NAT-PE System Flow


Now let us take a detailed look at the packet flow for NAT on the PE device. Packets can flow in both directions from hosts in global address space to hosts in private address space. Hosts in global address space are outside hosts, and hosts in private address space are inside hosts.

Inside-to-Outside Packet Flow

In this example, packets from a customer(s) in VPN A and VPN B with overlapping addresses are destined to the data center. After route lookup, NAT performs the translation (static or dynamic) and stores the VRF table ID in the translation entry.

Outside-to-Inside Packet Flow

NAT gets hold of the packet before routing and does a lookup on the translation table. NAT performs the reverse translation and sets the VRF table ID in the packet descriptor header.

This enables the subsequent route lookup to be done on the right forwarding information block.

If the outgoing interface is in a VRF on the same PE, the packet is forwarded an IP packet. If the destination is on a remote PE, the packet is imposed with an MPLS label and forwarded on the core facing interface.

Figure 13-4 depicts NAT and MPLS VPN shared services.

Figure 13-4. NAT and MPLS VPN for Shared Services





MPLS and Next-Generation Networks(c) Foundations for NGN and Enterprise Virtualization
MPLS and Next-Generation Networks: Foundations for NGN and Enterprise Virtualization
ISBN: 1587201208
EAN: 2147483647
Year: 2006
Pages: 162

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