BUG Report: 802.11s Mesh (V19.07.4)

I have spent 3 days checking my facts ... and pretty sure this is a bug.

To be clear - I do have the 802.11s mesh working in simple cases. It is on the 'edges' that I see problems.

Consider a case with:

  • 3 OpenWrt devices [A],[B],[C]
  • V19.07.4
  • All in a row
  • All have static IP addresses on the LAN-WLAN0 bridge
  • All OpenWrt devices have STP (spanning Tree) enabled on the LAN device. (But with only one LAN connection, this is irrelevant)
  • Only [C] has a hardwire link (Ethernet) to the local LAN. The LAN has a DHCP server.
  • Mesh radio links exist as follows
    • [A]-[B]
    • [A]-[C]
    • [B]-[C]
  • Connect a laptop to LAN port on [A]
  • Laptop gets DHCP address.
  • From the LAN can ping [A], [B], [C], and Laptop
  • SSH into each of [A], [B], [C]
    • each OpenWrt device can ping the other two OpenWrt devices
  • all good!

Now ... move [A] so that it is beyond the range of [C]

  • Mesh links exist only as follows
    • [A]-[B]
    • [B]-[C]
  • Connect a laptop to LAN port on [A]
  • Failure of Laptop to get DHCP address
  • SSH into each of [A], [B], [C]
    • each OpenWrt device can (still) ping the other two OpenWrt devices
  • From the LAN can ping [A], [B], but NOT [C] or laptop
  • Disabling STP does not change anything
  • :thinking:

So ... it appears that the 'design rule' for these mesh networks is as below (With a Portal Node being defined as one with a hardwire LAN connection)

ALL mesh Points must be in radio contact with ALL Portal Node (s)

This largely defeats the value of using the mesh.

This is a great pity as the other 802.11s mesh benefits are great.



I have nailed the problem down to ARP requests not being re-transmitted out the WLAN interface.

Others have reported same issue here https://bugs.openwrt.org/index.php?do=details&task_id=714, but worryingly ... no fix seems on its way.

BTW - I tried the 'hairpin' fix but it had no effect.

In a nutshell,
If [C] pings [A]

  • [C] issues an ARP broadcast :"Who has IP_A"
  • The ARP arrives on wlan0 of [B].
  • [B] does not reply (as it is not [C])
  • BUT !!!! [B] does NOT forward the ARP back out of wlan0, so the ARP request never reaches [A]
  • [A] never replies, so the ping fails

This is a pretty fundamental issue.

My assumption is that [B] should consult its mesh topology, and if it determines that there are MeshPoints that did not get the ARP broadcast, but which [B] does have a direct path to, then [B] should resend the ARP request out the same wireless interface it arrived.

This is a show stopper for me .. and means 802.11s on OpenWrt is quite broken.

This may be a better way of getting the attention of the developers
Reporting Bugs

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