IP NAT

[pbelletty]

Hi Folks,

I have a query on NAT that I hope you can help with.

Have static nat configured on a router. The inside interface is on
an Ethernet interface and the outside on a dialer interface.

When a packet transits the router NAT occurs however when I look in the
NAT table there is a dynamic entry.

Any idea why there is a dynamic entry?

Cheers,
Phil

[cadet alain]

Hi,

It is corresponding to the actual traffic matching the static nat. the static entries is a catch-all that is inserted into nat table when the static nat is configured contrary to dynamic nat where there is only a dynamic entry once there is traffic corresponding to nat config.

Regards.

Alain.

[pbelletty]

Hi Alain - Many thanks for the reply.

I am not sure what you are saying. Could you provide some more detail? Are you saying that the router will create dynamic nat entries when it matches a static nat?

My nat translation table shows the static nat entries. When a packet traverses the router that matches the static NAT it creates a dynamic entry in the table.

My understanding is that there should be no dynamic entry.

Here is the NAT table with no packets traversing the router.

engn-r-lasw-74#sh ip nat translations

Pro Inside global         Inside local          Outside local         Outside global

--- 172.24.59.25          194.223.243.1         ---                   ---

--- 172.24.59.26          194.223.243.2         ---                   ---

--- 172.24.59.29          194.223.243.3         ---                   ---

--- 172.24.59.30          194.223.243.4         ---                   ---

--- 172.24.59.33          194.223.243.5         ---                   ---

--- 172.24.59.37          194.223.243.7         ---                   ---

--- 172.24.59.38          194.223.243.8         ---                   ---

--- 172.24.59.41          194.223.243.9         ---                   ---

--- 172.24.59.42          194.223.243.10        ---                   ---

--- 172.24.59.45          194.223.243.11        ---                   ---

--- 172.24.59.46          194.223.243.12        ---                   ---

--- 172.24.59.49          194.223.243.13        ---                   ---

--- 172.24.59.50          194.223.243.14        ---                   ---

--- 172.24.109.121        194.223.243.15        ---                   ---

--- 172.24.109.122        194.223.243.16        ---                   ---

--- 172.24.109.126        194.223.243.17        ---                   ---

When a packet traverses the router I see..

engn-r-lasw-74# engn-r-lasw-74#sh ip nat translation
Pro Inside global         Inside local          Outside local         Outside global
--- 172.24.59.25          194.223.243.1         ---                   ---
--- 172.24.59.26          194.223.243.2         ---                   ---
--- 172.24.59.29          194.223.243.3         ---                   ---
--- 172.24.59.30          194.223.243.4         ---                   ---
--- 172.24.59.33          194.223.243.5         ---                   ---
--- 172.24.59.37          194.223.243.7         ---                   ---
--- 172.24.59.38          194.223.243.8         ---                   ---
--- 172.24.59.41          194.223.243.9         ---                   ---
--- 172.24.59.42          194.223.243.10        ---                   ---
--- 172.24.59.45          194.223.243.11        ---                   ---
--- 172.24.59.46          194.223.243.12        ---                   ---
--- 172.24.59.49          194.223.243.13        ---                   ---
tcp 172.24.59.50:52195    194.223.243.14:52195  193.113.131.49:5000   193.113.131.49:5000
tcp 172.24.59.50:52196    194.223.243.14:52196  193.113.131.49:5000   193.113.131.49:5000
--- 172.24.59.50          194.223.243.14        ---                   ---
--- 172.24.109.121        194.223.243.15        ---                   ---
--- 172.24.109.122        194.223.243.16        ---                   ---
--- 172.24.109.126        194.223.243.17        ---                   ---

Best regards,

Phil

[cadet alain · Accepted solution]

Hi Phil,

the staic nat creates static entries with no outside addresses and then once you get traffic then it will create an entry with all the addresses involved, what you call dynamic entries are these.It's a normal behaviour.

Regards.

Alain.

[pbelletty]

Hi Alain,

That's great - thanks for the explanation.

Regards,

Phil