MPLS- Label Spaces

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MPLS Label Spaces

■ Label distribution mode
■ Label retention mode
■ LSP control mode. 

Label Distribution Modes

The MPLS architecture has two modes to distribute label bindings:

■ Downstream-on-Demand (DoD) label distribution mode
■ Unsolicited Downstream (UD) label distribution mode

In the DoD mode, each LSR requests its next-hop (that is, downstream) LSR on an LSP, a label binding for that FEC. Each LSR receives one binding per FEC only from its downstream LSR on that FEC.

In the case of DoD, the LIB shows only one remote binding,

In the UD mode, each LSR distributes a binding to its adjacent LSRs,n the UD mode, an LSR receives a remote label binding from each adjacent LSR.

in the case of UD, the LIB shows more then one remote binding.

Cisco IOS, all interfaces,use the UD label distribution mode , except LC-ATM interfaces

Label Retention Modes

Two label retention modes are possible:
■ Liberal Label Retention (LLR) mode
      In LLR mode, an LSR keeps all received remote bindings in the LIB

■ Conservative Label Retention (CLR) mode.
    An LSR that is running this mode does not store all remote bindings in the    LIB, but it stores  only   the remote binding that is associated with the next-   hop LSR for a particular FEC.

In short, the LLR mode gives you quicker adaptation to routing changes, whereas CLR mode givesyou fewer labels to store and a better usage of the available memory on the router.

LSP Control Modes

LSRs can create a local binding for a FEC in two ways:

■ Independent LSP Control mode

■ Ordered LSP Control mode

The LSR can create a local binding for a FEC independently from the other LSRs. This is called  Independent LSP Control mode. In this control mode, each LSR creates a local binding for a  particular FEC as soon as it recognizes the FEC.

In Ordered LSP Control mode, an LSR only creates a local binding for a FEC if it recognizes that it is the egress LSR for the FEC or if the LSR has received a label binding from the next hop for this FEC.

The disadvantage of Independent LSP Control is that some LSRs begin to label switch packets before the complete LSP is set up end to end; therefore, the packet is not forwarded in the manner it should be