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MPLS question


Can someone tell me what is the correct answer for this question

Which statement pertains to the IP forwarding table ? (this question is pertaining to the topic on typical distribution over LC-ATM interfaces and VC merge)

A. The IP forwarding table is built only after the label request have been answered (with labels) from upstream LSRs

B. The IP forwarding table is built only after the label request have been answered (with labels) from downstream LSRs

my take for this question is B but the answers provided for this question is A.




Re: MPLS question

corerct answer is b becasue in atm table is bulit whn labels are answered.for more you can refer

vpn and mpls architecure

Community Member

Re: MPLS question


I have a related question.

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.

Thanks in advance.


Re: MPLS question

Hello Mohan,

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.

NB: When in doubt, read the RFC!

Hope this helps! Please use the rating system.

Regards, Martin

Community Member

Re: MPLS question

Thank you Martin. I read and understood the same from the MPLS and VPN book later.

Thank you for your time.

But my first question is still unanswered.

Thank you,


Re: MPLS question

Hi Mohan,

to answer your question, label allocation in

frame-mode MPLS: downstream unsolicited

cell-mode MPLS: downstream on demand

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).

Regards, Martin

Community Member

Re: MPLS question

Thanks Martin. That helps.



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