Cisco GRS Configuration Manual page 23

Implementing Carrier Grade NAT on Cisco IOS XR Software
3 Once the packet reaches the designated interface or VLAN on ASR9K, it is forwarded to the inside VRF
either through static routing or ACL-based forwarding (ABL). Based on this routing decision, the packet
that needs address translation is determined and is forwarded to the App SVI that is bound to the VRF.
4 The packet is forwarded by AppSVI1 through a default static route. The destination address and the port
get translated because of the CGN configuration applied on ISM.
5 The ISM applies NAT64 to the packet and a translation entry is created. The CGN determines the destination
address from the FIB Look Up. It pushes the packet to the egress port.
6 The packet is then forwarded to the egress port on the interface through App SVI2. The packet is forwarded
by App SVI2 through the default static route. Then the packet is sent to the public network.
7 The packets that do not need the address translation can bypass the App SVI and can be forwarded to the
destination through a different static route and a different egress port.
The following figure illustrates the path of the packet coming from the public network to the private network.
The packet goes through the following steps when it travels from the public network to the private network:
1 In the network shown in this figure, the packet travels from the host A (having the IP address 11.11.11.2)
in the public network to host B (having the IP address 3001:DB8:E0E:E03::) in the private network. The
public address has to be mapped to the private address by NAT64 that is implemented in ISM.
2 The packet enters through the ingress port on the Gigabit Ethernet (GigE) interface at Slot 3.
OL-32659-01
Cisco IOS XR Carrier Grade NAT Configuration Guide for the Cisco CRS Router, Release 5.2.x
Implementing NAT 64 over ISM
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