I have a Cable Modem which an approx. 1200 Mbit download speed connection to the internet. Unfortunately the modem splits the download into across ethernet ports - i.e. 600 MBit each.

The Cable Model does not support Link Aggregation (LACP). I've tried to connect the two ports from the Cable Modem to two ports on a Switch that does support LACP and is configured accordingly, but the result is that the network halts - a loop I assume.

Is there any way to combine the two 600MBit ports into one 1200 Connection? I believe the right term here is to Trunk the connection?

  • What's the model? How is your network layed out? – Zac67 Apr 26 '18 at 19:02

LACP is negotiated link aggregation (LAG) using a protocol. It needs to be actively supported on both sides. If one of the sides doesn't support LACP the trunk doesn't come up and you've got two separate ports. If the modem uses a simple switch for both ports (plus the internal one) and there's no spanning tree protocol running, this effectively creates a bridge loop, killing the network.

You'll need to find out what exactly the modem does.

You should try connecting two hosts, one on each port. If they can "see" each other (ping etc) the modem uses a simple bridge. With a bridge, you should be able to get up to 1 Gbit/s from each port alone.

Setting up a static, non-protocol trunk on the downlink switch might work. The trunk will make the switch regard both links as a single, logical port. It won't care where a frame came in, and it will never forward it back to the modem.

In the reverse direction, the switch will likely use some kind of SA/DA traffic distribution. This needs to be on MAC or IP addresses alone, don't activate TCP or UDP port distribution - that'll mess up the modem's MAC table.

However, the modem will likely reflect traffic not addressed to the modem itself back to the network. You might have a problem with the modem reflecting broadcasts back through the trunk but maybe you can live with that (possibly minimizing the broadcast domain for that segment).

A cleaner solution would be to set up a load-balancing router with two uplinks.


LACP needs to be configured at both ends of a connection.

Are you sure your internet speeds are above 1Gbps? If so, that's way fast.

As long as your internet speeds are below 600Mbps then you won't see any speed increase with a faster uplink to the modem.

My knowledge on Cable Modems is very limited.

  • Thank you. Yes, the connection is 1Gb. And yes it is fast. LACP can't be configured in the Modem/Router as it doesn't have the feature. What about VLAN or Trunking. Is there some trick I can apply here. – Jasper H Bojsen Apr 25 '18 at 11:57
  • No tricks for that I know of. I don't think you'll be able to get the modem to combine the two in parallel for a speed increase. You may be able to do some fancy load balancing from the switch. But it seems like a modem upgrade may be needed to fully utilise the line. – Huckleberry Finn Apr 25 '18 at 12:19
  • check for any firmware update which has the feature you require or you can try custom firmware which may have the feature dd-wrt.com/site/index – Vijay Muddu Apr 25 '18 at 12:24

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