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Community Member

differentaiated services

Hi,

From a cisco white paper I found the below.

"Two general approaches are used to mark MPLS traffic for QoS handling within an MPLS network in the first method, the DiffServ "coloring" information is carried in the experimental (EXP) field of the MPLS shim header. This field allows for eight different QoS markings. Label Switched Paths (LSPs) using this approach are called E-LSPs, signifying that QoS information is inferred from the EXP field.

Alternatively, IETF specifications allow for a second method of carrying the DiffServ information. Here, the label associated with each MPLS packet carries the portion of the DiffServ marking that specifies how a packet should be queued. The dropping precedence portion of the DiffServ marking is carried either in the EXP field, if an MPLS shim header is being used, or on fields available for this purpose on underlying technologies (for example, CLP bit for ATM and DE bit for Frame Relay). Switching paths within the MPLS network using this approach are called L-LSPs, signifying that QoS information is inferred, in part, from the MPLS label."

Can someon explain the second paragraph,and how this differs from diffserv to EXP mapping mentioned in the first paragraph.

Thanks in advance..

"

2 REPLIES

Re: differentaiated services

Hello,

ironically (but true): L-LSP differs from E-LSP in such way, that E-LSP is implemented and L-LSP is not.

Now in my own words (not fully RFC compatible ;-) ...

In IP DSCP: PPPDD0, where PPP=IP precedence/PHB and DD= drop precedence.

E-LSP in short: copy IP precedence (3 Bits) into the 3 MPLS EXP bits and apply DiffServ on a per interface level. There are no settings in the label, which would allow Weighted RED to see different drop precedence.

L-LSP in short: create one label per AF class (also for EF, BE) and copy drop precedence of the DSCP field in the IP header to the EXP bits. This allows Weighted RED to use the EXP bits (aka original drop precedence) and classification is based on the different labels.

Hope this helps! PLease rate all posts.

Regards, Martin

Community Member

Re: differentaiated services

hi martin,

hi martin,

thanks for ur reply

Just a query,

ironically (but true): L-LSP differs from E-LSP in such way, that E-LSP is implemented and L-LSP is not.

u meant to say that noone is using L-LSP in real time networks..?

Thanks

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