Basic Concepts Of Mpls - 3Com MSR 50 Series Configuration Manual

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Basic Concepts of MPLS

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ASICS ONFIGURATION
For details about MPLS architecture, refer to RFC 3031 "Multiprotocol Label
Switching Architecture".
FEC
As a forwarding technology based on classification, MPLS groups packets to be
forwarded in the same manner into a class called the forwarding equivalence class
(FEC). That is, packets of the same FEC are handled in the same way.
The classification of FECs is very flexible. It can be based on any combination of
source address, destination address, source port, destination port, protocol type
and VPN. For example, in the traditional IP forwarding using longest match, all
packets to the same destination belongs to the same FEC.
Label
A label is a short fixed length identifier for identifying a FEC. A FEC may
correspond to multiple labels in scenarios where, for example, load sharing is
required, while a label can only represent a single FEC.
A label is carried in the header of a packet. It does not contain any topology
information and is local significant.
A label is four octets, or 32 bits, in length.
Figure 377 Format of a label
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A label consists of four fields:
Label: Label value of 20 bits. Used as the pointer for forwarding.
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Exp: For QoS, three bits in length.
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S: Flag for indicating whether the label is at the bottom of the label stack, one
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bit in length. 1 indicates that the label is at the bottom of the label stack. This
field is very useful when there are multiple levels of MPLS labels.
TTL: Time to live (TTL) for the label. Eight bits in length. This field has the same
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meaning as that for an IP packet.
Similar to the VPI/VCI in ATM and the DLCI in frame relay, an MPLS label functions
as a connection identifier. If the link layer protocol has a label field like VPI/VCI in
ATM or DLCI in frame relay, the MPLS label is encapsulated in that field.
Otherwise, it is inserted between the data link layer header and the network layer
header as a shim. As such, an MPLS label can be supported by any link layer
protocol.
Figure 378 shows the place of a label in a packet.
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Label
Figure 377 illustrates its format.
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EXP S TTL
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