MPLS Label advertisement modes

Label advertising distribution Mode consists of two types. Upstream and downstream advertise bindings differently, depending on whether they are unsolicited or requested.

Fig 1.1-MPLS Label advertisement modes

DU mode
As soon as the LDP session is established, downstream LSRs advertise label binding information to their upstream counterparts unsolicited, while the upstream LSR keeps the label binding information and processes it based on its routing table. This is called as DU mode.

In other words, An LSR in DU mode advertises label binding information to its upstream LSR unsolicited after establishing an LDP session, while the upstream LSR keeps the information and processes it according to its routing table.

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DoD mode
The downstream LSR assigns a label to a FEC by sending a label request message containing the FEC's description to the upstream LSR, which encodes the binding information in a label mapping message.

Depending on the label distribution control mode the LSR is using, the downstream LSR will react with label binding information at different times:

• In ordered mode, an LSR only provides label binding information to its upstream LSR after receiving it from its downstream LSR.
• No matter whether it receives label binding information from its downstream LSR or not, an independent LSR responds immediately to its upstream LSR. When in independent mode, LSRs respond immediately to their upstream LSRs regardless of whether they receive label binding information from their downstream LSRs.