[FS#227] VLAN support mismatch between preinit and default network config

[LEDE Bugs]

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FS#227 - VLAN support mismatch between preinit and default network config
User who did this: Baptiste Jonglez (bjonglez)
Summary: PPPoE is broken on WRT1900ACS -> VLAN support mismatch between preinit and default network config
Task details edited:
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PPPoE is broken on WRT1900ACS

Upgraded from Lede r578 to latest Lede r1814 and PPPOE doesn't work anymore altough the pppd version and PPPoE version are the same: 
 - Linksys WRT1900ACS
 - LEDE reboot r1814

pppd debug log:

Plugin rp-pppoe.so loaded.
RP-PPPoE plugin version 3.8p compiled against pppd 2.4.7
Send PPPOE Discovery V1T1 PADI session 0x0 length 4
 dst ff:ff:ff:ff:ff:ff  src c2:56:27:ca:d7:d4
 [service-name]
Send PPPOE Discovery V1T1 PADI session 0x0 length 4
 dst ff:ff:ff:ff:ff:ff  src c2:56:27:ca:d7:d4
 [service-name]
Send PPPOE Discovery V1T1 PADI session 0x0 length 4
 dst ff:ff:ff:ff:ff:ff  src c2:56:27:ca:d7:d4
 [service-name]
Timeout waiting for PADO packets
Unable to complete PPPoE Discovery
Plugin rp-pppoe.so loaded.
RP-PPPoE plugin version 3.8p compiled against pppd 2.4.7
Send PPPOE Discovery V1T1 PADI session 0x0 length 4
 dst ff:ff:ff:ff:ff:ff  src c2:56:27:ca:d7:d4
 [service-name]
Send PPPOE Discovery V1T1 PADI session 0x0 length 4
 dst ff:ff:ff:ff:ff:ff  src c2:56:27:ca:d7:d4
 [service-name]
Send PPPOE Discovery V1T1 PADI session 0x0 length 4
 dst ff:ff:ff:ff:ff:ff  src c2:56:27:ca:d7:d4
 [service-name]

While on the same hardware running LEDE r578, the PPPoE module works as expected:

Plugin rp-pppoe.so loaded.
RP-PPPoE plugin version 3.8p compiled against pppd 2.4.7
Send PPPOE Discovery V1T1 PADI session 0x0 length 4
 dst ff:ff:ff:ff:ff:ff  src c2:56:27:ca:d7:d4
 [service-name]
Recv PPPOE Discovery V1T1 PADO session 0x0 length 40
 dst c2:56:27:ca:d7:d4  src a0:f3:e4:34:d8:21
 [service-name] [AC-name acc-aln1.hac] [AC-cookie  75 58 37 a5 ba 3c e4 a5 2a 61 bb 23 92 5c 1b dc]
Send PPPOE Discovery V1T1 PADR session 0x0 length 24
 dst a0:f3:e4:34:d8:21  src c2:56:27:ca:d7:d4
 [service-name] [AC-cookie  75 58 37 a5 ba 3c e4 a5 2a 61 bb 23 92 5c 1b dc]
Recv PPPOE Discovery V1T1 PADS session 0x30b length 4
 dst c2:56:27:ca:d7:d4  src a0:f3:e4:34:d8:21
 [service-name]
PADS: Service-Name: ''
PPP session is 779
Connected to a0:f3:e4:34:d8:21 via interface eth0
using channel 2
Using interface pppoe-wan
Connect: pppoe-wan  eth0
sent [LCP ConfReq id=0x1  ]
rcvd [LCP ConfReq id=0x66   ]
sent [LCP ConfAck id=0x66   ]
rcvd [LCP ConfAck id=0x1  ]
sent [LCP EchoReq id=0x0 magic=0xc6952556]
rcvd [CHAP Challenge id=0x1 , name = "acc-aln1.hac"]
sent [CHAP Response id=0x1 , name = "bthomehub at btbroadband.com"]
rcvd [LCP EchoRep id=0x0 magic=0x4cc73648]
rcvd [CHAP Success id=0x1 "CHAP authentication success"]
CHAP authentication succeeded: CHAP authentication success
CHAP authentication succeeded
peer from calling number A0:F3:E4:34:D8:21 authorized
sent [IPCP ConfReq id=0x1   ]
sent [IPV6CP ConfReq id=0x1 ]
rcvd [IPV6CP ConfReq id=0x7b ]
sent [IPV6CP ConfAck id=0x7b ]
rcvd [IPCP ConfReq id=0x38 ]
sent [IPCP ConfAck id=0x38 ]
rcvd [IPCP ConfNak id=0x1   ]
sent [IPCP ConfReq id=0x2   ]
rcvd [IPV6CP ConfAck id=0x1 ]
local  LL address fe80::c595:37d1:3987:1929
remote LL address fe80::0221:05ff:feb4:8824
Script /lib/netifd/ppp-up started (pid 2646)
rcvd [IPCP ConfAck id=0x2   ]
local  IP address 81.146.2.155
remote IP address 172.16.12.12
primary   DNS address 81.139.57.100
secondary DNS address 81.139.56.100
ppp.log
secondary DNS address 81.139.56.100
Script /lib/netifd/ppp-up started (pid 2653)
Script /lib/netifd/ppp-up finished (pid 2646), status = 0x9
Script /lib/netifd/ppp-up finished (pid 2653), status = 0x9
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Status: New -> Researching
Priority: Very Low -> Medium
Assigned To: [-] -> Mathias Kresin

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=227