Summary

  • RFC 7213 lets MPLS-TP nodes without an IP data plane learn a peer’s unicast MAC address and maximum frame size through a G-ACh advertisement.
  • The advertisement does not decide whether a link is point-to-point, whether an MFS mismatch should shut the link, or what addressing method applies after the information expires; those remain configured local decisions.
  • A safe implementation binds every learned value to authentication, lifetime, topology, visible change history and a tested fallback.

Discovery solves an address problem, not a policy problem

MPLS-TP can run between nodes that do not use an IP data plane. In that setting, ARP and IPv6 Neighbor Discovery may be absent, yet an Ethernet frame still needs a destination MAC address. RFC 7213 arranges the choices in an order: use an existing discovery mechanism such as ARP, Neighbor Discovery or LLDP when it is available; otherwise use the G-ACh Advertisement Protocol when both peers support it; only then fall back to static or non-negotiated alternatives.

That ordering matters because each alternative places cost and authority differently. Static configuration is locally controlled but administratively brittle. Replacing an interface can change its address and force a manual update at the peer, which is more awkward when another provider operates that peer. Broadcast can avoid knowing a unicast address but may distribute and process frames more widely than intended. The reserved MPLS-TP multicast address 01-00-5E-90-00-00 is a bounded placeholder only on a link known to be point-to-point.

The word “known” is the governance hinge. A provider-supplied circuit may appear point-to-point while its underlying Ethernet arrangement changes. For implementations that support these broadcast or multicast placeholder modes, RFC 7213 requires a means for the operator to declare a link point-to-point, and forbids those placeholders when the link is not known to have that topology. This requirement is conditional on those addressing modes; it does not make a GAP advertisement a topology certificate. The peer can advertise an address, but it cannot certify the receiver’s topology assumption.

The concrete advertisement carries two different facts

RFC 7213 defines GAP application 0x0001, Ethernet Interface Parameters. Its Source MAC Address object is type 0 with length 8; the value is a unicast MAC assigned to the sending interface in EUI-64 form, using the IEEE-defined mapping from the interface’s 48-bit MAC address. Its Maximum Frame Size TLV carries the largest Ethernet frame the sending interface says it can accept. GAP multicasts these advertisements on Ethernet to 01-00-5e-80-00-0d, so discovery does not depend on IP.

The two parameters do not have identical consequences. When the received Source MAC Address changes, the new address must be used. When a configured minimum frame size exceeds the peer’s advertised MFS, however, the operator must be notified and local policy chooses the response. One policy may shut the link and trigger end-to-end repair. Another may leave it up so that OAM can test the effective MFS. A remote fact is therefore mandatory input to a local decision, not the decision itself.

Multipoint links sharpen the boundary. A multipoint Ethernet section that carries a point-to-point MPLS-TP LSP must use unicast destination addresses. The operator must obtain them through static configuration or discovery; it cannot treat broad delivery as a harmless shortcut. Link shape determines which addressing behavior is authorized.

Lifetime is revocation, and fallback is part of the contract

Peer information is temporary. RFC 7213 inherits GAP lifetimes and recommends prompt advertisements and requests after reconfiguration, reboot or a detected disconnection. If the advertised Source MAC changes, the receiver switches to the new value. If its lifetime expires because advertisements stop, the peer disconnects or the TLV disappears, the old address must no longer be used. The node must return to the locally configured selection method for the no-advertisement case.

This fallback is not an implementation footnote. It defines what authority remains when peer evidence disappears. A system that learns an address but has never tested expiry behavior has only implemented acquisition, not revocation. Likewise, retained peer data must be discarded on restart, preventing yesterday’s observation from becoming today’s silent forwarding rule.

RFC 7213 also requires sent and received values to be inspectable. When received information changes local configuration, the reason must be clear; changed values must be used and made visible. If the link type changes and the addressing method is no longer compatible, the system must take the action configured for that condition. The audit trail must therefore join the advertisement, its authentication and lifetime, the topology declaration, the local rule and the resulting state transition.

The remaining cost belongs to the local authority

Authentication under RFC 7212 can mitigate an attacker changing the advertised MAC or MFS. It does not prove that a circuit is still point-to-point, choose the minimum acceptable frame size, decide whether OAM is sufficient, or determine who may receive multicast frames. On multipoint Ethernet, operators must restrict delivery to authorized ports or otherwise contain excessive distribution. Physical link security may also matter because even viewing the TLVs can disclose useful information.

The counterfactual shows why these distinctions belong in leadership review. A valid but stale MAC survives an interface replacement and keeps steering traffic to the wrong destination. A link declared point-to-point later becomes shared, turning a convenient multicast placeholder into unintended distribution. An MFS mismatch automatically shuts a service even though local policy intended verification first. None of these failures requires the RFC to be wrong; each arises when evidence is mistaken for authority.

Sources