Troubleshooting connectivity issues in Cisco 1751 - URGENT

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Feb 22nd, 2010
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I have a Cisco 1751 with a DSL interface card.  Every time we connect the DSL modem to the router, it appears to sync/bind, then the router appears to lose the connection.  The DSL modem has been replaced and verified this morning, so we are suspecting that something is going wrong in the router but are unsure as to how to troubleshoot/diagnose if it is an interface card going bad or not.  Need help with this urgently.


Thank you!

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dbass Mon, 02/22/2010 - 12:50
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You say that you have a DSL modem, and a 1751 with a DSL card.  What I've typically come across is DSL modems that have the DSL interface on one side and an Ethernet interface on the other.  The DSL interface on the router is a modem, so you don't need a modem sitting in front of it.


If the DSL modem has an DSL interface on one side and an Ethernet on the other, I would use it and plug the Ethernet port in to your "WAN" interface on the router and not worry about the DSL interface on the router.

RobGCisco Mon, 02/22/2010 - 13:47
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I mis-typed - it is an Ethernet port on the router.

DSL interface on modem with Ethernet output, going to Ethernet port on Router.

When the DSL modem is connected to the router, they appear to connect, then the connection drops.

Any help would be appreciated.


Rob

Paolo Bevilacqua Mon, 02/22/2010 - 13:30
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Please do not solicit for urgent help on a freely contributed forum like this.

If you need timely, guaranteded support, buy a support contract from Cisco or other company.


Beside this, like the dbass noted, you are giving contradictory information and it would be better if you leave the issue to a qualified technician.

Arup Dutta Mon, 02/22/2010 - 21:04
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hi,

   As u mention that u have DSL modem so frist you check your DSL connection by system.if it working properly then connect it by router ethernet interface and check the interface.by Sho ip int bri

dbass Mon, 02/22/2010 - 22:07
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Are you getting the correct link lights on the DSL modem...you should get one that indicates that the circuit is up?  If so, is the modem configured as a bridge or router?  Probably the easiest way to tell which one is to look at the interface on the 1751...if it is getting a private address, then it's probably configured as a router and if you're getting a public address then it's configured as a bridge.  If you aren't getting an IP, then I would say you most likely have a problem with the DSL modem or circuit and I would call the carrier to have them get you a new one.


Verify that when you connect the router to one of the switch ports on the DSL modem that you get a link light on it.


Also, maybe paste in the output of "sh int f0/0 (or whatever interface on the 1751 you're connecting to the DSL modem)" after you plug it in to the DSL modem and get a link.  Also post the output of "sh run int f0/0 (same interface as above)" as well.


Basically, this is what should happen:  turn off the DSL modem, plug the line in to it from the wall, then turn it back on...leave the router unplugged from the DSL modem.  Once the DSL modem is up it should get a link light on the WAN interface.  Once that is up plug the router interface in to one of the switch ports on the DSL modem...you should get a link light and see some activity lights blinking as well.  Do "sh int x0/0" or "sh ip int br" to make sure the interface you connected to the DSL modem got an IP address (it should be configured to get an address via DHCP).  You should have a default route configured on it that points out the interface...something like this: "ip route 0.0.0.0 0.0.0.0 f0/0".  If all that is in there, then ping 4.2.2.2.


HTH

RobGCisco Tue, 02/23/2010 - 07:19
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Thank you - it all appears to be working right now.


The DSL Modem is in bridge mode. What I was seeing was the DSL modem (WAN interface) would light up the DSL light solid (good), then the modem data light would light up solid, so it saw the router. The Router slot 0 (Ethernet 0/0) light would blink then light solid, so it appeared that they were synced and functional. The Router slot 1 (T1 CSU/DSU) light would be solid throughout this. The T1 is used for server traffic and the DSL for user traffic. After a 20-30 seconds, the Slot 0 (DSL) light would go out and the users couldn't get to the internet. Like I said, it appears to be working now, so I'm not sure if something a user was doing was taking the line down (i.e. streaming data and flooding the DSL).


The following are what the router is reporting now:


C1751>sh ip int br

Interface IP-Address OK? Method Status Protocol

Ethernet0/0 xx.xx.xx.xx YES DHCP up up

FastEthernet0/0 xx.xx.xx.xx YES NVRAM up up

Serial1/0 unassigned YES NVRAM up up

Serial1/0.1 xx.xx.xx.xx YES NVRAM up up

NVI0 unassigned YES unset up up


C1751>sh int e0/0

Ethernet0/0 is up, line protocol is up

Hardware is PQUICC Ethernet, address is xxxx.xxxx.xxxx (bia xxxx.xxxx.xxxx)

Description: DSL Interface

Internet address is 108.11.130.8/24

MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,

reliability 254/255, txload 1/255, rxload 8/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Half-duplex, 10BaseT

ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of "show interface" counters never

Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 329000 bits/sec, 40 packets/sec

5 minute output rate 50000 bits/sec, 29 packets/sec

289962 packets input, 235779145 bytes, 0 no buffer

Received 2 broadcasts, 0 runts, 0 giants, 0 throttles

2021 input errors, 0 CRC, 0 frame, 0 overrun, 2019 ignored

0 input packets with dribble condition detected

272719 packets output, 52899110 bytes, 0 underruns

2019 output errors, 9392 collisions, 1 interface resets

0 babbles, 1935 late collision, 45390 deferred

84 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out


C1751>sh int f0/0

FastEthernet0/0 is up, line protocol is up

Hardware is PQUICC_FEC, address is xxxx.xxxx.xxxx (bia xxxx.xxxx.xxxx)

Description: LAN Interface

Internet address is 65.196.40.225/27

MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,

reliability 128/255, txload 1/255, rxload 1/255

Encapsulation ARPA, loopback not set

Keepalive set (10 sec)

Full-duplex, 100Mb/s, 100BaseTX/FX

ARP type: ARPA, ARP Timeout 04:00:00

Last input 00:00:00, output 00:00:00, output hang never

Last clearing of "show interface" counters never

Input queue: 1/75/43/0 (size/max/drops/flushes); Total output drops: 0

Queueing strategy: fifo

Output queue: 0/40 (size/max)

5 minute input rate 101000 bits/sec, 55 packets/sec

5 minute output rate 558000 bits/sec, 72 packets/sec

910760 packets input, 268312980 bytes

Received 3059 broadcasts, 0 runts, 0 giants, 0 throttles

0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored

0 watchdog

0 input packets with dribble condition detected

986059 packets output, 468991452 bytes, 0 underruns

985078 output errors, 0 collisions, 2 interface resets

0 babbles, 0 late collision, 1265 deferred

988761 lost carrier, 0 no carrier

0 output buffer failures, 0 output buffers swapped out


If the problem comes back, is there an easy way to determine if the issue is caused by excessive traffic (i.e. DOS)? I know if I look at the interface (sh ip int br), I can see if the status & protocol are up, but if they are down, is there a way to tell why they went down (i.e. router error vs DSL Modem dumping connection)?


Thanks again,

Rob

Paolo Bevilacqua Tue, 02/23/2010 - 07:27
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You are using a wrong and too old router for the job. It is not keeping up with traffic and you adsl modem goes full-duplex while the 1751 does not.

So, it i expereinceing a lot of errors.

Get an 877 or 1801, used the emebedded ADSL itnerface.


  5 minute output rate 50000 bits/sec, 29 packets/sec

     289962 packets input, 235779145 bytes, 0 no buffer

     Received 2 broadcasts, 0 runts, 0 giants, 0 throttles

     2021 input errors, 0 CRC, 0 frame, 0 overrun, 2019 ignored

     0 input packets with dribble condition detected

     272719 packets output, 52899110 bytes, 0 underruns

     2019 output errors, 9392 collisions, 1 interface resets

     0 babbles, 1935 late collision, 45390 deferred

    84 lost carrier, 0 no carrier

dbass Tue, 02/23/2010 - 07:37
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"You are using a wrong and too old  router for the job. It is not keeping up with traffic and you adsl modem  goes full-duplex while the 1751 does not.

So, it i expereinceing a  lot of errors.

Get an 877 or 1801, used the emebedded ADSL  itnerface.

   5 minute output rate 50000 bits/sec, 29 packets/sec

     289962  packets input, 235779145 bytes, 0 no buffer

     Received 2  broadcasts, 0 runts, 0 giants, 0 throttles

     2021 input  errors, 0 CRC, 0 frame, 0 overrun, 2019 ignored

     0  input packets with dribble condition detected

     272719 packets  output, 52899110 bytes, 0 underruns

     2019 output errors, 9392  collisions, 1 interface resets

     0 babbles, 1935  late collision, 45390 deferred

    84 lost carrier, 0 no carrier"


Yeah, what he said :-)...didn't see this one before I hit post.

RobGCisco Tue, 02/23/2010 - 07:41
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Good to know - I knew this was an old router, and had my concerns about that. Right now our DSL is a 3MB line. I will have to check to see if we have a fractional or full T1.

The DSL connects to Ethernet0/0 interface and the T1 connects on Serial0/0


Is the 1751 able to go 10/full? In the Ethernet interface (not the FastEthernet interface)?

Also, since the Ethernet interface is actually a plugin card, is there a replacement card that could go higher?

dbass Tue, 02/23/2010 - 07:50
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Honestly I would just scrap the 1751.  If you are REALLY tight on money I would buy a 2621 XM off of ebay and it will be about as old, but have way more capability and 2 FE ports.  You can also use your T-1 wic that's in the 1751...as long as it's a wic and not a fixed port.


If you have a little more money to spend I would buy an 1841 or a 2801.  You will have to buy a T-1 wic to go in it bc it uses a newer version than what the 1700 or 2600 uses.


In regard to the DSL in or out of the router...  For me it depends on whether you can run the line thru some type of surge protection before plugging it in to the router.  I've had a bunch of ADSL wics get burnt out in thunderstorms.  For me, I just usually like to use the DSL modem and use it as a free surge protector...if it gets burnt out you just get a new one from the carrier.

dbass Tue, 02/23/2010 - 07:36
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What interface connects to the DSL modem?  It looks to me like there are some speed/duplex mismatches on your interfaces because there's lots of errors.


I think that is more than likely the cause of the interface dropping.  You can try to hard code both ends to 10/full and 100/full and see if that helps, but if you run at 10/half on the ethernet interface and it's connected to the DSL modem I don't think you're going to be able to fix it using that router.


At 10/half you will probably only be able to push 2-4 Mb thru the interface max...

Paolo Bevilacqua Tue, 02/23/2010 - 07:40
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to 10/full and 100/full and see if that helps


A 1751 cannot do 100 because it has Ethernet only. And, 10/full has never been a standard.


Conclusion, router from past century belongs in museum only.

RobGCisco Tue, 02/23/2010 - 07:49
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It is a little dusty - like something out of an Indiana Jones movie

If it belongs in a museum doesn't that mean it is MORE valuable - Ha, Ha, Ha!


Have you had better success with the 877 or 1801?

Or is there an upgraded Ethernet card we could use with our 1751?


BTW I have really appreciated all the help and feedback.

Paolo Bevilacqua Tue, 02/23/2010 - 07:51
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An 877 will be okay.

Too bad they made too many 17xx to be valuable .

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