topic Re: DHCP not working on bridge interface in i.MX Processors

DHCP not working on bridge interface

Phill231 — Sat, 19 Jul 2025 11:44:20 GMT

Hello,
I have a IMX6ULL based custom board with Yocto, and I am currently facing some problems with DHCP on a bridge interface. The IMX6ULL is connected via RMII to a Marvell 88E6320 switch, which has two external Ethernet ports. Below is my device tree configuration:

&fec1 { pinctrl-names = "default"; pinctrl-0 = <&pinctrl_enet1>; phy-mode = "rmii"; status = "okay"; fixed-link { speed = <100>; full-duplex; }; mdio { #address-cells = <1>; #size-cells = <0>; switch@0 { compatible = "marvell,mv88e6085"; reg = <0>; dsa,member = <0 0>; pinctrl-names = "default"; pinctrl-0 = <&pinctrl_marvell>; reset-gpios = <&gpio5 1 GPIO_ACTIVE_LOW>; status = "okay"; #address-cells = <1>; #size-cells = <0>; ports { #address-cells = <1>; #size-cells = <0>; port@3 { reg = <3>; label = "lan3"; }; port@4 { reg = <4>; label = "lan4"; }; port@6 { reg = <6>; label = "cpu"; ethernet = <&fec1>; phy-mode = "rmii"; fixed-link { speed = <100>; full-duplex; }; }; }; }; }; };

To create a bridge (with lan3 connected to my PC and lan4 connected to my home switch), I run the following commands:

ip link set lan3 up ip link set lan4 up ip link add br0 type bridge ip link set br0 up ip link set lan3 master br0 ip link set lan4 master br0

Then I try to request a DHCP lease and I get this output:

root@imx6ull-test:~# udhcpc -i br0 udhcpc: started, v1.36.1 Dropped protocol specifier '.udhcpc' from 'br0.udhcpc'. Using 'br0' (ifindex=6). udhcpc: broadcasting discover udhcpc: broadcasting discover udhcpc: broadcasting discover udhcpc: no lease, failing

However, if I restart the board and run DHCP directly on lan4:

ip link set lan4 up udhcpc -i lan4

I do get an IP address from my home DHCP server as expected.

Is there something wrong with how I'm configuring the bridge?

Re: DHCP not working on bridge interface

Zhiming_Liu — Thu, 24 Jul 2025 08:24:37 GMT

Hi,

Please refer kernel document:

https://www.kernel.org/doc/html/v5.10/networking/dsa/configuration.html

# tag traffic on CPU port ip link add link eth0 name eth0.1 type vlan id 1 # The master interface needs to be brought up before the slave ports. ip link set eth0 up ip link set eth0.1 up # bring up the slave interfaces ip link set lan1 up ip link set lan2 up ip link set lan3 up # create bridge ip link add name br0 type bridge # activate VLAN filtering ip link set dev br0 type bridge vlan_filtering 1 # add ports to bridge ip link set dev lan1 master br0 ip link set dev lan2 master br0 ip link set dev lan3 master br0 ip link set eth0.1 master br0 # tag traffic on ports bridge vlan add dev lan1 vid 1 pvid untagged bridge vlan add dev lan2 vid 1 pvid untagged bridge vlan add dev lan3 vid 1 pvid untagged # configure the bridge ip addr add 192.0.2.129/25 dev br0 # bring up the bridge ip link set dev br0 up

Best Regards,
Zhiming

Re: DHCP not working on bridge interface

Phill231 — Mon, 04 Aug 2025 09:26:47 GMT

Hello,
thank you for your reply.
I followed the documentation page you linked (https://www.kernel.org/doc/html/v5.10/networking/dsa/configuration.html#bridge), and the system works correctly with a static IP. However, if I try to use DHCP instead, I don't receive any response. These are the commands I used:

ip link set eth0 up ip link set lan3 up ip link set lan4 up ip link add name br0 type bridge ip link set dev lan3 master br0 ip link set dev lan4 master br0 ip link set dev br0 up root@imx6ull:~# udhcpc -i br0 udhcpc: started, v1.36.1 Dropped protocol specifier '.udhcpc' from 'br0.udhcpc'. Using 'br0' (ifindex=6). udhcpc: broadcasting discover udhcpc: broadcasting discover udhcpc: broadcasting discover udhcpc: broadcasting discover udhcpc: broadcasting discover udhcpc: broadcasting discover run-parts: /etc/udhcpc.d/99avahi-autoipd: exit status 1 udhcpc: broadcasting discover udhcpc: broadcasting discover udhcpc: broadcasting discover run-parts: /etc/udhcpc.d/99avahi-autoipd: exit status 1 udhcpc: broadcasting discover udhcpc: broadcasting discover udhcpc: broadcasting discover run-parts: /etc/udhcpc.d/99avahi-autoipd: exit status 1 udhcpc: broadcasting discover udhcpc: broadcasting discover udhcpc: broadcasting discover run-parts: /etc/udhcpc.d/99avahi-autoipd: exit status 1

I also tried using the commands you suggested, but I get the following error on the first command:

root@imx6ull:~# ip link add link eth0 name eth0.1 type vlan id 1 Error: dsa_core: DSA master cannot join unknown upper interfaces.

Re: DHCP not working on bridge interface

Phill231 — Mon, 04 Aug 2025 13:15:37 GMT

I have resolved the issue by enabling the CONFIG_BRIDGE_VLAN_FILTERING option in the kernel menuconfig.