Low speed on Cisco Router 1841 with SMC cable modem

Posted on 2012-09-20
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

    Try the steps here:

    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

    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:
    I do not understand what exact IOS can be compatible. Preferable Advanced IP Services IOS.
    LVL 13

    Expert Comment

    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.

    Author Comment

    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

    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

    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
    ip http authentication local
    ip http secure-server
    ip nat inside source list 100 interface FastEthernet0/0 overload
    LVL 3

    Accepted Solution

    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

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

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