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

14 Reply
Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-29 08:35:31

Hi @jakobp 

Thanks for posting in our business forum.

jakobp wrote

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.

RFC is a basic standard for vendors to comply with. But some vendors do not follow this order to announce or it does not affect the usage.

Will keep an eye on your ISP followup if you get back to us.

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Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-09-29 12:52:15

  @jakobp 

 

I've been doing some Wireshark captures with various devices to see how they process DHCP/ARP requests. 

My ISP uses PPPoE so I can't test with that, but I've tested connecting devices to the LAN on my Openwrt router.

 

Of the four devices, a Draytek 2830 router, TpLink W9970, Windows 11 & Debian Bookworm Linux system, only the Windows 11 system actually adheres to RFC5227 and sends ARP probes.

 

The Draytek, after getting the DHCP assisgnment, sends an ARP 'Who has' for its default gateway, which means its ARP announce for its allocated IP is delayed for around a second.

 

The TpLink W9970, after getting the DHCP assisgnment, sends an immediate ARP announce for its allocated IP in about 4 milliseconds.

 

The Debian system has a static IP, but on startup it initially sends an ARP 'Who has' for its default gateway. About 5 secsonds later the gateway sends a 'Who has' for the debian system IP and it responds. No ARP announcement is made, probably correct for a static IP.

 

The Win11, after getting the DHCP assisgnment, sends an ARP 'Who has' for its default gateway, having got a response to that , its sends out 3 ARP probes for its allocated IP and eventually send an ARP announcement some 4 seconds later.

 

Not sure what, if any, conclusions one can infer from that, but it would seem that some devices can send the ARP announce quicker than others.

 

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Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-10-05 09:22:28

  @MisterW 

 

Thank you all for your help and investigations. My ISP could fix it in their backend. Because I'm curious, I asked them if they could tell me what the issue was, so I'm waiting on their response.

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#14
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Re:ER605 v2 sending ARP announce after getting IP on WAN causes DHCP decline and not connecting to WAN
2024-10-05 09:47:23

  @jakobp 

 

I'd be interested to hear what they have to say.

 

Most ISPs use a DHCP Relay agent because their DHCP server is on an internal network and not accessible directly from a users connection.

 

I wonder whether the fact that the ER605 is sending the ARP announce so quickly is confusing their relay agent. It shouldn't be but who knows...

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