[PLUG] Lan loops - follow on PoE injecto

[Russell Senior]

According to https://openwrt.org/toh/gl.inet/gl-ar750s

The switch chip on his router looks like this:

port 0: CPU
port 1: WAN
port 2: LAN1
port 3: LAN2

The usual way OpenWrt VLANs these would be:

 VLAN 1: 0t 2 3
 VLAN 2: 0t 1

And eth0.2 would be the WAN interface, and eth0.1 would be part of the
bridge with wlan0 on the br-lan bridge. The brctl show command will output
something like this:

  # brctl show
  bridge name     bridge id               STP enabled     interfaces
  br-lan         7fff.uuvvwwxxyyzz       no              eth0.1
                                                         wlan0

But, again, the loop is happening within the switch chip. There is no loop
in the bridge.  (Aside: Because of the peculiarities of wifi, it's hard to
get bridge loops through wifi interfaces ... AP interfaces can bridge but
normal clients can't bridge because of the conflation of station BSSID and
source MAC).

In my TP-Link TP-WDR3600 v1, I have an Atheros AR9344 rev 2[1], which
embeds an Atheros AR8327 rev. 2 switch chip[2]. Port 0 connects to the CPU,
port 1 connects to the WAN port, and ports 2,3,4,5 connect to the four LAN
ports.

Sketchy links to datasheets (typically not made available to people without
an NDA, with some exceptions, I'd guess these are leaked):
[1] http://microchip.ho.ua/data/AR9344.pdf
[2] https://datasheetspdf.com/pdf-file/771154/Atheros/AR8327/1

Keep in mind that just because the datasheet says a feature is available,
does not mean that the feature works reliably or that the operating system
driver supports or uses the feature. To tell what the switch chip is going
to do without testing, you are going to need to: a) read these datasheets
and correctly interpret them; b) assume the datasheets are correct; and c)
read the OpenWrt driver to see which bits are toggled where, under what
circumstances, and with what intentions. Call me goofy, but just plugging
in some wires and watching seems less prone to error.

On Thu, Dec 26, 2019 at 9:20 PM Mike C. <mconnors1 at gmail.com> wrote:

> >
> > Ooh, try this: shell in and run: swconfig dev switch0 show | grep por
> >
>
> The "brctl show" command will also provide some very useful info such as
> which interfaces are bridged and what the state of Spanning Tree is on
> those interfaces.
>
> # brctl show
> bridge name     bridge id               STP enabled     interfaces
> dev             8000.0050568954bc       no              eth0
>                                                         eth1
>
> prod 8000.000000000000 no
>
> I think what needs to happen here to successfully recreate a layer 2 LAN
> loop is to directly connect 2 ports that are in the same bridge.
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