Cisco 1721, T1, and PAT

Posted on 2004-03-24
Last Modified: 2010-03-17
I have a new Cisco 1721 router, a freshly hot T1 line, and about 40 workstations.

At this time the internal network is getting IP addresses from our server that is running DHCP, and assigning addresses in the range of -

My new ISP has given me the following information
 CSU Info:
Clock Source: Internal
Channels 1-24
Channel rate: 64K
Framing ESF
Line coding: B8zs

Router info:
Encapsulation: PPP
Ip Address
default gateway:
Subnet mask:

LAN side:
IP Address:
Subnet mask:
Usable addresses : to .197 with a subnebt mask of

Using this information, I really need to know the configuration for using this Cisco router.

If I am correct I will have to use PAT because I have so few IP addresses available, and would like to reserve one of them in case I have to set up a SMTP server later.

Please Help !

Thank you!
Question by:tommycole

Accepted Solution

Jharper earned 400 total points
ID: 10677851
This config should get you started.



Current configuration:
! Last configuration change at 13:10:54 cst Tue Feb 24 2004 by jharper
! NVRAM config last updated at 13:11:11 cst Tue Feb 24 2004 by jharper
version 12.1
service timestamps debug datetime msec localtime
service timestamps log datetime msec localtime
service password-encryption
hostname yourhostname
enable secret 5 xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
memory-size iomem 15
clock timezone cst -6
clock summer-time cdt recurring
ip subnet-zero
ip name-server Your DNS Server
interface FastEthernet0/0
 ip address
 ip nat inside
 duplex auto
 speed auto
 no cdp enable
interface Serial0/0
 description Office Internet Link - Account #
 ip address
 ip nat outside
 no cdp enable
ip nat pool p1 netmask
ip nat inside source list 1 pool p1 overload
ip classless
ip route
no ip http server
no cdp run
line con 0
 login authentication No_Auth
 transport input none
line aux 0
line vty 0 4
 password 7 XXXXXXXXX
no scheduler allocate

Author Comment

ID: 10682689
I don't know why, but that won't work.  I have emaied the tech support for the ISP to make sure the circuit is still turned up.

Thanks anyway.  I am sure the problem lies elsewhere than in your IOS script.

Expert Comment

ID: 10683111
What do you get with a show interface s0? Are the interface and line protocol up?

Author Comment

ID: 10684002
Yes, The FastEthernet0 and Serial0 are both up.
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Expert Comment

ID: 10687263
Can you ping out to the internet from your router?  Sometimes they forget to route your block of internal IPs to your router.  If you can ping from inside the router into the Intenet it means that they need to route these IPs to your router and your PAT address won't work until they complete this.

LVL 12

Assisted Solution

Scotty_cisco earned 100 total points
ID: 10709250
There is no access-list defining traffic to nat that is why you are not geting out

access-list 1 permit

in the config that you are already using

Expert Comment

ID: 10727469
You're right, I accidently cut that access-list out when I was cleaning it up for him.


Author Comment

ID: 11043496
Thank you all very much !

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