Label Distribution Protocol
7210 SAS M, T, X, R6, Mxp MPLS Configuration Guide Page 241
The tunnel table route export policy in LDP instructs LDP to listen to BGP route entries in the
CPM Tunnel Table. If a /32 BGP labeled route matches a prefix entry in the export policy, LDP
originates an LDP FEC for the prefix, stitches it to the BGP labeled route, and re-distributes the
LDP FEC its iBGP neighbors.
LDP Downstream-on-Demand (DoD)
The user enables the use by an LDP session of the Downstream-on-Demand (DoD) label
distribution using the command "config>router>ldp>peer-parameters>peer>dod-label-
distribution".
When this option is enabled, LDP will set the A-bit in the Label Initialization message, when the
LDP session to the peer is established. When both peers set the A-bit, both uses the DoD label
distribution method over the LDP session [rfc5036].
This feature can only be enabled on a link level LDP session and applies to prefix labels only, and
not service labels.
Single-Hop LDP DoD Procedures
As soon as the link LDP session comes up, the 7210 SAS sends a label request to the DoD peer for
the FEC prefix corresponding to the peer’s LSR-id. The DoD peer LSR-id is found in the basic
Hello discovery messages the peer used to establish the Hello adjacency with the 7210.
Similarly, if the 7210 and the directly attached DoD peer enter into the extended discovery and
establish a targeted LDP session, the 7210 immediately sends a label request for the FEC prefix
corresponding to the peer’s LSR-id found in the extended discovery messages.
However, the 7210 node does not advertise any <FEC, label> bindings, including the FEC of its
own LSR-id, unless the DoD peer requested it through a Label Request Message.
When the DoD peer sends a label request for any FEC prefix, the 7210 replies with a <FEC, label>
binding for that prefix if the FEC was already activated on the 7210. If not, the 7210 replies with a
notification message containing the status code of “no route”. The 7210 does not attempt in the
latter case to send a label request to the next-hop for the FEC prefix when the LDP session to this
next-hop uses the DoD label distribution mode. Thus, the reference to single-hop LDP DoD
procedures.
The single-hop LDP DoD procedures makes sure the 7210 has a label for the LDP DoD peer,
whenever it is needed.
The 7210 needs a label of directly attached DoD peer in the following cases:
• A BGP labeled route for the peer’s prefix from RTM to its BGP neighbors through iBGP.
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