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3.5.11. Label Release Message

An LSR sends a Label Release message to an LDP peer to signal the peer that the LSR no longer needs specific FEC-label mappings previously requested of and/or advertised by the peer.

The encoding for the Label Release Message is:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|   Label Release (0x0403)   |      Message Length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Message ID                                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     FEC TLV                                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Label TLV (optional)                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                     Optional Parameters                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Message ID 32-bit value used to identify this message.

FEC TLV Identifies the FEC for which the FEC-label mapping is being released.

Optional Parameters This variable length field contains 0 or more parameters, each encoded as a TLV. The optional parameters are:

  Optional Parameter    Length       Value

  Label TLV             variable     See below

The encodings for Label TLVs are found in Section "Label TLVs".

Label If present, the label being released (see procedures below).

3.5.11.1. Label Release Message Procedures​

An LSR transmits a Label Release message to a peer when it no longer needs a label previously received from or requested of that peer.

An LSR MUST transmit a Label Release message under any of the following conditions:

  1. The LSR that sent the label mapping is no longer the next hop for the mapped FEC, and the LSR is configured for conservative operation.

  2. The LSR receives a label mapping from an LSR that is not the next hop for the FEC, and the LSR is configured for conservative operation.

  3. The LSR receives a Label Withdraw message.

Note that if an LSR is configured for "liberal mode", a release message will never be transmitted in the case of conditions (1) and (2) as specified above. In this case, the upstream LSR keeps each unused label, so that it can immediately be used later if the downstream peer becomes the next hop for the FEC.

The FEC TLV specifies the FEC for which labels are to be released. If no Label TLV follows the FEC, all labels associated with the FEC are to be released; otherwise, only the label specified in the optional Label TLV is to be released.

The FEC TLV may contain the Wildcard FEC Element; if so, it may contain no other FEC Elements. In this case, if the Label Release message contains an optional Label TLV, then the label is to be released for all FECs to which it is bound. If there is not an optional Label TLV in the Label Release message, then the sending LSR

is releasing all label mappings previously learned from the receiving LSR.

See Appendix A, "LDP Label Distribution Procedures", for more details.