This function explains controlling of state advertisements of non-negotiated Label Distribution Protocol (LDP) applications.
This implementation is in conformance with RFC 7473 (Controlling State Advertisements of Non-negotiated LDP Applications).
The main purpose of documenting this function is to use it in a Multipoint LDP (mLDP)-only environment, wherein participating
routers don’t need to exchange any unicast binding information.
Non-Negotiated LDP Applications
The LDP capabilities framework enables LDP applications’ capabilities exchange and negotiation, thereby enabling LSRs to send
necessary LDP state. However, for the applications that existed prior to the definition of the framework (called non-negotiated LDP applications), there is no capability negotiation done. When an LDP session comes up, an LDP speaker may unnecessarily
advertise its local state (without waiting for any capabilities exchange and negotiation). In other words, even when the peer
session is established for Multipoint LDP (mLDP), the LSR advertises the state for these early LDP applications.
One example is IPv4/IPv6 Prefix LSPs Setup (used to set up Label Switched Paths [LSPs] for IP prefixes). Another example is L2VPN P2P FEC 128 and FEC 129 PWs Signaling (an LDP application that signals point-to-point [P2P] Pseudowires [PWs] for Layer 2 Virtual Private Networks [L2VPNs]).
In an mLDP-only setup, you can disable these non-negotiated LDP applications and avoid unnecessary LDP state advertisement.
An LDP speaker that only runs mLDP announces to its peer(s) its disinterest (or non-support) in non-negotiated LDP applications.
That is, it announces to its peers its disinterest to set up IP Prefix LSPs or to signal L2VPN P2P PW, at the time of session
Upon receipt of such a capability, the receiving LDP speaker, if supporting the capability, disables the advertisement of
the state related to the application towards the sender of the capability. This new capability can also be sent later in a
Capability message, either to disable a previously enabled application’s state advertisement, or to enable a previously disabled
application’s state advertisement.
As a result, the flow of LDP state information in an mLDP-only setup is faster. When routers come up after a network event,
the network convergence time is fast too.
IP Address Bindings In An mLDP Setup
An LSR typically uses peer IP address(es) to map an IP routing next hop to an LDP peer in order to implement its control plane
procedures. mLDP uses a peer’s IP address(es) to determine its upstream LSR to reach the root node, and to select the forwarding
interface towards its downstream LSR. Hence, in an mLDP-only network, while it is desirable to disable advertisement of label
bindings for IP (unicast) prefixes, disabling advertisement of IP address bindings will break mLDP functionality.
Uninteresting State - For the Prefix-LSP LDP application, uninteresting state refers to any state related to IP Prefix FEC, such as FEC label bindings and LDP Status. IP address bindings are not
considered as an uninteresting state.
For the P2P-PW application LDP application, uninteresting state refers to any state related to P2P PW FEC 128 or FEC 129, such as FEC label bindings, MAC address withdrawal, and LDP
Control State Advertisement
To control advertisement of uninteresting state of non-negotiated LDP applications, the capability parameter TLV State Advertisement Control Capability is used. This TLV is only present in the Initialization and Capability messages, and the TLV can hold one or more State Advertisement
Control (SAC) Elements.
As an example, consider two LSRs, S (LDP speaker) and P (LDP peer), that support all non-negotiated applications. S is participating
(or set to participate) in an mLDP-only setup. Pointers for this scenario:
By default, the LSRs will advertise state for all LDP applications to their peers, as soon as an LDP session is established.
The capabilities sac mldp-only function is enabled on S.
P receives an update from S via a Capability message that specifies to disable all four non-negotiated applications states.
P’s outbound policy towards S blocks and disables state for the unneeded applications.
S only receives mLDP advertisements from specific mLDP-participating peers.