H3C Technologies H3C S7500E Series Switches - Manual (Page 44)

2-2
z
Presenting between the link layer and the network layer, MPLS can work above various link layer
protocols such as PPP, ATM, frame relay, and Ethernet, provide connection-oriented services for
various network layer protocols such as IPv4, IPv6, and IPX, and is compatible with the
mainstream network technologies.
z
As MPLS is connection-oriented and supports label stack, it is used in various services, such as
VPN, traffic engineering, and QoS.
Basic Concepts of MPLS
FEC
As a forwarding technology based on classification, MPLS groups packets with the same
characteristics (such as packets with the same destination or service class) into a class, called a
forwarding equivalence class (FEC). Packets of the same FEC are handled in the same way on an
MPLS network. At present, the device supports classifying FECs according to the network layer
destination addresses of packets.
Label
A label is a short, fixed length identifier for identifying a single FEC. A label is locally significant and
must be locally unique.
Figure 2-1 Format of a label
As shown in Figure 2-1 , a label is encapsulated between the Layer 2 header and Layer 3 header of a
packet. A label is four bytes in length and consists of four fields:
z
Label: 20 bits in length. Label value for identifying a FEC.
z
Exp: Three bits in length. Reserved field, usually used for CoS.
z
S: One bit in length. MPLS supports multiple levels of labels. This field is used to indicate whether
a label is at the bottom of the label stack. 1 indicates that the label is at the bottom of the label
stack.
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TTL: Eight bits in length. Like the homonymous IP header field, it is used to prevent loops.
LSR
A label switching router (LSR) is a fundamental component on an MPLS network. LSRs support label
distribution and label swapping.
LER
A label edge router (LER) resides at the edge of an MPLS network and is connected with another
network.
LSP
A label switched path (LSP) is the path along which packets of a FEC travel through an MPLS
network.