Hi there,

After getting fiber to the home I switched to the ER605v2 but cannot get it to connect to the WAN. It just didn't get an IP from my ISP's DHCP. After trying a lot of things and contacting my ISP I still haven't gotten it to work. Last weekend I spent some more time on that and did some packet inspection via interface mirroring and found some interesting things.

The router is doing a normal DHCP discover and recieves a IP normally. After the server sends the DHCP ACK the router then sends an ARP gratuitous annouce that it now has that IP adress. This causes my ISP to send an ARP response with their MAC adress and the router's IP address as sender and the routers MAC and IP as dest which indicates an ARP collision (which it isn't, cause it's sending the MAC of the WAN port). This causes the router to send a DHCP decline though, cause I assume it thinks the IP is already in use. Maybe this is causing some ARP protection to kick in on my ISP. This repeats over and over again. I'm no network expert, I just did some research and found that regarding RFC 5227 it should apparently first do an ARP probe to check if the IP is use before announcing itself. Also the ISP is doing an ARP probe before it sends the router the IP so it can't be duplicate.

I also traced when I hooked my laptop up to the modem directly and it just sends an ARP probe after recieving the IP. There i get no ISP response so it just works.

Overview of the packet trace (replaced actual adresses with placeholders):

No.    Time    Source    Destination    Protocol    Length    Info
31419    255.089218    0.0.0.0    255.255.255.255    DHCP    342    DHCP Discover - Transaction ID 0x88269d46
31641    256.094493    ISP-IP    255.255.255.255    DHCP    437    DHCP Offer    - Transaction ID 0x88269d46
31847    257.119227    0.0.0.0    255.255.255.255    DHCP    342    DHCP Request  - Transaction ID 0x88269d46
31849    257.147154    ISP-IP    255.255.255.255    DHCP    437    DHCP ACK      - Transaction ID 0x88269d46
32098    258.187002    WAN-MAC    Broadcast    ARP    60    ARP Announcement for WAN-IP
32099    258.188406    ISP-MAC    WAN-MAC    ARP    60    Gratuitous ARP for WAN-IP (Reply) (duplicate use of WAN-IP detected!)
32101    258.249445    0.0.0.0    255.255.255.255    DHCP    342    DHCP Decline  - Transaction ID 0x4830755f

The ARP response from my ISP:

Address Resolution Protocol (reply/gratuitous ARP)
    Hardware type: Ethernet (1)
    Protocol type: IPv4 (0x0800)
    Hardware size: 6
    Protocol size: 4
    Opcode: reply (2)
    [Is gratuitous: True]
    Sender MAC address: ISP-MAC
    Sender IP address: WAN-IP
    Target MAC address: WAN-MAC
    Target IP address: WAN-IP
[Duplicate IP address detected for WAN-IP (ISP-MAC) - also in use by WAN-MAC (frame 32098)]
    [Frame showing earlier use of IP address: 32098]
    [Seconds since earlier frame seen: 0]
 

I'm not sure if this is a standard practice so my question is if this is a firmware bug or if my ISP is just sending a nonsense response?

I'm at my wits end and I'll also forward this to my ISP.

Any help is appreciated and please correct me if my assumptions are wrong.