NAT Timeout

The ip nat translation {dns-timeout | finrst-timeout | icmp-timeout | port-timeout | pptp-timeout | syn-timeout | tcp-timeout | timeout | udp-timeout} {sec | never} global configuration command changes the amount of time which Network Address Translation (NAT) translations time out.

Router#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Router(config)#ip nat translation ?
  dns-timeout     Specify timeout for NAT DNS flows
  finrst-timeout  Specify timeout for NAT TCP flows after a FIN or RST
  icmp-timeout    Specify timeout for NAT ICMP flows
  max-entries     Specify maximum number of NAT entries
  port-timeout    Specify timeout for NAT TCP/UDP port specific flows
  pptp-timeout    Specify timeout for NAT PPTP flows
  syn-timeout     Specify timeout for NAT TCP flows after a SYN and no 
                  further data
  tcp-timeout     Specify timeout for NAT TCP flows
  timeout         Specify timeout for dynamic NAT translations
  udp-timeout     Specify timeout for NAT UDP flows

Router(config)#

Below lists the defaults value for the corresponding NAT timeouts:

dns-timeout	60 seconds (1 minute)
finrst-timeout	60 seconds (1 minute)
icmp-timeout	60 seconds (1 minute)
pptp-timeout	86400 seconds (24 hours)
syn-timeout	60 seconds (1 minute)
tcp-timeout	86400 seconds (24 hours)
timeout	86400 seconds (24 hours)
udp-timeout	300 seconds (5 minutes)

Below shows that the NAT timeout for an ICMP session is 1 minute:

Router#sh ip nat translations
Pro Inside global      Inside local       Outside local      Outside global
--- 100.100.100.21     192.168.2.21       ---                ---
--- 100.100.100.22     192.168.2.22       ---                ---
Router#
Router#sh ip nat translations
Pro Inside global      Inside local       Outside local      Outside global
icmp 100.100.100.21:6  192.168.2.21:6     172.16.2.21:6      172.16.2.21:6
--- 100.100.100.21     192.168.2.21       ---                ---
--- 100.100.100.22     192.168.2.22       ---                ---
Router#
00:51:44: NAT*: s=192.168.2.21->100.100.100.21, d=172.16.2.21 [30]
00:51:44: NAT*: s=172.16.2.21, d=100.100.100.21->192.168.2.21 [30]
00:52:44: NAT: expiring 100.100.100.21 (192.168.2.21) icmp 6 (6)
Router#