The method used determines how QoS is enforced within the MPLS core.
With E-LSPs, the DSCP is mapped to an EXP value in the MPLS header. Within the MPLS core, the P routers will look at the EXP bits when determining what sort of per-hop behaviour should be provided to the packet.
With L-LSPs, the DSCP is used to select a particular LSP on which the packet will be sent. The LSPs in the network are pre-configured to offer different classes of service. Therefore, within the core, prioritisation is done between the LSPs and the P routers do not have to look at the EXP bits. An example of this is where you configure 8 LSPs to every destination, where each corresponds to a DSCP class-selector value. So you could map packets with CS1 to the first LSP, CS2 to the second LSP and so on...
in addition to the statements of Paresh I would like to add, that to my knowledge E-LSP (utilizing exp bits for QoS classes) is the only thing implemented within IOS.
L-LSP would provide the full DSCP picture in such a way, that the class (like af2) is determined from the label/LSP and the drop preference is mapped into the exp bits. So here af21, af22 and af23 traffic would be sent down the same LSP and exp bits would be 1, 2 and 3 respectively.
With E-LSP there are no drop preference bits available.
In this case, when I do configured the marking and set MPLS exp on the IOS, the E-LSP is "auto" on and marking DSCP for classes such as CS,AF will also turn on the L-LSP? Kind of confuse this morning :)
Nothing is on be default, really. You can mark the EXP bits in your MPLS header but they don't really mean anything until you configure appropriate per-hop behaviour within your MPLS core using service policies. However, when you do translate your DSCP markings to the EXP bits of the attached MPLS header, and you configure appropriate per-hop behaviour within your network, then you are using E-LSPs. There is no command or anything that will turn on E-LSPs.
Now, as Martin mentioned, frame-mode MPLS in IOS does not support L-LSPs yet. In fact, I recall seeing a Networkers presentation which stated that there had been no demand for frame-mode L-LSPs so far.
So they are really two different concepts and configured in two different ways (if available).
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