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Cisco ASA Series CLI Configuration Guide
Chapter 1 Information About NAT
Routing NAT Packets
free addresses, a consideration if you are using a 1:1 translation like dynamic NAT or static NAT.
Dynamic PAT greatly extends the number of translations you can use with a small number of
addresses, so even if the available addresses on the outside network is small, this method can be
used. For PAT, you can even use the IP address of the mapped interface.
Note
If you configure the mapped interface to be any interface, and you specify a mapped address
on the same network as one of the mapped interfaces, then if an ARP request for that mapped
address comes in on a
different
interface, then you need to manually configure an ARP entry
for that network on the ingress interface, specifying its MAC address (see the
arp
command). Typically, if you specify any interface for the mapped interface, then you use a
unique network for the mapped addresses, so this situation would not occur.
•
Addresses on a unique network.
If you need more addresses than are available on the mapped interface network, you can identify
addresses on a different subnet. The upstream router needs a static route for the mapped addresses
that points to the ASA. Alternatively for routed mode, you can configure a static route on the ASA
for the mapped addresses, and then redistribute the route using your routing protocol. For
transparent mode, if the real host is directly-connected, configure the static route on the upstream
router to point to the ASA: specify the bridge group IP address. For remote hosts in transparent
mode, in the static route on the upstream router, you can alternatively specify the downstream router
IP address.
•
The same address as the real address (identity NAT).
The default behavior for identity NAT has proxy ARP enabled, matching other static NAT rules. You
can disable proxy ARP if desired.
Note
: You can also disable proxy ARP for regular static NAT if
desired, in which case you need to be sure to have proper routes on the upstream router.
Normally for identity NAT, proxy ARP is not required, and in some cases can cause connectivity
issues. For example, if you configure a broad identity NAT rule for “any” IP address, then leaving
proxy ARP enabled can cause problems for hosts on the network directly-connected to the mapped
interface. In this case, when a host on the mapped network wants to communicate with another host
on the same network, then the address in the ARP request matches the NAT rule (which matches
“any” address). The ASA will then proxy ARP for the address, even though the packet is not actually
destined for the ASA. (Note that this problem occurs even if you have a twice NAT rule; although
the NAT rule must match both the source and destination addresses, the proxy ARP decision is made
only on the “source” address). If the ASA ARP response is received before the actual host ARP
response, then traffic will be mistakenly sent to the ASA (see
Summary of Contents for 5505 - ASA Firewall Edition Bundle
Page 28: ...Glossary GL 24 Cisco ASA Series CLI Configuration Guide ...
Page 61: ...P A R T 1 Getting Started with the ASA ...
Page 62: ......
Page 219: ...P A R T 2 Configuring High Availability and Scalability ...
Page 220: ......
Page 403: ...P A R T 2 Configuring Interfaces ...
Page 404: ......
Page 499: ...P A R T 2 Configuring Basic Settings ...
Page 500: ......
Page 533: ...P A R T 2 Configuring Objects and Access Lists ...
Page 534: ......
Page 601: ...P A R T 2 Configuring IP Routing ...
Page 602: ......
Page 745: ...P A R T 2 Configuring Network Address Translation ...
Page 746: ......
Page 845: ...P A R T 2 Configuring AAA Servers and the Local Database ...
Page 846: ......
Page 981: ...P A R T 2 Configuring Access Control ...
Page 982: ......
Page 1061: ...P A R T 2 Configuring Service Policies Using the Modular Policy Framework ...
Page 1062: ......
Page 1093: ...P A R T 2 Configuring Application Inspection ...
Page 1094: ......
Page 1191: ...P A R T 2 Configuring Unified Communications ...
Page 1192: ......
Page 1333: ...P A R T 2 Configuring Connection Settings and QoS ...
Page 1334: ......
Page 1379: ...P A R T 2 Configuring Advanced Network Protection ...
Page 1380: ......
Page 1475: ...P A R T 2 Configuring Modules ...
Page 1476: ......
Page 1549: ...P A R T 2 Configuring VPN ...
Page 1550: ......
Page 1965: ...P A R T 2 Configuring Logging SNMP and Smart Call Home ...
Page 1966: ......
Page 2059: ...P A R T 2 System Administration ...
Page 2060: ......
Page 2098: ...1 8 Cisco ASA Series CLI Configuration Guide Chapter 1 Troubleshooting Viewing the Coredump ...
Page 2099: ...P A R T 2 Reference ...
Page 2100: ......