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MPLS Fundamentals by Luc De Ghein - CCIE No. 1897

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VC-Merge

As mentioned, the upstream LSR requests a label for a prefix from its downstream LSR and so on, until it reaches the egress LSR. However, without VC-merge, the label requests are propagated from the ingress LSR to the egress LSR, even if an intermediate LSR has already received an outgoing label from its downstream LSR for that prefix. Look at Figure 5-10, where the ATM LSR brussels-atm has already received a label for the prefix 10.200.253.6/32 from its downstream LSR brussels. This first label was outgoing label 1/34 from brussels-atm to brussels. For the traffic from washington-atm to brussels, a second label will be requested from brussels by brussels-atm. This is the label 1/33.

Figure 5-10. Two Upstream LSRs

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