 3 TARGET 646
```

## SEE ALSO

- spec — [RFC 5036 — LDP Specification](https://www.rfc-editor.org/rfc/rfc5036.html) (rfc-editor.org)
- source — [FRRouting (implements ldpd)](https://github.com/FRRouting/frr) (github.com)
- iana — [IANA Service Name and Transport Protocol Port Number Registry — 646](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=646) (iana.org)

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HTML: <https://whatport.net/646/> · JSON: <https://whatport.net/646.json> · full dataset: <https://whatport.net/ports.json>