Easy question! 1841 Router

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Dec 8th, 2008
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I don't know ANYTHING about routers and my boss wants to know why we have what looks to be 2 T1 lines going into the 1841 router and only 1 network cable coming out to the network. I don't have a clue but am thinking maybe its a redundancy thing?


I have attached a couple of pictures showing the router and cables.


The red cable is plugged into the port labeled FE 0/0 and the other end of it goes into my network hub/switch.

The blue cables are plugged into slots 0 and 1 which are labeled T1 DSU/CSU. The other end of those cables are plugged into T1 lines.

He wants to know why we can't just get rid of one of the T1 lines!





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Jon Marshall Mon, 12/08/2008 - 12:28
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Could you post the config of the router - it could well be that both T1's are in use.


Do you know how to obtain config of router ?


If not do you know an IP address of the router you can telnet into


OR


do you have a laptop with a COM port and one of the Cisco blue rollover cables ?


Jon

shelly_ratliff Mon, 12/08/2008 - 12:50
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Jon,

I don't know how to obtain the configuration of a Cisco router. I don't know the ip address and it is not on the same network segment as my desktop computer. I will have to bring in my laptop tomorrow. I have the blue rollover cable which you describe. I don't know any passwords to this router. Am I going to be able to at least display the configuration or copy it?



Jon Marshall Mon, 12/08/2008 - 13:02
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Shelly


We can talk you through connecting up to the router but if you don't know the passwords then apart from trying the obvious such as "cisco" or any or passwords you may use internally the only way to get access to the running config would be to do a password recovery. This does a reload of the router so there would be an outage.


Jon

shelly_ratliff Mon, 12/08/2008 - 14:33
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I located a printout of the configuration.


It is described as DS1 3Meg Managed ISP via multi-t1.


Without retyping the entire configuration, the things that stand out to me are as follows:


Interface Multilink1

description : MDG: 3M: S3/0/0/23:0 ....

no cdp enable

ppp multiling

no ppp multilink fragmentation

multilink-group 1

sh

exit



Description 1.544M 1 of 2

.....

Description 1.544M 2 of 2

.....


It would appear to me that there are 2 T1 lines combined by the router into 1 internet connection. Would you agree?

1. What would happen if I unplugged 1 of the 2? I would assume that I will loose the internet connection? If not, how could I tell if one of the lines is "out of order"? The T1 appears to be coming out of a phone jack right into the router. Isn't there usually a smartjack somewhere close by that you can check for errors?

jgreenwoodii Mon, 12/08/2008 - 12:30
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To answer the question it depends on the setup of your router's connection to the internet. The two T1's could be bundled together to provide you a 3Mbps Internet connection. Or they could be individual T1's to provide Internet redundancy in case one of your T1 goes down. There could be a # of reasons but ultimately he should consult with your Network Administrator/IT department. It could also be your Internet Service Provider's equipment as well so be careful.


HTH


Jonathan

Richard Burts Mon, 12/08/2008 - 12:37
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Shelly


One thing that your boss should cosider in this is that if he does remove 1 of the T1s he will most likely cut in half his bandwidth to the outside world.


The cable connected to the Fe0/0 provides 100 Mbps (or perhaps 10 Mbps depending on what equipment it connects to) providing access to the inside network. The T1 provides 1.5 Mbps each. As the previous posts have pointed out we could tell more definitively if we saw the configuration of the router. But in eliminating a T1 your outside access probably goes from 3.0 to 1.5.


And the other aspect to consider is that the 2 T1s probably give some redundancy for connectivity to outside. If 1 T1 is eliminated you will introduce a significant single point of failure.


So I suggest that your boss consider carefully the implications before deciding to eliminate a T1.


HTH


Rick

shelly_ratliff Mon, 12/08/2008 - 12:41
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Nothing drives me crazier then the standard "contact your administrator" answer. I am the LAN Administrator! The guy asking the question is the Manager of IS!!!!!


I know a bit about "routers" (having installed about 300 Linksys and Netgear) but I don't know anything about Cisco routers or how they interface with the T1. Generally, in a small network like ours this is handled by a consultant since it is setup once and generally needs little or no ongoing maintenance. The person who set this up is no longer available so I have been asked to "look into it".



Jon Marshall Mon, 12/08/2008 - 12:44
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Okay, but to be fair to Jonathan you did say you knew nothing about routers :-)


We really need to look at the config to even begin to help you. This router is under your companies control ?


If so, do you know how to log into a Cisco router ?


Jon

jgreenwoodii Mon, 12/08/2008 - 13:02
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Not sure if my previous post made it but I apologize Shelly, I was just responding to the first 6 words in your original post:) Sounds like from previous post your in good hands, hope all works out for you.


R/


Jonathan

shelly_ratliff Mon, 12/08/2008 - 14:11
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Apology accepted. Guess I should have been clearer. I know nothing about Cisco routers!



shelly_ratliff Mon, 12/08/2008 - 14:36
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I located a printout of the configuration.


It is described as DS1 3Meg Managed ISP via multi-t1.


Without retyping the entire configuration, the things that stand out to me are as follows:


Interface Multilink1

description : MDG: 3M: S3/0/0/23:0 ....

no cdp enable

ppp multiling

no ppp multilink fragmentation

multilink-group 1

sh

exit



Description 1.544M 1 of 2

.....

Description 1.544M 2 of 2

.....


It would appear to me that there are 2 T1 lines combined by the router into 1 internet connection. Would you agree?

1. What would happen if I unplugged 1 of the 2? I would assume that I will loose the internet connection? If not, how could I tell if one of the lines is "out of order"? The T1 appears to be coming out of a phone jack right into the router. Isn't there usually a smartjack somewhere close by that you can check for errors?

Richard Burts Mon, 12/08/2008 - 14:57
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Shelly


I am glad that you found a copy of the router configuration. It makes this discussion easier.


Yes that means that both of the T1s are logically combined to provide a single logical connection to the outside world.


If you unplugged 1 of the 2 T1s I would expect the logical interface to remain up and active (but operating at only 50% of normal capacity). Without the ability to access the router and monitor its components the main way to tell if one of the T1s has failed is to wait for the provider to call and inform you of the problem (and given the performance of many providers I would not want to depend on that)


Usually there is a smartjack. It may be close to where the T1 service enters the building and the circuit may have been "extended" from the smartjack to your router interface.


HTH


Rick

shelly_ratliff Mon, 12/08/2008 - 15:16
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Thank you.


Now that being said, my boss is worried because he is under the impression that the T1 is very "big" and 2 of them combined surely would be more than 1 cat5 at 100mb can make available to the network via the switch! LOL! Am I missing something here? Isn't the T1 1.5m so 2x 1.5m = 3m? Isn't this 3m "slower" than the 100m port going out, therefore isn't there little likelihood of this one port to the network will ever cause a bottleneck?



Joseph W. Doherty Mon, 12/08/2008 - 17:26
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You're correct. The dual bonded T1 will provide 3 Mbps vs. FastEthernet's 100 Mbps so you would normally expect the WAN to bottleneck first. (BTW: However, Ethernet could be running at 10 Mbps - if 10, and if at half duplex, 3 Mbps could stain the Ethernet connection.)

Richard Burts Mon, 12/08/2008 - 20:10
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Shelly


I made the point in a previous post that there is significant imbalance between the bandwidth of your LAN (perhaps 100 Mbps) and the bandwidth of the existing (2*T1) WAN, wich at its very best is 3 Mbps. If your boss eliminates a T1 he is cutting one half of the bandwidth to the Internet.


I sympathesize that he may believe that T1 is "big". Some of us who started with dial up circuits and with switched 56K circuits used to think of T1 as "big" (yes I know that I am dating myself). Is there a diplomatic way to tell him that the environment has changed? T1 is no longer "big". The arithmetic is quite plain: the LAN (at 10 Mbps or 100 Mbps) is somewhere between 3 and 33 times more than the current WAN (at 3 Mbps). And eliminating the second T1 makes it much worse.


HTH


Rick

Richard Burts Mon, 12/08/2008 - 14:51
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Shelly


In the short term it may be that you now have sufficient information to give your boss some answers to his question. Points that you could make about why you do not want to remove the "extra" T1 include:

- loss of bandwidth for access to the outside world.

- loss of redundancy in the connection to outside.

- introduction of a significant single point of failure in the network (a T1 is much more likely to suffer a problem which interrupts transmission of data than is the router, your hub/switch, or your LAN network).


In the longer term I would worry about having a router which provides network connectivity and which you do not know how to access or to manage.


We could probably figure out the IP address of the FE interface of the router. It is probably the default gateway of the switch, or if your network is a single VLAN network (flat network) it is likely that your PC default gateway is the router interface address. But unless the previous consultant used some very obvious passwords, as suggested in a previous post, you will probably need to do the password recovery procedure on your router. We could provide you with links to documentation describing how to do it, or you might want to ask your boss to provide some funds so that you could engage a consultant to come in and assist with this. (It is not difficult once you know what you are doing, but it may be a bit daunting to someone who knows little or nothing about Cisco routers).


HTH


Rick

shelly_ratliff Tue, 12/09/2008 - 07:25
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Thank you and everyone for your input. I agree, having such an important internet/network interface that no one knows how to support is nuts! I will get a Cisco Consultant out here to document this very important piece of the network puzzle.



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