The MPLS WG Archive

[David Charlap <david.charlap@marconi.com>]

Mayank Kumar wrote:
> ur argument makes sense. Thank you . But i do not understand what do you
> mean by VCID's. Is it some combination of VPI and VCI.

Yes.  It's the combined VPI/VCI value that represents a single ATM 
connection.

> When u say that VCID's are
> allocated on aper interface basis , do u mean that vcid's are unique for a
> specific interface only and thus there can be two identical VCID's on two
> different atm interfaces or for that matter ports.

Yes.  The same VCID can exist on two different ports.  When this 
happens, there is absolutely no relationship between the two connections.

> Also then i can infer one more thing that, the platform wide label space
> contains only shim-labels (that means the generic encapsulation specified in
> rfc 3032). Also  the pw wide label space contains just the 20 bit labels .

This is my understanding.  If platform-wide labels are implemented on a 
platform, then they are used only on interfaces where shim headers are 
used to store the label.  And shim headers only store 20-bit labels.

> It does not make sense that the label space contain the 3 bit EXP and the S
> and the TTl fields too.

EXP and TTL fields are not part of the labels.  While your forwarding 
hardware may manipulate all three of these, they serve different purposes.

Labels serve to identify the route from ingress to egress (and may also 
identify QoS, in the case of an IntServ LSP or a DiffServ L-LSP.)

The EXP bits serve to identify the QoS in the case of a DiffServ E-LSP.

The TTL is decremented as packets are forwarded and only affects traffic 
flow if it decrements to zero.

-- David