NAT precedence

Unanswered Question
husycisco Tue, 01/01/2008 - 19:15
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Hi Aksher

You have never rated the useful posts of experts here. Please rate useful. You have NEVER! rated. Please show respect. Rating a post does not cost any fee.


JORGE RODRIGUEZ Tue, 01/01/2008 - 21:16
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Agree.. we are all always in the look to help out others, in return it is good to use the rating system which in fact helps netpros to improve even more in assisting others.

Christopher Dreier Tue, 01/01/2008 - 21:46
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Here is the order of operations for NAT on the firewall:

1. nat 0 access-list (nat-exempt)

2. Match existing xlates

3. Match static commands

a. Static NAT with and without access-list

b. Static PAT with and without access-list

4. Match nat commands

a. nat [id] access-list (first match)

b. nat [id] [address] [mask] (best match)

i. If the ID is 0, create an identity xlate

ii. Use global pool for dynamic NAT

iii. Use global pool for dynamic PAT

Be aware that in your second example, you are not referencing the ACL listed directly below it. The space inside the parenthesis is used to reference an interface. (2 would fall in to the OoO 4bi and 1) would fall in to the OoO 4a



Christopher Dreier Wed, 01/02/2008 - 06:16
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It means any existing xlate currently in the xlate table.


You have the following command in your config:

static (inside,outside) netmask

You build an xlate with this static. You then experience traffic that would use this same static, but you didn't check the xlate table. - Problem!

So you should always check to see if an xlate exists in the table before attempting to create a translation based on a NAT rule.


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