R4 br-lan can't pppoe?

moore · November 26, 2025, 3:07am

This feature was eventually implemented here.

openwrt/openwrt/blob/v24.10.0/target/linux/generic/pending-6.6/705-net-dsa-tag_mtk-add-padding-for-tx-packets.patch

From: Felix Fietkau <[email protected]>
Date: Fri, 6 May 2022 21:38:42 +0200
Subject: [PATCH] net: dsa: tag_mtk: add padding for tx packets

Padding for transmitted packets needs to account for the special tag.
With not enough padding, garbage bytes are inserted by the switch at the
end of small packets.

Fixes: 5cd8985a1909 ("net-next: dsa: add Mediatek tag RX/TX handler")
Signed-off-by: Felix Fietkau <[email protected]>
---

--- a/net/dsa/tag_mtk.c
+++ b/net/dsa/tag_mtk.c
@@ -29,6 +29,13 @@ static struct sk_buff *mtk_tag_xmit(stru
 
 	skb_set_queue_mapping(skb, dp->index);
 
+	/* The Ethernet switch we are interfaced with needs packets to be at
+	 * least 64 bytes (including FCS) otherwise their padding might be

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slavb18 · October 4, 2026, 5:57pm

Tested both official 25.12.5 and custom 24.10 with this patch - PPPoE on WAN port still fails with “Timeout waiting for PADO packets” (while an older OpenWrt 19 router and Ubuntu PC connect instantly on the same line).

It's really hard to believe that such a popular flagship board still cannot establish a basic PPPoE connection on WAN.

frank-w · October 4, 2026, 6:22pm

Do you send it over wan directly or over an vlan? You can setup a local pppoe server on another host an read packets via tcpdump to see if vlan tags and pppoe packets are leaving R4

https://wiki.fw-web.de/doku.php?id=en:bpi-r2:network:start#server1

slavb18 · October 4, 2026, 6:41pm

It is sent directly over WAN untagged (no VLAN). On Ubuntu and OpenWrt 19 it connects on the raw physical interface without VLAN.

Thanks for the pppoe-server idea - I will connect R4’s WAN port directly to a PC NIC, run pppoe-server and tcpdump on the PC side to inspect the exact egress frames and see if any unexpected tags/padding issues are present.

slavb18 · October 6, 2026, 4:36pm

I completed the local PPPoE server test: BPI-R4 connects successfully to pppoe-server on Ubuntu. The capture on the PC shows untagged, correctly padded PADI frames.

However, through my C-Data FD511G-X ONU, R4 sends PADI but receives no PADO. Ubuntu and my old OpenWrt router connect through the same ONU without problems. Disabling flow control and testing at 100 Mbps did not help. What would you suggest checking next?

Would a compatible ONU SFP module in the R4 bypass this problem, or could it behave the same way? I was considering a GPON stick, but the C-Data status page reports EPON, so I assume I would need an EPON module supported and provisioned by my ISP. Does this approach make sense?