At the entry of a MPLS tunnel, which packet fields are used to determine the label of an incoming packet?

Here it says that there are other attributes that are taken into account besides the IP destination address, without specifying what else exactly:

Ingress routers at the edge of the MPLS network classify each packet potentially using a range of attributes, not just the packet's destination address, to determine which LSP to use. Inside the network, the MPLS routers use only the LSP labels to forward the packet to the egress router.

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    Why is my question off-topic? – Ricky Robinson Jan 7 '14 at 17:26
  • The question is extremely vague about what MPLS services are even running; thus it's impossible to say how labels are allocated. – Mike Pennington Jan 19 '14 at 9:52
  • It is, as you say, extremely vague, because I wanted to know if any two packets with the same IP destination followed the same MPLS tunnel. That's all. From your comment I might deduce that this depends on the MPLS implementation. – Ricky Robinson Jan 19 '14 at 12:50
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    I'm sorry, you didn't ask for whether two packets with the same IP destination are forwarded through the same tunnel. You asked for which packet fields are used to determine the label of an incoming packet. What you're missing is that mpls is a vague term... it isn't a single service, it includes a family of services, and at least three different protocols are used to bind labels to those services (LDP, RSVP, BGP, to name some off the top of my head) – Mike Pennington Jan 19 '14 at 13:03

Good question, possibly better served on the Network Engineering beta SE site.

As far as I know the RFC's don't specify what attributes an LSR takes into account to form the label inside the MPLS-SHIM. (http://www.ietf.org/rfc/rfc3031.txt?number=3031) ...However LDP (label distribution protocol) which is typically the protocol used to determine the MPLS label has it's own RFC (3036) found here: http://tools.ietf.org/search/rfc3036 but even there I don't see a specification on what attributes are required/optional to determine a label ID.

For instance Juniper's attributes it takes into account are found here: http://www.juniper.net/techpubs/en_US/junose14.3/information-products/topic-collections/swconfig-bgp-mpls/index.html?topic-49178.html

Reading the RFCs it looks like they allow for either static or dynamic labeling to setup the label as long as the downstream routers can understand the label and match it to the appropriate NHLFE (otherwise the norm is to discard the packet). The dynamic ones use standards like LDP (label distribution protocol), BGP, or RSVP.

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  • Thank you for your detailed answer. Is there a way to move the question to that SE site you mentioned? – Ricky Robinson Jan 7 '14 at 15:05
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    The question was deleted from Network Engineering... – Ricky Robinson Jan 7 '14 at 17:27
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    @RickyRobinson - I don't know why it was deleted on the NE site. I asked Tom to move it there, which he did. Perhaps the mods there didn't think it was applicable (which is pretty freaking stupid IMO). I'll see about getting it re-opened here. – TheCleaner Jan 7 '14 at 17:30

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