Reconfigure Cisco Router 1841 with new T1 Lines

Unanswered Question
May 6th, 2010


I am really very much interested in Network Administration,and recently got a job in the same.We have remote locations having installed with new T1 lines, which means new circuits and new IP Space. The circuits have been delivered and are ready for us to schedule turn up with the carrier. The Cisco routers 1841 at those locations will need to be reconfigured for the new IP space associated with the circuits. Once this is complete we need to turn the service up. We are doing this remotely from the main office location. We will want to do this on a scheduled cutover with the vendor asap as the circuits havestarted billing.

IP information for the new connections has been provided for us with what we will need to swing the connections over to. Please find the attached IP information I was provided with. At the time of conversion we need to move the T1 service cable from 1 carriers demarc to the other.

As far as I know, I think I should just change the IP Address information and Subnet mask information in the Serial interface which is a T1 interface, and then change the static route information. I don't think I need to touch any other data in config file.

But the problem is when I try to change the config on the serial interface using the following I am loosing the connection with the router:

Router_1841#conf t
Enter configuration commands, one per line.  End with  CNTL/Z.
Router_1841(config)#int s0/0/0
Router_1841(config-if)#ip address yyy.yyy.yyy.yyy

where, being the new IP  address
yyy.yyy.yyy.yyy being the subnet mask
As soon as I enter this command, that is it, I am  lost with the connection of router. I can connect to it only after I asksomeone  at the location to reboot it.
Can someone please help me with this issue. I will  be really really very helpful. Also, the vendors are in a hurry to schedule the  cutover asap.
I will be very glad if someone can correct me if i am wrong, and let me  know the correct way to reconfigure the routers.

Thank you very much.



I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Kimberly Adams Thu, 05/06/2010 - 14:02


How are the remote locations currently connecting to the host site now?  This will help in finding a way to assist you in your migration.


Kimberly Thu, 05/06/2010 - 15:16


Thanks muchfor the replyu. Can you be more specific in how are they connected?

They are in WAN with Citrix environment.Is that what u r asking me. Pleaselet me know.



Kimberly Adams Thu, 05/06/2010 - 15:22


The remote sites are they currently using a T1 but you are just changing providers?  This is what I was referring to, the physical transport connecting the remote sites to the host location, ie ... Point-to-Point T1, MPLS, or Frame Relay.  Do you have more then one WIC card (Serial Interface) in the routers at the remotes or just one?  Is there any Dialup at the remote stores that you may be able to connect into for this change?

I know, I have a bunch of questions.  Maybe you could attach a copy of your configuration, which would also help to answer some of these questions.


Kimberly Wed, 05/12/2010 - 09:37


Sorry for the delays. Thank you so much. Yes, the remote sites are currently using a T1 but just changing providers.

physical transport connecting the remote sites to the host location is Frame Relay.

Only one WIC card (Serial Interface) in the routers at the remotes.

I am ssh ing into the router to make the changes.

Current serial interface config:

interface Serial0/0/0

description Global Crossing T1 Interface

ip address yyy.yyy.yyy.yyy                                // - old ip address, yyy.yyy.yyy.yyy - old subnet mask

ip nbar protocol-discovery

ip nat outside

ip virtual-reassembly

service-module t1 timeslots 1-24

crypto map VPN_MAP

max-reserved-bandwidth 100

service-policy output citrix-llq

Can you please provide me the commands to configure the Cisco 1841 router with the following options:

ip address bbb.bbb.bbb.bbb                         // - new ip address, bbb.bbb.bbb.bbb - new subnet mask








Thanks & Regards,


normanjeter Fri, 05/07/2010 - 15:01

If I'm reading your post correctly, you are telnetting (or ssh'ing) in to the router remotely, across the WAN link that you're changing, and trying to reconfigure the T1 connection information. The reason you are being asked how the current WAN connection is configured is because it really sounds like it is a single T1 WIC in the unit with a point-to-point connection setup. The reason you are losing connection is because when you change the IP info of the T1 connection and hit enter it is changing to the new connection with a new IP address, and new Subnet, and new IP space. This, unfortunately, is the connection you are currently linked to and connecting through. Now if you had a second T1 WIC in the router you could make all your configs on THAT WAN interface, hook up your new T1 to it, and not lose connection. Same with a Modem, you can Dial into the modem and connect to the router to make your changes and not lose connectivity. With a single T1 connection and a single T1 WIC though, your kinda in a bad spot with no other method of connecting to the unit. As a possibility, you could Proceed to make any ACL changes connected to the outside interface (specifying the new IP address and subnet) make the changes, and after you hit enter and get disconnected you should be able to reestablish connection by using the NEW ip address instead of the old one. This is a bit of a longshot though because any changes you make on the Serial interface are going to affect your current session to the unit. Quick and simple solution? Get an external modem, hook it up to the Auxillary port and configure the router to accept connections via that. There are lots of guides out there on how to connect to your router through a modem connection.

Hope this helps!

P.S. If you use a Modem don't make the mistake of forgetting to make sure the phone jack is an Analog one. That can be a real forehead slapper when you figure out thats why its not getting a dial-tone. lol... I had a similar situation where I was TOLD it was an Analog line but it wasn't really. Wed, 05/12/2010 - 09:44


Thanks very much. Yes, thereis only one T1 WICconnection, that is thereason, as soon as i change the ip address and hit enter, i am losing connection. Can you please provide me a solution for this.

Can I make changes to the startup config, i mean can i make changes to the config.txt and upload it to startup-config using TFTP?

The only thing I am worried is, if evrything works fine, happy. But if something fails, then i am not sure if there is any chance to upload old configuration back to the router?

Thanks & Regards,


Kimberly Adams Wed, 05/12/2010 - 10:00


There really is not a solution for what you are trying to do but using a dialup connection.  There are products out there that are like a terminal server (Digi is one for example) that give you the ability to have a dialup back door for circuit changes and outages and hooked directly to the console port of your router.  I have used these extensively for circuit changes and ISP changes when that is the only path into the remote network.  Yes Norman is correct, all you need is an analog phone line and you are off and running.

Please let me know if you have any furhter questions.


Richard Burts Wed, 05/12/2010 - 12:33


Part of the issue, as others have commented, is that you are trying to change the connection while you are still using the connection. So part of the solution is that you need to make the change only when the remote office is ready to switch the connection to the new provider. So timing of the changes is critical.

It is possible to do what you suggest and to put the changes into startup config. (put the entire config into a text file, make the changes in interface configuration and anything else that needs to change such as static route, save the changed text file, and TFTP the text file to startup config on the remote router. the new config is loaded when the rotuer reboots) The weakness in this approach is as you suggest, that if something does not work right in the changes that there is not a way to easily go back to the original config.

I have an approach that you might consider. At the time when the remote office is ready to switch to the new provider: make a text file which has the configuration changes in the interface config and any other changes such as static routes, TFTP the text file to running config on the remote router. As the TFTP file makes changes in the router config the existing connection will stop working. Have the people in the remote office switch the T1 connection from the old provider to the new provider. Hopefully the new connection will come up and work. If the new connection does not work, then have the people in the remote office switch the T1 back to the old provider and reboot the router. That will put the router back to the original config and the old connection should work.


Rick Thu, 05/13/2010 - 13:46

Thanks Rick. I am going to giveit a try.

Thanks & Regards,


normanjeter Thu, 05/13/2010 - 14:44


Rick's suggestion is a good one,only it can be time consuming if you are still kinda new to the IOS commands, it is real easy to make a mistake when manually editing the config file in a text editor.  The easiest way, for future purposes, is to pull an analog phone line into the server room and hook up an external 56k modem, plugged into the AUX port. The config for this is simple and you can actually use Hyperterminal to Dial directly into the modem and get a console window into the router. Using this method you can make all the configs and have them do the changeover and never lose connection. Since you are only transmitting basic text through the phone line you can actually get some pretty decent response times. Its not like dialing up to the internet. If you need the config for the router to use the modem let me know. We are using several 1841's and this is the method I use to make config changes whenever we have to changeover a circuit, have issues with the line, or need to reboot the router remotely.


Richard Burts Fri, 05/14/2010 - 04:56


I congratulate you and your organization/company for the willingness to provision an analog line in the server room of every remote site and an analog modem for every router. I would agree with you that if Anisha can get her organization/company to do this that having remote out of band management is the better way to handle this kind of change. I offered my suggestion as an alternative since, at least so far, Anisha's organization/company has not provisioned remote out of band management.


There is another suggestion that I would offer for situations where you are using TFTP to send config changes to a remote router. Before you initiate the TFTP you can schedule a reload on the router. There is a command:

reload in

which schedules a reload. For example reload in 30 will schedule a reload in 30 minutes. If you schedule a reload, use TFTP to transmit config changes, and it turns out that the config change does not work as expected, then the reload will cause the router to reboot and return to the working configuration.

If you use the scheduled reload and your config change is successful (things work as expected) then use the command reload cancel to cancel the scheduled reload.

In your particular case here, where you do need someone at the remote site to be actively involved, you probably do not need the scheduled reload. Since there needs to be someone at the remote site who will be moving the connection then that person can reload the router is the switch in providers does not work. But when there is not necessarily a person at the remote site involved then the scheduled reload can be very helpful.


Rick Fri, 05/14/2010 - 06:41

Norman & Rick

Can you please explain a bit more about the process you suggested me. I don't have any tech people at the remote location, so I need to explain them very clearly what they should do. And I am new to Out of band management. Can you please explain me in detail.

Is it just like getting a normal 56k Modem and connecting the analog phone line to it, and connecting the aux port of the router to the Modem?

Thank you very much.



Richard Burts Fri, 05/14/2010 - 11:57


Since Norman has more recent experience with remote Out of Band management than I do I will leave the explanations about remote Out of Band management to Norman.

As I think about your situation I realize that we may not be thinking about the complete issue. Your description has been about the remote side and how to manage that. But there is probably a companion set of issues about the Main Office side. Does the change of provider at the remote also mean that there is a new T1 coming into your router at the Main Office? Do you need to manage changes at the Mail Office at the same time that you are making changes at the remote? Will the changed connection for a remote use the same interface on the  main site router or would it be different?

The approach that I suggested is relatively straightforward, especially in terms of what the remote people must do so it does not take a lot of technical skill on the part of the remote people. I will go through that approach with a bit more explanation:

- you will prepare a text file with the config changes that you need to make. This can be done ahead of time.

- at the time that you are ready to switch the provider connections at a remote site you will telnet or SSH to the remote router.

- I suggest that you do a copy run start (or the equivalent command write net) so that the startup config has a fresh copy of what is running on the router. (this provides some protection in case you do need to reboot the router - it is not required but I believe that it is a good idea)

- in your session on the remote router use the command copy tftp running-config to copy the text file with the config changes into the running config of the remote router.

- when the copy tftp changes the address of the interface of the remote then you should lose your connection to the remote office.

- at that point the people in the remote office should move the connection for the T1 from the dmarc of the original provider to the dmarc of the new provider. (note that there are a couple of assumptions here 1) that the original dmarc and the new dmarc are close enough together that the cable can just be moved. if the dmarc are not close together then some cabling change may be required 2) assuming that the new T1 is provisioned the same way that the old T1 was provisioned)

- if you need to make changes in the main office this would be the time to make them (change config, move cable, etc).

- when both the main office and the remote are connected to the new circuit it should come up. So you need to do some testing to verify that you have end to end connectivity, that devices at the remote can access resources at the main office, etc

- if the new circuit has come up successfully then you need to save the config changes (remote site and main office) using the command copy run start (or write mem).

- if the new circuit has not come up successfully then have the people at the remote site move the cable back to the original T1 and have them power down and poser back up the remote router. you would also need to undo any changes that you made at the main site.



ronaldobf Sat, 05/15/2010 - 23:21

A suggestion, since you don't  have a technician at the remote site, is that you can ask someone at the site to bring a notebook with a cable (better if this is a console cable) and plug it to the equipment. The person at the site must have internet access. Then, you can use NetMeeting or VNC (there is a VNC portable) or any other remote access software.

You will be able to configure it on the fly by yourself.

If you don't have a person available, I recommend you to schedule a reload on the equipment before making any change so if you lose connectivity, when reloading it will be back with the old config. For this to work, you must not save the config until you are sure the new configuration works fine. If so, cancel the reload with "reload cancel" command.

With the steps above, you don't need help from a local person at the site in rebooting the equipment.

Remember to schedule a proper tech window with a good time frame so you can avoid any impact to the users. Don't forget to include a time window for the fall back, in case of any problems during the change, you can use this time to put back the old configs.

I guess a provider technician will be at the site to install the new demark and modem, etc... you may use him to change the cabling to the new demark. If that's possible, I recommend you to have the new configuration in hand (already validated in the past) so as soon as he change to the new demark, you put the new config and test the connectivity.


normanjeter Thu, 05/20/2010 - 09:15

I like the scheduled reboot idea. I was actually unaware of that little gem of a command.

As far as the Out of band connection goes, The setupis pretty simple:

1: Pull an analog Phone line to the server room.

2: get an external 56k modem (I prefer USRobotics)

3: get a serial (db-9) to Serial (db-25) modem cable and connect to your pc's Serial Port.

4: configure the modem Using the info on this page:

5: Get a Modem cable that goes from 25-pin male to rj-45

6:  Plug into AUX port

7: Add the necessary lines to your Config file:

line aux 0
login local
modem InOut
modem autoconfigure type usr_courier    <-- Replace usr_courier with whatever model you get, though the courier is a nice one to use.
transport input all
escape-character BREAK
autoselect ppp
stopbits 1
speed 115200
flowcontrol hardware

This should get you going for dialing into the modem to connect to the router while doing the changeover. Let me know if you have any questions.

Norman Thu, 05/20/2010 - 18:02

Thank you very much Norman. I will give a try with this. I will let you know if I have any questions.

Thanks & Regards,



This Discussion