Cisco 7604 Configuration Guide - Page 307
NAT in Transparent Mode
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Chapter 16 Configuring NAT Figure 16-1 NAT Example: Routed Mode Web Server www.cisco.com NAT Overview Originating Packet Translation 10.1.2.27 209.165.201.10 Outside 209.165.201.2 10.1.2.1 Responding Packet Undo Translation 209.165.201.10 10.1.2.27 Inside 132925 10.1.2.27 See the following commands for this example: hostname(config)# nat (inside) 1 10.1.2.0 255.255.255.0 hostname(config)# global (outside) 1 209.165.201.1-209.165.201.15 NAT in Transparent Mode Using NAT in transparent mode eliminates the need for the upstream or downstream routers to perform NAT for their networks. For example, a transparent firewall FWSM is useful between two VRFs so you can establish BGP neighbor relations between the VRFs and the global table. However, NAT per VRF might not be supported. In this case, using NAT in transparent mode is essential. NAT in transparent mode has the following requirements and limitations: • When the mapped addresses are not on the same network as the transparent firewall, then on the upstream router, you need to add a static route for the mapped addresses that points to the downstream router (through the FWSM). • If the real destination address is not directly-connected to the FWSM, then you also need to add a static route on the FWSM for the real destination address that points to the downstream router. Without NAT, traffic from the upstream router to the downstream router does not need any routes on the FWSM because it uses the MAC address table. NAT, however, causes the FWSM to use a route lookup instead of a MAC address lookup, so it needs a static route to the downstream router. • The alias command is not supported. • Because the transparent firewall does not have any interface IP addresses, you cannot use interface PAT. • ARP inspection is not supported. Moreover, if for some reason a host on one side of the firewall sends an ARP request to a host on the other side of the firewall, and the initiating host real address is mapped to a different address on the same subnet, then the real address remains visible in the ARP request. OL-20748-01 Catalyst 6500 Series Switch and Cisco 7600 Series Router Firewall Services Module Configuration Guide using ASDM 16-3
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