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DES-7200 Configuration Guide Chapter 1 MPLS Configuration
1-9
hop device. In independent DU mode, one LSR can announce the label mapping of an FEC at
any time deemed as proper.
In ordered control mode, one LSR binds an FEC to a label and sends the binding upstream only
when the FEC has the next-hop label mapping or the LSR is the egress of the FEC. Otherwise,
the LSR does not bind the FEC to a label, or send the binding to an upstream LSR until receiving
the label mapping of the FEC from a downstream LSR. In ordered control and DU mode, one
LSR announces the label to an upstream LSR only when the LSR is the egress of the FEC or the
LSR receives the label distributed by a downstream LSR. If the label distribution mode of the
downstream LSR is DOD, the LSR, either in DOD or DU mode, passes on the label request from
an upstream LSR to downstream devices.
1.1.6.2 LSP Loop Control
During the LSP setup process, the loop detection mechanism must be provided to ensure timely
detection of any loops formed by the LSP. There are two methods to avoid LSP loops: the
maximum number of hops and path vector.
In the former mode, the messages that transmit label binding information record the number of
bypassing LSRs. The number increases by one after every LSR. If the number exceeds the
specified maximum value, the system considers that a loop occurs and terminates the LSP.
In the latter mode, the messages that transmit label binding information record the IDs of
bypassing LSRs. The ID of an LSR is recorded to the vector table of the message after each
LSR. Upon receipt of a label binding message, an LSR checks whether its own ID is included in
the vector table. If not, the LSR adds its own ID to the record when distributing the message; if
yes, the LSR considers that a loop occurs and terminates the LSP.
1.1.7 MPLS Applications
Thanks to the combination of Layer 2 switching and Layer 3 routing technologies, the MPLS
technology improves the forwarding rate of packets. With the development of the
Application-Specific Integrated Circuit (ASIC) technologies, the forwarding rate is no longer a
bottleneck in network development. As a result, the edges of MPLS in enhancing forwarding
rates are not remarkable. Due to the innate advantage of combining Layer 2 switching and Layer
3 routing technologies, however, MPLS still has unprecedented edges over other technologies in
terms of virtual private networks (VPNs) and traffic engineering (TE). In this context, MPLS
receives more and more attention. The MPLS applications also gradually shift to the application
areas of MPLS VPN and MPLS TE.