Strange, I found that this service may not have been started at all!

Changing the systemd defaults is a thing I hesitate to do! It will have a ton of unexpected sideeffects. I guess you have an error in your entire setup as multiple sites of mine work flawlessly

I want to do it and have to re explore the cause of the failure, but I haven't found out why dhcp6c can't start, I have to start it manually.

Loaded: loaded (/lib/systemd/system/[email protected]; disabled; vendor preset: enabled)
Active: inactive (dead)
  Docs: man:dhcp6c(8)
        man:dhcp6c.conf(5)

By the way, a new problem has also been identified, see T2681

PPPoE is configured as follows (account password is hidden):

pppoe pppoe0 {
    authentication {
        password pass
        user userid
    }
    default-route force
    description ISP
    dhcpv6-options {
        prefix-delegation {
            interface br1 {
                address 101
                sla-id 2
                sla-len 8
            }
            interface br2 {
                address 101
                sla-id 1
                sla-len 8
            }
            length 56
        }
    }
    firewall {
        in {
            ipv6-name WAN-IN
            name wan
        }
        local {
            ipv6-name WAN-LOCAL
            name wan-local
        }
    }
    idle-timeout 30
    ipv6 {
        address {
            autoconf
        }
        enable
    }
    mtu 1492
    source-interface eth5
}

@c-po The problem was found when upgrading to the latest version, and the reason is not clear.

Oddly enough, whether I change the service settings or change the default settings of systemd, there are problems.

@c-po The basic cause of the failure has been determined.This time, the problem may be relatively serious, because it is not the configuration of the service, but the execution of commit will not start the dhcp6c service at all.

Since I don't know how the designer calls dhcp6c, it will take more time if I try to find out by myself, so I hope @c-po can start to investigate in person.

The minimum scale test code command is as follows:

sudo systemctl stop dhcp6c@pppoe0
set interfaces bridge br3
set interfaces pppoe pppoe0 dhcpv6-options prefix-delegation interface br3 sla-len 8
set interfaces pppoe pppoe0 dhcpv6-options prefix-delegation interface br3 sla-id 3
set interfaces pppoe pppoe0 dhcpv6-options prefix-delegation interface br3 address 101

If it is normal, after br3 is created, dhcp6c should be restarted after commit because the configuration of dhcp6c is changed, but it is not restarted.

dhcp6c@pppoe0 It is still in the following state:

● [email protected] - WIDE DHCPv6 client on pppoe0

Loaded: loaded (/lib/systemd/system/[email protected]; disabled; vendor preset: enabled)
Active: inactive (dead)
  Docs: man:dhcp6c(8)
        man:dhcp6c.conf(5)

@c-po When you submit the following commit, the problem seems to be solved, when I need to do further testing tomorrow, waiting for tomorrow's daily build

https://github.com/vyos/vyos-1x/commit/03fb974b8f5ca09159780e2daee755fbcf807837

There is already a new build containing the fix.