Some sites not loading on Airtel - issue and fix

[pxgmr]

I am using my own ONT (TP-Link XZ000-G7) and router (OpenWrt). I updated my router which required unfortunately a restart after 50 days of uptime with no issues.

Since that restart, I was facing an incredibly annoying issue:

• Some sites weren't loading, and I was getting ERR_TIMED_OUT instead.
• Browser VPN extensions (e.g., Windscribe) weren't working
• Restarts of the PC, ONT, router, etc. fixed nothing
• Payment gateway pages (especially) for OTP in apps didn't load

Now I did some basic troubleshooting:

• WireGuard VPN (Cloudflare Warp) wasn't affected at all. Everything worked.
• GoodbyeDPI did nothing. The sites weren't censored, and they loaded fine on 5G too.
• DNS lookups were working fine. I use DNS over TLS.
• Both IPv4 and IPv6 had connectivity.
• Could ping all of these sites despite the HTTP timeout.
• Checked all devices - the same sites were broken on all of them (so issue wasn't device specific)
• Turned off IP blocklists and DNS adblock

So, after scratching my head and consulting AI, I figured out the fix:

• Set MTU to 1480 for PPPoE in your router.

This one was very weird because I have been using 1492 for many, many years now. Did a lot of testing with IPv6 MTU, IPv4 MTU, mini jumbo frames, etc.

And even when troubleshooting earlier I checked with 8.8.8.8 and it worked just fine with 1492 (ping 8.8.8.8 -f -l 1464). But the sites that were broken did not work with 1492 immediately started working with 1480 MTU.

If you are stuck with the provided Airtel router and are facing the same issue, then you can set the MTU manually on most operating systems but good luck setting the MTU on your smart TV.

This could just be an issue that I created myself (given my setup) but given that it started happening recently, I think someone incompetent at Airtel is setting up tunnels or vlans somewhere without accounting for their packet overhead.

Another note: Using the ONT with transparent mode adds another 4 bytes of overhead due to the VLAN tag. But the ONT also supports baby jumbo frames so I was able to use 1504 MTU on ethernet. Of course, Airtel doesn't support baby jumbo frames so was still restricted to 1492 for PPPoE until recently.

 

[hariselio]

In Mikrotik router, MTU got automatically changed to 1480 from last week or two, till last month i remember seeing 1492. For ACT pppoe, it is showing 1492 and Airtel pppoe it is 1480. Isn't OpenWrt will automatically does similar adjustment based on config from ISP side?

 

[ka2107]

I am also using OpenWrt router (currently running OpenWrt 24.10.4) along with my own ONT (Netlink HG323RGW, bought and used for BSNL Fiber before Airtel Fiber).

In my router, when I go to OpenWrt LuCI, Network ==>> Interfaces ==>> Devices Tab, I see the PPPoE Tunnel Device MTU is set to 1480 bytes by OpenWrt itself. I am NOT manually overriding the MTU value of the PPPoE tunnel device in /etc/config/network config file.

 

[pxgmr]

I have MTUs set manually for troubleshooting after tinkering like a year ago.

Clearly something changed in the routing recently as 1492 was working fine for all sites. It's really incompetent for them to downgrade the MTU like that.

 

[x86_64]

yeah i also noticed.
only 1452 bytes of icmp payload can be sent without fragmenting.
that means 1452 icmp payload + 8 icmp header + 20 ipv4 header == 1480 bytes MTU.

root@deb-airtel:~# ping -M do -s 1452 1.1.1.1
PING 1.1.1.1 (1.1.1.1) 1452(1480) bytes of data.
1460 bytes from 1.1.1.1: icmp_seq=1 ttl=59 time=19.6 ms

root@deb-airtel:~# ping -M do -s 1453 1.1.1.1
PING 1.1.1.1 (1.1.1.1) 1453(1481) bytes of data.
ping: sendmsg: Message too long
ping: local error: message too long, mtu=1480

but their crap router web interface still shows 1492 MTU which is incorrect.