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Mobile Backhaul by Esa Metsala, Juha Salmelin

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7.5 MPLS Resilience

In the following section resilience topics related to MPLS are discussed. Further reading can be found e.g. by De Ghein [38], Minei and Lucek [39] and Guichard, Le Faucheur and Vasseur [40].

7.5.1 Label Allocation

With IP forwarding, MPLS Labels are assigned by LDP [41] to the destination networks. Information of the destination networks is obtained via an IGP.

For the label distribution two operating modes exist: downstream on demand and unsolicitated downstream. In both cases labels are allocated locally and LDP delivers the locally assigned label to the upstream node. In downstream on demand mode the label is allocated only as a response to a request. In unsolicitated downstream the downstream LSR allocates the labels to all destination networks it knows of without a specific request.

When labels are allocated to each destination network and all upstream neighbours are informed of the labels, how does the upstream LSR decide what to do with multiple labels to the same destination network? With conservative label retention labels that have not been requested, will not be kept. In liberal retention mode all labels that have been received are kept.

In RFC3037 [42], a motivation is given that downstream on demand with conservative label retention should be used when labels are a scarce resource, and should be conserved. ATM and cell mode –MPLS is given as an example. In frame mode MPLS this is not the case. Standard allows all combinations.

Liberal retention ...

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