Level 2: Rookie

HFC NBN Drop outs frequently - DHCP issue

Since I switch to HFC NBN from HFC cable, my home Internet becomes so unstable. 

Actually it became worse from 26 Oct 11:30am, Internet dropped out 1 or 2 times each hour. It went ok after 5pm. 

27 Oct not much complains, Internet only dropped once in the morning. 

28 Oct it started to drop out again every hour from 9am. 

The NBN black connection box all lights are green stable during the drop outs. 

Before 27 Oct, I use my own Mikrotik router connects to NBN black box directly getting DHCP IP addresses. During that day, I saw the WAN Internet IP addresses changing almost every hour (Telstra NBN DHCP lease is 1 hour). If it failed to receive response from Telstra DHCP server. Then after DHCP lease expired, it has to bring down the WAN interface to force a DHCP discover / bind process. So the Internet will be dropped out around 5 minutes each time. 

I called Telstra 26 Oct, and was advised to use Telstra Smart Modem (first gen) not my personal router. Fine. After I changed to Telstra Smart Modem late evening that day, Internet became stable. And I can see the DHCP on Telstra Smart Modem got a one hour DHCP lease at first, then it will renew a one day DHCP lease. So the following day 27 Oct, my home Internet was quite stable. 

But from 28 Oct morning, Telstra Smart Modem DHCP behaviors the same. It cannot properly get response from DHCP server in the mid-point of DHCP lease, after DHCP expires, it loses Internet IP address, and fall back to 4G backup. Then took 10 minutes to roll back to primary ethernet link after it gets the new Internet IP again. 

So this is not a problem on my Mikrotik router, not a problem on Telstra Smart Modem, obviously a conjunction on Telstra DHCP server side. 

I called Telstra couple times, and seems like staff on call not really understand technical. Just follow the instruction to perform useless troubleshooting steps. 

I think it may worth to leave a post here, so someone from Telstra tech or backend engineer can comment and try to fix this issue asap. Feel free to DM me if you need further information. Thanks. 

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Level 2: Rookie

Re: HFC NBN Drop outs frequently - DHCP issue

After couple days observation, I find Telstra DHCP server has issue on client DHCP renew. 

1. A new DHCP discover request will get a ONE hour DHCP lease at the beginning. 

2. Client (Modem) will try to renew DHCP at mid-point of that hour (after 30 min), if the DHCP renew successful, the DHCP lease will change to ONE day (24 hours). 

3. Client (Modem) will try to renew DHCP at mid-point of that day (after 12 hours). It always fails at this phase these days. Since the client (modem) not receive the DHCP ACK (response), the Internet will drop out at around 15 hours of that 24 hours DHCP lease and loses the IP. 

The client (modem) has to bring down the interface to do a DHCP discover again in order to get a NEW IP address. Then repeat the 1 step. Get a ONE hour DHCP lease. It will renew a ONE day DHCP lease after 30 min. Then drop out after another 15 hours. 

So the Internet IP address will change at least once every 15.5 hours.


I'd like to share some screenshots of the related info.

First One Day DHCP lease


This was the first ONE day DHCP lease. It supposed to renew and kept its IP address at around 10pm 28 Oct, but failed. 


Not able to renew DHCP after 15 hours

At around 10:15pm, the client (modem) tried to renew DHCP but ends with (runs L3DHCPMain.check(ETH.timeout)). It kept trying since then, but all failed with timeout. 

 The next day after dropout. New IP

So after 15 hours (0:50am 29 Oct) still not able to renew Internet IP. The dropout began. 0:55am it failed over to 4G backup. Around 1:50am, the primary Internet got a new IP (ONE hour DHCP lease) then failed back to primary Internet link. 2:27 the DHCP renew was successful, and extend to a ONE day DHCP lease. 


I can expect the next dropout will happen at 17:30 this afternoon. 


Is this something Telstra can explain? 


Level 24: Supreme Being
Level 24: Supreme Being

Re: HFC NBN Drop outs frequently - DHCP issue

If you want this fixed you will need to lodge a complaint with Telstra. The call centre staff will not know what your talking about and are reluctant to escalate to some one who does. If you lodge a complaint your case manager should hopefully excalate to some one who understands the problem and can investigate and fix. You can lodge acomplainy by ringing 132200 and when asked for reason say complaint or use online form at link below.




Level 2: Rookie

Re: HFC NBN Drop outs frequently - DHCP issue

Thanks, SR1-2242623909779 complaint lodged.

Level 2: Rookie

Re: HFC NBN Drop outs frequently - DHCP issue


As I suggested, Telstra offered me static IP to test if this solve the problem. 

The Internet seemed to become stable the first day of using static IP. But dropout still happened after day one. 

The Telstra static IP is delivered thru DHCP. I don't think I can manually set static IP on WAN interface as Telstra use IPoE to authenticate users. 

So the dropout still can be forecast between each DHCP lease. 

Static IP DHCP lease is different to default dynamic one. The static IP DHCP is one hour lease first, then 4 hours lease afterwards. Dynamic IP DHCP is one hour lease first, then 1 day lease afterwards. 

If conjunction happens on Telstra side, your modem / router not able to properly communicate to DHCP server and get lease renewed, then the dropout will definitely happens.  Because of IPoE, if DHCP fails, your Internet will pause (not authenticated). Then that NBN Arris connection box will bring down all interfaces to force modem / router to perform a DHCP DISCOVER to get a new DHCP lease. Then the Internet flows again. 


Lot of users even not noticed the dropout as they may not actively use Internet or monitoring the connectivity. Telstra Smart Modem will fail back to 4G mobile network during the dropout. So the dropout usually only lasts 5 minutes. But if user has some service running at home and needs remote access, he/she will definitely notice that public IP address is keep changing. Static IP will help on this case but the dropout still happens unless Telstra tech investigate the root cause of DHCP and IPoE conjunction issue on their end. 

Level 24: Supreme Being
Level 24: Supreme Being

Re: HFC NBN Drop outs frequently - DHCP issue

This problem does not occur on all NBN services. My NBN Services does not drop out every day. There might be a fault with the Telstra server you are connected to.

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