MPLS and VPN Architecture book says that the label allocation in Cell Mode MPLS is downstream-on-demand. Page 56 & 57.
MPLS configuration on Cisco IOS Software book says the label allocation method in cell mode is same as frame-mode.
I think the first one is correct. Can anyone let me know what the right answer is?
Also, the second book shows in fig 1-17 (page 23) that an MPLS label is inserted between AAL5 header and IP header. I don't think this is the case with cell-mode MPLS. Where is this label used at the egress? Can anyone explain why this label in inserted and what is the purpose?
My understanding is; if a pure IP packet is coming into the router then that IP packet is broken down into cells and are transmitted accoriding to the VPI/VCI information received from the downstream ATM LSR. If the received packet is an MPLS packet with a label, a mapping between this label and the VPI/VCI will already exist in the LFIB and the packet will be broken down into cells and will be transmitted according to the VPI/VCI values. So,there is no need to insert a label here. Only if you use routed VC (just like a point-to-point), then you need to insert a label, in which case it is not considered a cell-mode MPLS, but frame mode MPLS.
Please correct me if I am wrong. I am new to MPLS and could be very wrong here.
this can be answered directly from the RFC 3035 "MPLS using LDP and ATM VC Switching", Section 9:
" Note that if a packet has a label stack with only one entry, this
requires it to have a single-entry shim (4 bytes), even though the
actual label value is encoded into the VPI/VCI field. This is done
to ensure that the packet always has a shim. Otherwise, there would
be no way to determine whether it had one or not, i.e., no way to
determine whether there are additional label stack entries."
The actual section is longer and discusses more cases (worth reading), but the above excerpt is stating the underlying issue, why the RFC mandates the use of a label between AAL5 frame and its payload.
In other words, if there is a label stack (two or more) the AAL5 frame is the only way to send it. If there is only one label in the sending LSR, then the information about the encapsulation is transported. The label will be the explicit null label also allowing to preserve the experimental bits for proper QoS treatment.
The reason for the different approach in cell-mode is: the number of VCs supported on an interface is limited (order of thousands). Thus a LSP is only built if there is traffic. For the same reason VC merge was introduced.
After all cell-mode MPLS is not relevant in todays networks (exceptions might exist).
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