Forward Equivalence Class; Figure 6 Mpls Label - Zte ZXR10 8900 Series User Manual

ZXR10 8900 Series User Manual (MPLS Volume)
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A label contains four fields:
� Label field: 20 bits long, used to forward pointers.
� Exp field: 3 bits long, reserved for defining experiments.
�
S field: 1 bit long. MPLS supports multiple layers of labels.
When the value of this field is 1, it means that this entry is
a label that locates the bottom of the label stack. When the
value of this field is 0, it means that this entry is a label that
locates any position of the label stack except the bottom.
� TTL field: 8 bits long, used for coding time to live.
MPLS supports different protocols in data link layer. MPLS label is
encapsulated after data link layer message and before layer 3 data.
Each protocol has different protocol number defined for MPLS.
In Ethernet networks, MPLS packets are identified by value 0x8847
(for unicast) and value 0x8848 (for multicast). In PPP, a new NCP,
MPLSCP, is identified by value 0x8281.
The S field of a label is shown in
message can be nested with multiple labels.
F 6 MPLS L
IGURE ABEL
When a message is encapsulated with multiple labels, LSR deals
the message with First In First Out (FIFO) rule. That is,
forwards the message according to the top label.

Forward Equivalence Class

Forward Equivalence Class (FEC) is a representation of a group of
packets that share the same requirements for their transport. All
packets in such a group are provided with the same treatment.
During labeling binding of LDP, different FECs correspond to differ-
ent labels. Each node in MPLS domain identifiers the FEC to which
the packet belongs according to the label.
When two groups of packets with the same source address and dif-
ferent destination addresses enter a MPLS domain, MPLS judges
the groups according to FEC and MPLS finds that the two groups
belong to two FECs. Therefore, MPLS treats the groups in differ-
ent ways, for example, in different paths and reserving different
resources. The two groups are classified into different classes at
the ingress of the MPLS domain and encapsulated different labels.
The nodes in the MPLS domain forward the groups of packets ac-
cording to labels. When the packets leave the MPLS domain, the
Figure 6. In a MPLS domain, a
LSR just