Cisco GRS Configuration Manual page 20

Implementing NAT 44 over ISM
1 In the network shown in this figure, the packet travels from the host A (having the IP address 10.222.5.55)
in the private network to host B (having the IP address 5.5.5.2) in the public network. The private address
has to be mapped to the public address by NAT44 that is implemented in ISM.
2 The packet enters through the ingress port on the Gigabit Ethernet (GigE) interface at Slot 0. While using
NAT44, it is mandatory that the packet enters through VRF.
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). After the inside VRF determines that the
packet needs address translation, it is forwarded to the App SVI that is bound to the VRF.
4 The packet is forwarded by AppSVI1 through a default static route (ivrf1). The destination address and
the port get translated because of the CGN configuration applied on ISM.
5 The ISM applies NAT44 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. An inside VRF is
mapped to an outside VRF. The outside VRF is associated with this interface. The packet is forwarded by
App SVI2 through the default static route (ovrf1). 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:
Cisco IOS XR Carrier Grade NAT Configuration Guide for the Cisco CRS Router, Release 5.2.x
10
Implementing Carrier Grade NAT on Cisco IOS XR Software
OL-32659-01