[PLUG] Lan loops - follow on PoE injecto

Russell Senior russell at personaltelco.net
Thu Dec 26 02:12:10 UTC 2019


This behavior is going to depend on the switch chip embedded in the SoC on
Tomas's device. The Linux kernel or its bridging behavior won't be involved
until the traffic leaves the switch. In openwrt, there is a standard
interface for configuring the switch, called swconfig, although the future
belongs to something called DSA, or distributed switch architecture. A few
devices have DSA drivers, but most are still on swconfig. Because of the
uncertainty over what the specific switch does, empiricism wins here. It
does what it does. Short of a part number and a detailed datasheet, testing
is almost certainly the most expedient approach.

On Wed, Dec 25, 2019, 17:59 Mike C. <mconnors1 at gmail.com> wrote:

> Date: Wed, 25 Dec 2019 12:25:35 -0800
> Subject: Re: [PLUG] Lan loops - follow on PoE injector
> On Wed, Dec 25, 2019 at 9:33 AM Russell Senior <russell at personaltelco.net>
> wrote:
>
> > Only one way to be sure! ;-)
> >
> >
> Said the person on top of the building when contemplating if the fall would
> kill him, while another person used factual data and applied higher level
> abstract concepts such as math & physics to determine the most likely
> outcome.  J/K Ha!
>
> The OP is running OpenWrt on the router so this makes it far more
> interesting, especially for Network Nerds like me, as OpenWrt is highly
> configurable providing ample opportunities for experimentation.
>
> According to this table from OpenWrt documentation, if the device type is
> set to "bridge" then Spanning Tree Protocol can either be enabled or
> disabled. (See screenshot "STP")
>
> Even more interesting is that you might be able to even get a diagram of
> the switch/router backplane that will show you how the ports are physically
> & logically connected. Such as this diagram I found in the OpenWrt
> documentation. (See attached screenshot "Diagram")
>
> OpenWrt documentation refers to Asus WL-500G as a switch, however based on
> the diagram the 4 LAN ports are physically bridged off Eth0 and the only
> actual switching that happens appears to be between Eth0 (LAN) and Eth2
> (WiFi).
>
> I suspect this is a pretty common architecture for SOHO network gear.
> Physically wired up, the LAN ports are a multi-port bridge. It's designed
> to bridge 4 physical LANs into one logical LAN (VLAN 0) the default
> management vlan.
>
> OpenWrt would most certainly set the interface type to bridge and enable
> STP thereby thwarting any layer 2 looping mischief.
>
> But just disable STP in OpenWrt and have some fun!
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