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Cisco ASA 5500 Series Configuration Guide using ASDM
Chapter 34 Configuring Twice NAT (ASA 8.3 and Later)
Configuring Twice NAT
Configuring Dynamic NAT or Dynamic PAT Using a PAT Pool
This section describes how to configure twice NAT for dynamic NAT or for dynamic PAT using a PAT
pool. For more information, see the “Dynamic NAT” section on page 32-8 or the “Dynamic PAT” section
on page 32-10.
Guidelines
For a PAT pool:
• If available, the real source port number is used for the mapped port. However, if the real port is not
available, by default the mapped ports are chosen from the same range of ports as the real port
number: 0 to 511, 512 to 1023, and 1024 to 65535. Therefore, ports below 1024 have only a small
PAT pool that can be used. (8.4(3) and later, not including 8.5(1) or 8.6(1)) If you have a lot of traffic
that uses the lower port ranges, you can now specify for a PAT pool a flat range of ports to be used
instead of the three unequal-sized tiers: either 1024 to 65535, or 1 to 65535.
• (8.4(3) and later, not including 8.5(1) or 8.6(1)) If you use the same PAT pool object in two separate
rules, then be sure to specify the same options for each rule. For example, if one rule specifies
extended PAT and a flat range, then the other rule must also specify extended PAT and a flat range.
For extended PAT for a PAT pool (8.4(3) and later, not including 8.5(1) or 8.6(1)):
• Many application inspections do not support extended PAT. See the “Default Settings” section on
page 46-4 in Chapter 46, “Getting Started with Application Layer Protocol Inspection,” for a
complete list of unsupported inspections.
• If you enable extended PAT for a dynamic PAT rule, then you cannot also use an address in the PAT
pool as the PAT address in a separate static NAT with port translation rule. For example, if the PAT
pool includes 10.1.1.1, then you cannot create a static NAT-with-port-translation rule using 10.1.1.1
as the PAT address.
• If you use a PAT pool and specify an interface for fallback, you cannot specify extended PAT.
• For VoIP deployments that use ICE or TURN, do not use extended PAT. ICE and TURN rely on the
PAT binding to be the same for all destinations.
For round robin for a PAT pool:
• (8.4(3) and later, not including 8.5(1) or 8.6(1)) If a host has an existing connection, then subsequent
connections from that host will use the same PAT IP address if ports are available. Note: This
“stickiness” does not survive a failover. If the ASA fails over, then subsequent connections from a
host may not use the initial IP address.
• (8.4(2), 8.5(1), and 8.6(1)) If a host has an existing connection, then subsequent connections from
that host will likely use different PAT addresses for each connection because of the round robin
allocation. In this case, you may have problems when accessing two websites that exchange
information about the host, for example an e-commerce site and a payment site. When these sites
see two different IP addresses for what is supposed to be a single host, the transaction may fail.
• Round robin, especially when combined with extended PAT, can consume a large amount of
memory. Because NAT pools are created for every mapped protocol/IP address/port range, round
robin results in a large number of concurrent NAT pools, which use memory. Extended PAT results
in an even larger number of concurrent NAT pools.