Airport Base Station Cutting Internet Speed in Half

The Arris/Motorola SB61XX series of modems are incompatible with the 802.11ac AirPort base stations (potentially, the 802.11n models as well.) Neither are bad in themselves, they just don't "play nice" together.

To test for incompatibility, check the ARP cache. It should only contain around a dozen entries. Considerably more could potentially indicate an issue. Entries are dynamic and are added on a "as-needed" basis and they represent mappings for routers and hosts that a device is actively using. Entries have a typical lifespan of around 10-20 minutes.

To check, enter the following command to display all of the currently cached ARP entries in Terminal: arp -al

Aren't almost all new/modern wireless routers/base stations 802.11 a/b/g/n/ac?

Yes, but not all modems & routers are fully compatible. Most are, but there are exceptions. There are numerous posts here at the Apple Support Communities specifically about the Arris / Motorola SB61XX series and 802.11ac AirPort base stations (some 802.11n models as well) about loss of connectivity or degraded throughput performance with this combination of networking equipment.

The basic recommendation is to either replace the base station or the modem. We see much lower compatibility issues with using modems provided by Cisco or Zoom (there are others as well). Regardless, you should use only modems that your ISP supports.

I ran that command in terminal and got a lot more than a dozen entries...do you need to see what some of them where?

No, I do not need to see them ... and you shouldn't post them anyway as the results include the MAC hardware addresses of your devices.

The total number is not as important as it should reflect the total number of active devices on your current network. For example, I have three base stations and about a half-dozen devices connected at any given time. My ARP cache shows around 20-25 entries. Yours would be dependent on the number of clients on your network. However, if i got 50+ entries here, I would suspect a compatibility issue between my "main" router and the modem.

You can also clear out the ARP cache, with the following Terminal command: arp -d -a

Then wait a few days, and then, use the arp -al command again to see what entries have been added since.

I have Comcast service in Tucson AZ. I use an Arris SB6141 cable modem with the latest Airport Extreme base station and have not experienced any issues with speed problems. I get similar speeds with the wired ethernet connection to the modem, the wired ethernet connection to the Airport Extreme and the wireless connection from the Airport. I also have service with Cox in Southern California. In that location I use an Arris SB6190 with the latest Airport Extreme. Speed results are similar for ethernet to the modem, ethernet to the Airport Extreme, and wireless to the Airport Extreme. The only issue I've had with the Cox service is the wireless connection is significantly faster on the 5Ghz network from the Airport.

Hope this is useful.

I think that your arris modem only has a 100MB ethernet port, not a Gigabit, because with the protocol overhead that would give you about 93 MB/s throughput.

Can confirm. I've been doing some serious testing with my ISP's modem which is connected to an AirPort Extreme in bridge mode.

AirPort Extreme is indeed throttling speeds. The moment I connect directly to the modem, speeds double or treble.