Low speed on Cisco Router 1841 with SMC cable modem

Posted on 2012-09-20
Medium Priority
Last Modified: 2012-10-03
We have Cisco router 1841
IOS  c1841-advipservicesk9-mz.124-25e.bin
Or c1841-advsecurityk9-mz.124-22.T4.bin
We connected this router to business Cable Modem SMCD3GN2-BIZ with speed up to 50Mb.
However our speed is in no more than 2 Mb. Though through any new Linksys or Netgear is 40Mb.
 We had a similar issue before with Comcast SMC older modem, and found that IOS c1841-advipservicesk9-mz.124-25e.bin on the router resolved this very weird problem. Now we tried several different IOS, but no result.

Did anyone else encounter this problem too?
Thank you for advance.
Question by:csinow
LVL 13

Assisted Solution

themrrobert earned 750 total points
ID: 38419920
Try the steps here: http://www.dslreports.com/faq/16583

It seems that this issue could be caused by a great many issues, including config files, ios version, and IP settings. I found several posts and sites with questions regarding Cisco 1841 slowness, but the link I posted here has the most relevant and useful information.

Author Comment

ID: 38419994
I'm posting more information.
This 1841 router worked in production environment in a different location with Comcast modem SMCD3G-CCR. Speed was 25Mb.
We use the same config  in 4 location. Cisco says regarding compatibilities with DOCSIS. This link: http://www.cisco.com/en/US/docs/cable/cmts/wideband/solution/guide/release_2.0/append.html.
I do not understand what exact IOS can be compatible. Preferable Advanced IP Services IOS.
LVL 13

Expert Comment

ID: 38420104
It sounds like maybe you have to change the duplex settings and possibly the even more complex settings.

Check for loops in your network and config, and follow through the steps I listed, that should help you.
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Author Comment

ID: 38420365
Even there is no network. Just modem, router, an laptop. All settings on interfaces are without a firewall. It is basic as possible. The same speed no more the 4Mb. i found IOS c1841-advipservicesk9-mz.124-25f. Speed a little bit increased.
LVL 12

Expert Comment

ID: 38420484
Do you have any incrementing counters on any interfaces. I have used 1841 on connections over 100 mbps. My average packet size was about 800 bytes.

Author Comment

ID: 38420515
Here is router interfaces config:
interface FastEthernet0/0
 description OUTSIDE
 ip address 97.32.xx.xx
 no ip redirects
 no ip unreachables
 no ip proxy-arp
 ip nat outside
 ip virtual-reassembly
 speed 100
interface FastEthernet0/1
 description INSIDE
 ip address
 ip nat inside
 ip virtual-reassembly
 speed 100
ip forward-protocol nd
ip route 97.32.ss.ss
ip http authentication local
ip http secure-server
ip nat inside source list 100 interface FastEthernet0/0 overload

Accepted Solution

vipelite earned 750 total points
ID: 38457695
Can you post the command:

sh int fa0/0
sh int fa0/1

As well as what's the memory size on this router? or I/O maybe too low and I notice you're using the command ip virtual-reassembly could potential slow down your router.

But these are just speculations... what would help if you reproduce the problem and run commands such as:

sh processes CPU sorted | ex 0.00 to assure you're router isn't do anything funny.

Also, based on your isolation between netgear/lynksis hub switches which works fine.. could be a duplex issue since hub/switches use half-duplex may try half-duplex towards the modem. You never know it's worth a try! =p

Also, do you need to add any type of bandwidth on the interface facing modem? may want to look into this as well bandwidth 50000 under the interface. May require for the modem to communicate on the higher level protocols.

Post your results and let's see if we can narrow this down..

Author Closing Comment

ID: 38459104
Problem has been solved. We put WAN interface f0/0 with speed auto! Speed went up to 50Mb

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