HP 12500 Series Configuration Manual page 10

Figure 1 Format of a label
0
Layer 2 header
As shown in
packet. A label is four bytes in length and consists of four fields:
Label—20 bits in length. Label value for identifying a FEC.
•
Exp—Three bits in length. Reserved field, usually used for CoS.
•
S—One bit in length. MPLS supports multiple levels of labels. This field indicates whether a label is
•
at the bottom of the label stack. 1 indicates that the label is at the bottom of the label stack.
TTL—Eight bits in length. Like the homonymous IP header field, it is used to prevent loops.
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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.
An LSP is a unidirectional path from the ingress of an MPLS network to the egress. On an LSP, two
neighboring LSRs are called the "upstream LSR" and "downstream LSR" respectively. In
the downstream LSR of LSR A; LSR A is the upstream LSR of LSR B.
Figure 2 Diagram for an LSP
LFIB
Labeled packets are forwarded according to the Label Forwarding Information Base (LFIB).
Control plane and forwarding plane
An MPLS node consists of two planes, control plane and forwarding plane.
• The control plane assigns labels, selects routes, creates the LFIB, and establishes and removes LSPs.
The forwarding plane forwards packets according to the LFIB.
•
Label
Label Layer 3 header
Figure 1, a label is encapsulated between the Layer 2 header and Layer 3 header of a
19 22 23
Exp S TTL
Layer 3 data
2
31
Figure 2, LSR B is