MPLS Guide Label Distribution Protocol
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Since TCP is back-pressured by the receiver, it is necessary to be able to push that
back-pressure all the way into the protocol. Packets that cannot be sent are buffered
on the session object and reattempted as the back-pressure eases.
5.1.5 Label Exchange
Label exchange is initiated by the service manager. When an SDP is attached to a
service (that is, once the service gets a transport tunnel), a message is sent from the
service manager to LDP. This causes a label mapping message to be sent.
Additionally, when the SDP binding is removed from the service, the VC label is
withdrawn. The peer must send a label release to confirm that the label is not in use.
5.1.5.1 Implicit Null Label
The implicit null label option enables an eLER to receive MPLS packets from the
previous-hop LSR without the outer LSP label.
The implicit null label is signaled by the eLER to the previous-hop LSR during FEC
signaling by the LDP control protocol. When the implicit null label is signaled to the
LSR, it pops the outer label before sending the MPLS packet to the eLER; this is
known as penultimate hop popping.
The implicit null label option can be enabled for all LDP FECs for which the router is
the eLER by using the implicit-null-label command in the config>router>ldp
context.
If the implicit null configuration is changed, LDP withdraws all the FECs and
readvertises them using the new label value.
5.1.5.2 Other Reasons for Label Actions
Label actions can also occur for the following reasons:
• MTU changes — LDP withdraws the previously assigned label and resignals the
FEC with the new Maximum Transmission Unit (MTU) in the interface parameter
• clear labels — when a service manager command is issued to clear the labels,
the labels are withdrawn and new label mappings are issued
• SDP down — when an SDP goes administratively down, the VC label
associated with that SDP for each service is withdrawn
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