Help with NAT overload (PAT)

[eduardo]

Dear Collegues:

As a practice I have created a topology in which I am trying to run NAT from RA. The purpose is to translate subnet IPs 172.28.1.16/28 and 172.28.1.32/28 into the serial 0/3/0 ip address into the serial 0/3/0 IP which is 172.28.1.5. When I ping host AP (ip address 10.0.0.18) from host PC Call Center 2 (ip address 172.28.1.34) it is 100 % reachable. When I check the translations in RA I see that the translation is fine:

--------------------------------------------------------
RA#show ip nat translation
Pro Inside global Inside local Outside local Outside global
icmp 172.28.1.5:3207 172.28.1.34:3207 10.0.0.18:3207 10.0.0.18:3207
icmp 172.28.1.5:3208 172.28.1.34:3208 10.0.0.18:3208 10.0.0.18:3208
--------------------------------------------------------

However when I check the statistics it shows me the following results:

--------------------------------------------------------
RA# sho ip nat statistics
Total translations: 51 (0 static, 51 dynamic, 51 extended)
Outside Interfaces: Serial0/3/0
Inside Interfaces: FastEthernet0/0.2 , FastEthernet0/0.3
Hits: 2513 Misses: 3355
Expired translations: 3271
Dynamic mappings:
--------------------------------------------------------

And the counter of hits , misses and expired translations they are increasing progresively with every packet I sent from the source to the destination. My questions:

1) Is my NAT properly configured ?
2) Does anyone knows the posibles causes of why the expiration translation counter might increase in a router ?

Here I show my nat configuration:

TECH-LOGS:
----------
ip nat inside source list 1 interface Serial0/3/0 overload
!
access-list 1 permit 172.28.1.16 0.0.0.15
access-list 1 permit 172.28.1.32 0.0.0.15
----------
interface Serial0/3/0
ip address 172.28.1.5 255.255.255.252
ip nat outside
----------
interface FastEthernet0/0.2
encapsulation dot1Q 2
ip address 172.28.1.17 255.255.255.240
ip nat inside
----------
interface FastEthernet0/0.3
encapsulation dot1Q 3
ip address 172.28.1.33 255.255.255.240
ip nat inside
----------

For further information feel free to check the attached PKA file.

Greetings and thanks ,