100baseT/Full (100 MBit/s) support on Archer C7 v5

I want to use 100baseT/Full (100 MBit/s) on a Archer C7 v5 (with a Atheros AR8327N switch chipset) running OpenWrt 19.07.2 because it's connected to my router through a CAT 5 cable that's built into the wall.

According to ethtool the access point only supports the 1000baseT/Full link mode and also uses it. However, according to the data sheet, the chipset has support for the full variety of link modes, including 100baseT/Full and 10baseT/Full.

Is there a way to add the lower-throughput link modes to the list of supported link modes? I would be happy to open a pull request against the OpenWrt if that's necessary and someone could give me some pointers on what to change.

Output of ethtool eth0
# ethtool eth0
Settings for eth0:
	Supported ports: [ ]
	Supported link modes:   1000baseT/Full 
	Supported pause frame use: No
	Supports auto-negotiation: No
	Supported FEC modes: Not reported
	Advertised link modes:  1000baseT/Full 
	Advertised pause frame use: No
	Advertised auto-negotiation: No
	Advertised FEC modes: Not reported
	Speed: 1000Mb/s
	Duplex: Full
	Port: MII
	PHYAD: 0
	Transceiver: internal
	Auto-negotiation: on
	Current message level: 0x000000ff (255)
			       drv probe link timer ifdown ifup rx_err tx_err
	Link detected: yes

ethtool configures CPU ports. On the Archer C7 the eth0 port is a GMII direct connection from the CPU chip to the switch chip. It should always run at 1000 Mb.

swconfig is the userspace tool that configures the switch chip. It does not yet support all the options in all hardware such as forcing the line speed. See swconfig help.

Thanks for the information. That makes sense. Is there a way to force the link speed using some other tool?

I have an Archer C7 V5 running OpenWrt 19.07.02.

A RPI3B (with 100 Mbit ethernet) is connected to one of the Archer's LAN ports and it works. Therefore I guess, that both sides (Archer and RPI3B) agreed on 100 Mbit. I did not use ethtool or some other tool to configure the ethernet speed, just connecting both devices.

Turns out that swconfig did indeed show a link speed of 100Mbit/s. I was just running into a bufferbloat which significantly slowed the overall connection speed.

Thanks for your help!

You're welcome.

Yes the default (and only) setting is that the switch port will auto-negotiate the highest speed supported by both ends.

I thought the OP's question was how to force a lower speed for more reliable operation on dodgy cabling. Though category 5 (pre- 5e) cable can be reliable at 1000 Mb especially if the length is much less than 100 meters.