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 SAS and the directly attached DoD peer enter into the extended discovery and establish a targeted LDP session, the 7210 SAS 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 SAS 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 SAS replies with a <FEC, label> binding for that prefix if the FEC was already activated on the 7210 SAS. If not, the 7210 SAS replies with a notification message containing the status code of ‟no route”. The 7210 SAS 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 SAS has a label for the LDP DoD peer, whenever it is needed.
The 7210 SAS 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.
When it receives a label request message from a directly attached DoD peer for the prefix of another directly attached DoD peer. In this case the DoD peers are trying to establish a SDP among themselves.
Trying to establish a SDP to a directly attached LDP DoD peer.
The 7210 SAS also supports sending and receiving the Label Abort Request Message as described. This message is used to abort an outstanding request for a label in case no response was received from the peer within a finite amount of time.