Mpls Configuration Basics; Label Advertisement And Management - 3Com MSR 50 Series Configuration Manual

1318 C 76: MPLS B
HAPTER
MPLS Configuration
Basics
Label Advertisement
and Management
C
ASICS ONFIGURATION
The Diff-Serv model maps a service to a certain service class at the network edge
according to the QoS requirement of the service. The DS field (derived from the
TOS field) in the IP packet identifies the service uniquely. Then, each node in the
backbone network performs the preset service policies to diversified services
according to the field to ensure the corresponding QoS.
The QoS classification and label mechanism in Diff-Serv is similar to the MPLS label
distribution. In fact, the MPLS-based Diff-Serv is implemented by integrating the
DS distribution into the MPLS label distribution.
In MPLS, the decision to assign a particular label to a particular FEC is made by the
downstream LSR. The downstream LSR informs the upstream LSR of the
assignment. That is, labels are advertised in the upstream direction.
Label advertisement mode
Two label advertisement modes are available:
Downstream on demand (DoD): In this mode, a downstream LSR binds a label
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to a particular FEC and advertises the binding only when it receives a label
request from its upstream LSR.
Downstream unsolicited (DU): In this mode, a downstream LSR does not wait
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for any label request from an upstream LSR before binding a label to a
particular FEC.
An upstream LSR and its downstream LSR must use the same label advertisement
mode; otherwise, no LSP can be established normally. For more information, refer
to "LDP Label Distribution" on page
Label distribution control mode
There are two label distribution control modes:
Independent: In this mode, an LSR can notify label binding messages upstream
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anytime. The drawback of this mode is that an LSR may have advertised to the
upstream LSR the binding of a label to a particular FEC when it receives a
binding from its downstream LSR.
Ordered: In this mode, an LSR can send label binding messages about a FEC
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upstream only when it receives a specific label binding message from the next
hop for a FEC or the LSR itself is the egress node of the FEC.
Label retention mode
Label retention mode dictates how to process a label to FEC binding that is
received by an LSR but not useful at the moment.
There are two label retention modes:
Liberal: In this mode, an LSR keeps any received label to FEC binding regardless
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of whether the binding is from its next hop for the FEC or not.
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