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I have a cisco 7206 vxr with 15.0 ios(c7200p-adventerprisek9-mz.151-3.S0a.bin) .I configured the overloaded nat there. some of confs:

ip nat pool nat a.b.c.2 a.b.c.254 netmask 255.255.255.0

ip nat inside source list nat pool nat overload

Also i do when needed ip nat inside and outside commands.

But the sh ip nat statistics gives :

Total active translations: 278405 (0 static, 278405 dynamic; 278460 extended)
Outside interfaces:
  GigabitEthernet0/1.42, GigabitEthernet0/3, GigabitEthernet0/3.9
  GigabitEthernet0/3.52
Inside interfaces: 
  GigabitEthernet0/1.45
Hits: 1325770289  Misses: 0
CEF Translated packets: 1248777875, CEF Punted packets: 1186298958
Expired translations: 569696201
Dynamic mappings:
-- Inside Source
[Id: 1] access-list nat pool nat refcount 278416
 pool nat: netmask 255.255.255.0
    start **a.b.c.2 end a.b.c.254**
    type generic, total addresses 253, **allocated 22 (8%)**, misses 0
nat-limit statistics:
 max entry: max allowed 0, used 0, missed 0

What is algorith of allocating public ip?(where i can read info about it)?

How i can utilizate all of ips?

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1 Answer 1

I experimented with this on 12.4T earlier this year. The short version is that IOS does not round-robin through the NAT pool. When you use NAT overload (essentially, PAT), IOS will only roll over to the next address in the pool if the requested IP/port combination is unavailable. This has the effect of causing most traffic to get NAT'd to the first address in the pool, the next most to the second address, and so on. This is addressed in Cisco's NAT FAQ.

The only Cisco platform I'm aware of that can perform true round-robin IP allocation from a NAT pool is the ASA running 8.4(3) or later. We've tested it and it does appear to work as advertised.

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> I experimented with this on 12.4T earlier this year. ok.Is it ASR asr1000rp2-advipservicesk9.03.04.02.S.151-3.S2.bin supports or later ioses true round-robin IP allocation for nat? –  naimson Dec 4 '12 at 7:10
    
I don't have anything running 15.x code, so I couldn't say. However, I don't see anything about NAT in the 15.1S release notes. –  James Sneeringer Dec 4 '12 at 15:16
    
ok.thanks.Can you suggest me another docs,or threads about this problem? –  naimson Dec 4 '12 at 20:51
    
The NAT FAQ I linked to in my answer is the best authoritative source I could find that describes how NAT pools behave in IOS. –  James Sneeringer Dec 4 '12 at 21:02
    
Is there way to more "sparse" nat public ip concentration except this one: splice a big private subnet into a big number of little with different ip nat pool nat ? Maybe i can play with nat parameters to achieve some effect? –  naimson Dec 5 '12 at 13:42

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