Juniper BGP, Configuration Manual

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Figure 75: Traffic Across the MPLS Backbone of a BGP/MPLS VPN
Host 1 constructs an IP packet with the address of Host 2 as the final destination,
and sends the packet to router CE 1. CE 1 encapsulates the packet appropriately and
forwards it to PE 1.
PE 1 receives the packet from CE 1. Based on the interface the packet came in on,
PE 1 determines that it must use the forwarding table for VRF A to route the packet.
PE 1 looks up the destination address of Host 2 in the forwarding table of VRF A and
finds the following instructions:
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Push label 16; that is, prepend it to the data packet. This innermost label identifies
the VRF on PE 2, where the final destination and interface lookup takes place.
Label 16 was previously allocated by PE 2 and communicated to PE 1 by MP-BGP.
VRF A shows this label as part of the NLRI for destination address Host 2.
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Push label 21 and forward the MPLS-encapsulated data packet to router P 1.
Label 21 is prepended to label 16; the labels are
stacked . Label 21 becomes the
outermost label and is assigned to the first segment—PE 1–P 1—in the
label-switched path from PE 1 to PE 2. The LSP was previously configured.
P 1 receives the data packet from PE 1 and pops label 21. P 1 looks up label 21 in
its forwarding table and determines it must push label 19 on the stack, and forwards
the data packet to P 2.
P 2 receives the data packet from P 1 and pops label 19. P 2 looks up label 19 in its
forwarding table and determines it must push label 46 on the stack, and forwards
the data packet to PE 2.
PE 2 receives the data packet from P 2, and looks up label 46. PE 2 determines it is
the egress router of the LSP and must pop label 46. Then it proceeds to look up the
next label, label 16, and determines that the packet goes to VRF A. Then the IP
address is looked up in VRF A to determine the destination and outgoing interface
for the packet. PE 2 forwards the packet to CE 6.
Transporting Packets Across an IP Backbone with MPLS
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Chapter 5: Configuring BGP-MPLS Applications