3Com 3C13636 Configuration Manual page 833

3Com Router 3000 Ethernet Family
Configuration Guide
Label is located between the link layer header and the network layer packet, with the
length of 4 bytes. A label contains four fields:
Label: label value, 20bits, used as the pointer for forwarding.
Exp: 3bits, reserved for test.
S: 1bit, MPLS supports hierarchical label structure, i.e., multi-layer label. Value 1 refers
to the label of bottom layer.
TTL: 8 bits, similar to TTL in IP packets.
III. Label operations
1) Label mapping
There are two types of label mapping: one is label mapping at ingress routers and the
other is label mapping in MPLS domain.
The first type is also called ingress LSR, in which input packets are grouped on a
certain principle into multiple FECs. The corresponding labels are mapped to these
FECs and the mapping results are recorded into label information base (LIB). In simple
words, it is to assign a label to a FEC.
The second type is also called incoming label mapping (ILM), that is, to map each input
label to a series of next hop label forwarding entries (NHLFE). The packets are
forwarded along the paths based on the mapping results.
2) Label encapsulation
Label encapsulation in different media is illustrated in Figure 1-2.
Ethernet/SONET/
SDH packets
Frame-mode
ATM packets
Cell-mode ATM
packets
Figure 1-2 Label position in packet
3) Label assignment and distribution
Label distribution refers to the process for a FEC to create corresponding label
switching path (LSP).
In the MPLS architecture, the decision to bind a particular label to a particular FEC is
made by downstream LSR, and the downstream LSR notifies the upstream LSR. That
is to say, the label is specified by the downstream LSR, and the label is distributed from
downstream to upstream.
Two label distribution modes are available in MPLS: Downstream unsolicited (DU)
mode and downstream on demand (DoD) mode.
Ethernet header
Label
/PPPheader
ATM header
Label
VPI/VCI
3Com Corporation
L3 data
L3 data
L3 data
1-2
Chapter 1 MPLS Architecture