AX7501 DCHP problem

igor_k

Hello,

I think i found a bug and I can't find the easy way to report it. So I will give a try here.
Problem: Changing DCHP v4 settings makes router unusable.
Description: After changing the DHCP config to "DCHP relay", the internet connection becomes permanently broken(timeout). Attempt to change back the DHCP settings does not restore internet connectivity.
Test setup:Router: Default config, IPv6 disabled.
Laptop: wired, with static IP 192.168.1.222 mask 255.255.255.0 gateway 192.168.1.1, DNS 192.168.1.1
Mobile phone on the Wifi, with dynamic IP.
Changing router settings:
DHCP Enabled: Everything works as expected: I can ping/login to router from laptop, internet works on both devices.
DHCP Disabled: Everything works as expected: Mobile can't connect to Wifi (no DHCP on Wifi), I can ping/login to router from laptop, internet works on laptop.
DHCP Relay: Mobile connects to Wifi, I can see its lease on my DHCP server, I can ping/login to router from laptop, internet does not work on both devices.
Change back to any option does not solve issue. Only Master reset.

Model NameAX7501-B1Firmware VersionV5.17(ABWW.4)C0
I would appreciate any resolution hints, thanks in advance!
Igor.

tonygibbs16

Hello @igor_k

Welcome to the forum.

I think that you might have a misunderstanding about how DHCP relay works in general.

The following quote from CBT Nuggets might help you:

"What is a DHCP Relay?

Quick Definition: A DHCP relay, or DHCP relay agent, is a device on a network that forwards DHCP-related communications to and from DHCP clients and DHCP servers. DHCP relays are necessary on larger networks with multiple subnets because of the nature of DHCP requests. DHCP relay agents pass along DHCP packets between clients and servers to expand the size and scale of a DHCP-enabled network."

from https://www.cbtnuggets.com/blog/technology/networking/how-to-use-the-ipv6-dhcp-relay but it applies to IPv4 as well.

DHCP relay is needed because DHCP uses broadcast addresses for parts of DHCP Discover Offer Request Acknowledgement (DORA) process, see https://en.wikipedia.org/wiki/Dynamic_Host_Configuration_Protocol#:~:text=DHCP%20operations%20fall%20into%20four,with%20clients%20broadcasting%20a%20request.

and Routers do not forward broadcast datagrams between subnets.

This means that if you have a DHCP server on the same subnet (e.g. the 192.168.1.0/24 subnet in your case) as your DHCP clients then you do not need to enable DHCP relay.

- I think that you just need to have your DHCP server at 192.168.1.100 and disable the DHCP server on your AX7501 home gateway, so that there is just one DHCP server for your hosts to find/discover.

I think that this should work for you.

Please let us know how you get on.

Kind regards,

Tony

igor_k

Hello tonygibbs16,

Thank you for the prompt reply and for very informative links!

I think you might have misunderstood the subject of my message. It is a bug report, stating that setting "DHCP relay" option breaks the router primary function irrecoverably. I didn't describe the "why" because it is irrelevant for the subject and not because I dont understand what I am doing.
I changed the config to as close to defaults as possible for finding out what exactly causes router to die - i had to master reset the box and reconfigure all for 10+ times... In reality I have 3 subnets with a common DMZ, so a single DHCP will be very handy.
I do agree with you, for the purpose of switching to another DHCP, "disable" option shall be good enough. However, if you set "disable DHCP", who will attribute IP addresses of devices in the guest network? Shall not that case be addressed with DHCP relaying? :-) I just checked, with "DHCP disabled" the Guest feature is broken - they get auto-ip and no internet access.

Best regards,

Igor.