ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN

ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN

ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-16 16:21:54
Model: ER605 (TL-R605)  
Hardware Version: V2
Firmware Version: 2.2.6

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.

  0      
  0      
#1
Options
14 Reply
Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-17 07:55:00

  @jakobp 

 

Looks like your ISP might be insisting on the connection being from a 'fixed' MAC address. Try using the MAC address clone function on the ER605 ( manual page 33 ) to set the same MAC address as the PC that works.

  0  
  0  
#2
Options
Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-17 08:52:14

  @MisterW 

 

Thanks for the reply. No my ISP is not enforcing a certain MAC as I have tried with different devices and got a connection. It only seems that getting a public facing IP is bound to the routers MAC address. 

 

And sorry I should have mentioned the things I already tried in my post.

 

Things I already tried and didn't work:

  • Downgrading firmware
  • Running on a controller
  • Running in standalone
  • Factory reset
  • Cloning my Laptops MAC
  • Using a random MAC
  • Using unicast DHCP
  • Restarting Modem and router
  • Using a different cable

 

I also did packet inspection while trying it with different MACs and unicast DHCP with the same behavior as in my post.

 

I couldn't think of anything else to try. The only way I got it working in the meantime is getting an IP on my Laptop and using that as a static address. But that is not the ideal way to do it. Currently I'm running an older router running openWRT (using the ER605's WAN MAC) which works without issues.

  0  
  0  
#3
Options
Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-17 09:23:09
It does look like something not quite right. Strange that everything else seems to work ok apart from the ER605 ? TBH I've not much experience of ISPs using DHCP , mine uses PPPoE. I've dropped you a PM
  0  
  0  
#4
Options
Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-23 06:53:49

Hi @jakobp 

Thanks for posting in our business forum.

After the WAN finishes the DHCP, it will send an ARP announcement to check if there is any conflict.

ISP replies the gratuitous ARP which means there is an IP conflict so our router declines it.

This is the expected behavior.

 

Do you get the same IP address as your other router which works normally?

Best Regards! If you are new to the forum, please read: Howto - A Guide to Use Forum Effectively. Read Before You Post. Look for a model? Search your model NOW Official and Beta firmware. NEW features! Subscribe for the latest update!Download Beta Here☚ ☛ ★ Configuration Guide ★ ☚ ☛ ★ Knowledge Base ★ ☚ ☛ ★ Troubleshooting ★ ☚ ● Be kind and nice. ● Stay on the topic. ● Post details. ● Search first. ● Please don't take it for granted. ● No email confidentiality should be violated. ● S/N, MAC, and your true public IP should be mosaiced.
  0  
  0  
#5
Options
Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-23 08:03:33

  @jakobp 

 

Is there a chance you could get a packet capture using the router that works, so we can see any difference in the DHCP/ARP conversations

  0  
  0  
#6
Options
Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-27 13:34:22

  @MisterW 

 

I sadly couldn't get port mirroring working quickly in openWRT so I did a capture on my laptop hooked up to the fiber modem directly. Laptop is running Windows. It gets the IP normally by DHCP, then sends three ARP probes for the IP and finally an ARP annouce after it didn't get a reply to the probes (No duplicate IP ARP reply from the ISP). This seems to be in line with the ARP RFC from what I understand.

Heres the overview of the capture:

2    5.911216    0.0.0.0    255.255.255.255    DHCP    342    DHCP Discover - Transaction ID 0xe830ae03
3    5.914730    ISP_DHCP_Server    WAN_IP    DHCP    405    DHCP Offer    - Transaction ID 0xe830ae03
4    5.915349    0.0.0.0    255.255.255.255    DHCP    360    DHCP Request  - Transaction ID 0xe830ae03
5    5.931066    ISP_DHCP_Server    WAN_IP    DHCP    420    DHCP ACK      - Transaction ID 0xe830ae03
8    7.253867    Laptop_MAC    Broadcast    ARP    42    Who has WAN_IP? (ARP Probe)
9    8.255726    Laptop_MAC    Broadcast    ARP    42    Who has WAN_IP? (ARP Probe)
11    9.256186    Laptop_MAC    Broadcast    ARP    42    Who has WAN_IP? (ARP Probe)
12    10.253051    Laptop_MAC    Broadcast    ARP    42    ARP Announcement for WAN_IP

  0  
  0  
#7
Options
Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-27 13:41:20 - last edited 2024-09-27 13:42:10

Hi  @Clive_A 

 

Thanks for the reply.

 

From what I understand it should ARP probe the IP a few times before it sends the ARP announce. At least form what I read in the RFC. I'm no expert though, just trying to understand what might be going wrong.

 

Yes I get an IP address in the same network segment. I can also see the non working router going through that DHCP offer/decline every few seconds trying with different adresses.

 

My ISP is also looking into the issue now, so maybe they can also resolve the issue on their end.

  0  
  0  
#8
Options
Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-27 14:06:42 - last edited 2024-09-27 14:18:20

  @jakobp 

 

Interesting!

I'll have to have a good read through the RFC myself.

 

I wonder if, in the case of the ER605, the ARP announce is sent when the ISP systems are still finalising the IP allocation, and so its not sure who's been allocated it!.

Obviously in the Windows case the ARP probes will delay the sending of the announce.

 

As a matter of interest , is it a UK ISP ? and if so which ?

 

I have a colleague using Virgin media in the UK, they use DHCP and he's not having any problems with the ER605

 

I sadly couldn't get port mirroring working quickly in openWRT

 

When I've needed to capture on Openwrt I've used Wireshark. Provided you can ssh into Openwrt, then you can set it (wireshark) to run tcpdump remotely on the Openwrt device and capture straight into Wireshark

 

  0  
  0  
#9
Options
Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-27 14:20:39

  @MisterW 

 

Yeah the RFC was kind of interesting. I've never dug that deep for an network issue before. At least I learnt something new.

 

Hmm you could be onto something, didn't think of that yet. Maybe it is a kind of race condition.

 

I'm located in Austria, it's a local fiber to the home provider.

  0  
  0  
#10
Options
Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-27 14:27:10

  @jakobp 

 

I'm located in Austria, it's a local fiber to the home provider.

 

That might mean they will at least look into the problem. I was afraid that it was going to be a UK ISP using the BT/Openreach network. There'd be no chance of getting the monolith that is Openreach to investigate.

 

 

  0  
  0  
#11
Options