RT2600ac breaks HyperBackup (MTU Size?)

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RT2600ac breaks HyperBackup (MTU Size?)

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  1. RT2600ac
Operating system
  1. macOS
Mobile operating system
  1. iOS
I do HyperBackups every night to a remote target-DS - it used to work fine for years now.

Recently, I replaced the old router (Netgear) on the target-site with a new Synology RT2600ac.
Since this "upgrade" I get HyperBackup errors and also the entire WAN access breaks down (on the target site) for ~20min after HyperBackup starts.

On the source-site I get the following message:
[Network][Daten_Essentials_level2] Failed to access the backup destination. (Failed to transfer data to the target. Maybe MTU size is not appropriate for the network.)

The RT2600ac default MTU value is: 1492 - which I didn't change.

Data-Replications works fine without any issues (same source / target NAS like HyperBackup).

Any hints where to start the trial/error game?
Thank you!

PS: source is a coax (cable) line; target ADSL;
Hi there! I'm a little off topic, but I have a RT2600 as well. You mention above that you didn't change the MTU size. I'm wondering if there is a way to actually to this? Do you know?

If it's saying that the MTU is too large, then it stands to reason, changing it to something smaller (based on ping tests, I show my optimum is 1472) might work. Have you run a ping test to check yours?

My writing this is two pronged. First to try to help you and second, to see if you've come across a way to change it on the 2600. In my hunting, I've not found a way. I'm fine with using ssh to change it via cli, but haven't found a way through cli either.

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Last edited:
ping from the exact interface:
ping -c 3 -M do -s <MTUsize> -I <exact interface for the test> <target>
MTUsize is the value tested MTU size between you and target (try from 1470, for VPN you need lower value from 1200). But it is up to E2E route configuration.
exact interface for the test is the IP address of tested NAS interface, e.g.
target is IP address or domain (synoforum.com)

then you can tune the MTU in the NAS or in the Router by:
ifconfig <interface> mtu <value>
don’t forget to add +28 to final MTU value achieved from the test, eg. 1470 + 28= 1498

the interface you can check by:
note, there isn’t the IP address as is for the ping command
-- post merged: --

PS: source is a coax (cable) line; target ADSL;

Seems to be the ADSL is the bottleneck and the problem core.
Because this kind of interface is frequently running on really old infrastructure (from the main switch to the last mile). When you will add there QoS from the IPS also , some services could be difficult to operate.
What about test of different period for the transfer (from 2:00am)?
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In the Target site:
try to enable ICMP in firewall, just for the source IP address. You can avoid possible Black hole event in this case = dropped connection.
In NAS side you can setup this by Firewall rule:
Custom port: ICMP
Source: your source IP address

I don’t know how to setup in the Syno routers, because I’m Ubiquiti guy.
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