Juniper JUNOSE 11.2.X BGP AND MPLS, Руководство по настройке

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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:
•
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.
•
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.
CE 6 receives the IP packet from PE 2 and looks up the destination address Host 2.
Subsequent forwarding to Host 2 occurs by means of the IGP in the customer site.
The network structure shown in Figure 75 on page 396 consists of two VPNs, A and B. VPN
A comprises CE 1, CE 5, and CE 6. VPN B comprises CE 2, CE 3, and CE 4. CE 1 has data
traffic destined for both CE 5 and CE 6. Because both of these destination sites are within
the same VPN, PE 1 uses the same forwarding table, in VRF A, to do the lookups and
MPLS encapsulation. The innermost label determines the destination VRF and is the
same for all packets in that VPN, even if they are destined for different CE routers. CE 2
and CE 3 have traffic destined for CE 4. Because these all are in VPN B, PE 1 uses a different
forwarding table, in VRF B, for looking up destinations for traffic originating with these
sites. However, both VPNs use the same LSP, because both VPNs use the same ingress
(PE 1) to and the same egress (PE 2) from the service provider core. Remember that the
illustrated LSP carries data traffic only from PE 1 to PE 2. Traffic from PE 2 to PE 1 requires
a different LSP.
397 Copyright © 2010, Juniper Networks, Inc.
Chapter 6: Configuring BGP-MPLS Applications