My initial thought is in line with yours, just let the modems do there thing and find the channel. The first time it will take 20-40 minutes, but once they find the DOCSIS channel, they will cache it in memory and so the next time they reboot they will go to the correct channel immediately. If the modems are not doing this, talk to your modem vendor and tell them to fix their firmware.

You can talk to your Cisco support engineer and they are able to add custom code for modem steering in special cases like this. The challenge is keeping the custom code in place as the IOS is upgraded. You may loose the custom code when you upgrade and have to get it added back in. So it is always best to deal with these scenarios directly.

Another option would be if you can get the modem to register on any of the primary capable DOCSIS channels and then steer it via OUI mapping in the CMTS or by downloading a config file during DHCP that tells the modem to move frequency. Again, these become more cumbersome methods, but would reduce the 20-40 minute time.

-Brady

On this particular topic, here’s our dilemma:

We have 3 DSs per DSG (4 or 8 channels w/ Cisco 5×20 line card in M-CMTS mode). Therefore, we can’t make every DS channel primary capable – not allowed. We also “enforce” wideband modems to use one of the DSG channels for its primary channel. This has created a problem with new wideband CM’s DS selection process to the point where it can take up to an hour for modems to find a legit primary channel – average is 20 – 40 minutes or so.

My initial thought is to not enforce the process, but allow load balancing and D2 modems to use the modular channels (DSG channels) as needed. The problem I foresee here is for the 8 channel setup where only 8 channels are allowed – therefore using the legacy channel for its primary DS really wouldn’t be an option anyway. (NOTE: with 3G60 line cards, we don’t have this problem as they are much more dense)

Do you have any suggestions on how to improve the DS selection process with this setup?

Thanks for any information provided.

There is nothing in the DOCSIS spec that talks about this. But you raise a good point. First, most modem manufacturers provide a search table that makes the cable modem search algorithm begin around 500 MHz. Why? Because many legacy Downstream plants start their DOCSIS channels above 500 MHz, so it reduces the time to find a DOCSIS channel and lock. Starting at 100 MHz would increase the time. So if you put a DOCSIS channel below 500 MHz, say around 400 MHz, this will take the longest time to register. So in the case of load balancing, first time registration would be long if you take this route. However, a cable modem does have memory. Typically they will remember at least the last two channels they have locked on. So once they found that 400 MHz channel and the possible upper channel for load balancing, you should be okay. But if you have have more than two, all bets are off depending upon your cable modem vendor.

Remember that you can enter in known frequencies into the config file. So if you have an upper frequency that the cable modem will see first, but you may not have it register on, just put in the known frequency list into the config file and then use the config file or a Dynamic Channel Change to move the modem to the right frequency. This will save your modem a lot time in searching for the correct DOCSIS frequency. Planning ahead is always better than letting the modem wonder around your network.

Regards,
-Brady