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MPLS Overview (part two)

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MPLS Overview (part one)


LSP Setup

MPLS packets need to pre-allocate a good label to establish an LSP, to forward packets. Divided into static and dynamic LSP LSP LSP two kinds.


Static LSP establishment:

User static LSP is assigned for each FEC labels established manually. Since each node of the static LSP can not perceive the entire LSP to each other, so that the static LSP is a local concept.


Static LSP without using a label distribution protocol to exchange control packets, thus consuming little resources for simple topology and stability of small networks. But established by statically allocated label LSP can not dynamically adjust according to changes in the network topology, require administrator intervention.


When configuring a static LSP, the administrator needs to manually allocate labels for each LSR, following the rule is: the value of the previous node label is equal to the value of the next node tag.

Dynamic LSP establishment:

Dynamic LSP label distribution protocol


Dynamic LSP established by dynamic label distribution protocol. Label distribution protocols are MPLS control protocol (also referred to as signaling protocol), responsible for the FEC classification, labeling and distribution of LSP establishment and maintenance of a series of operations.


The basic dynamic LSP establishment process


Tags, distributed by the downstream LSR dispensing direction from downstream to upstream. Below, on the basis of dividing FEC IP routing table on the downstream LSR, the label allocation according to FEC, advertised to the upstream LSR, in order to establish and label forwarding table LSP.


Figure: The basic dynamic process of establishing the LSP

MPLS forwarding

Basic MPLS forwarding process:

basic concept

In the MPLS forwarding process related to the basic concepts involved are as follows:


Tag includes a label press-operation type (Push), label switching (Swap) and pop-up tags (Pop), which is the basic operation of the label forwarding.


Push: When the IP packets MPLS domain, MPLS edge device the packet is inserted into a new label between Layer header and IP header; intermediate devices as needed or MPLS, label stack adds a new label (i.e., inlay sleeve packaging).


Swap: When a packet forwarding in the MPLS domain, according to the label forwarding table, next hop assigned label, replace the top label MPLS packets.


Pop: When packets leave the MPLS domain, the MPLS packet label peeled off.


In the last hop node, the label has no value. In this case, you can use the penultimate hop popping characteristic PHP (Penultimate Hop Popping), in the penultimate hop at the label pop-up, reduce the burden on the last hop. The last-hop IP forwarding node directly or label forwarding the next layer.


By default, the label, the device supports PHP PHP-Egress node assigned to the penultimate hop is 3.


The basic forwarding process:

LSP to support PHP example to illustrate the basic MPLS forwarding process.

Figure: MPLS basic forwarding process


As shown above, the MPLS label distribution has completed, the establishment of an LSP, the destination address which is The basic MPLS forwarding process is as follows:


Ingress node receives the IP packet destination address is, the label Z and pushed forward.

Transit nodes receiving the labeled packet, label switching, the label into the label Y. Z

Transit penultimate hop node receives the packet with the label Y. Because Egress give its label value is 3, the operation for PHP, Y pops the label and forwards the packet. Egress issued from the penultimate jump packets are transferred in the form of IP packets.

Egress node receives the IP packet and forwards it to the destination


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