Router(OpenWrt) <----> Modem(OpenWrt) mini-jumbo frames

Installing and Using OpenWrt Network and Wireless Configuration
1

Mini-Jumbo frames was working fine when I was using a none OpenWrt modem. But since the switch I can't seem to get it working again.
I keep getting Request timed out.

ping 8.8.8.8 -f -l 1472

Pinging google.com [8.8.8.8] with 1472 bytes of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 8.8.8.8:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

though the Router pppoe-wan interface is using mtu 1500

pppoe-wan: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc cake state UNKNOWN mode DEFAULT group default qlen 3
    link/ppp

And

daemon.debug pppd[3029]: rcvd [LCP ConfReq id=0x2 <mru 1500> <auth pap> <magic 0x396dcdec>]
daemon.debug pppd[3029]: sent [LCP ConfAck id=0x2 <mru 1500> <auth pap> <magic 0x396dcdec>]

I did what https://forum.openwrt.org/u/trismo suggested here https://forum.openwrt.org/t/solved-turning-a-xdsl-lede-router-functionally-into-a-bridged-vdsl-modem/6973/2
So the Modem dsl0 interface is using mtu 1508

dsl0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1508 qdisc fq_codel master br-wan state UP qlen 1000
    link/ether xx:xx:xx:xx:xx:xx brd ff:ff:ff:ff:ff:ff

But it didn't make any difference.

And http://www.letmecheck.it/mtu-test.php gives me

Sending 32 bytes to <IP address>  <-  not fragmented

Sending 750 bytes to <IP address>  <-  not fragmented

Sending 1125 bytes to <IP address>  <-  not fragmented

Sending 1313 bytes to <IP address>  <-  not fragmented

Sending 1407 bytes to <IP address>  <-  not fragmented

Sending 1454 bytes to <IP address>  <-  not fragmented

Sending 1478 bytes to <IP address>  <-  FRAGMENTED!

Sending 1466 bytes to <IP address>  <-  FRAGMENTED!

Sending 1460 bytes to <IP address>  <-  not fragmented

Sending 1463 bytes to <IP address>  <-  not fragmented

Sending 1465 bytes to <IP address>  <-  FRAGMENTED!

Sending 1464 bytes to <IP address>  <-  not fragmented

Sending 1465 bytes to <IP address>  <-  FRAGMENTED!

Sending 1464 bytes to <IP address>  <-  not fragmented


From the tests we did, we can assume that 1464 bytes is the largest unfragmented packet
size. The MTU size would be 1492, made up from 1464 payload and 28 ICMP/IP Headers
and payload information. 
The maximum MTU size for <IP address> is:    1492

So any suggestions or ideas on how to get the mini-jumbo frames to work again with an OpenWrt Modem in full bridge mode?

2

no bright ideas yet?

4
  • Can you try using -I (uppercase i) to explicitly specify dsl0 during pings?
  • Please run this ping on the router (as you have to specify dsl0, which you configured the larger MTU)
  • Please provide the MTU of the device from which you ran ping, as both -f and -l (lowercase L) are invalid on the OpenWrt-default ping command
  • Please provide the MTU of the LAN (or other interface) that the test device was connected to on the router (i.e. ifconfig br-lan | grep MTU)
  • I'm not sure its advisable to flood ping Alphabet Corp

ping 8.8.8.8 -s 1472 -I <IP_of_dsl0>

  • What device are you using?
  • Is this device's hardware switch chip capable of jumbo frames?
5

Windows 10 default MTU 1500

# ifconfig br-lan | grep MTU
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1


PING 8.8.8.8 (8.8.8.8) from 192.168.2.100: 1472 data bytes
ping: can't set multicast source interface

I had the Router running with mini-jumbo frames when it was hooked to a normal modem.
As for the current modem it's Tplink W8970

6

Can you change the MTU on LAN and the Windows client to match the MTU that you desire?

They must have the larger MTU so they in turn don't fragment.

7

They are set to MTU of 1500, that's the point of having mini-jumbo frames so that PPPoE frames match the MTU on Lan and windows.

8

And i thought you wanted 1508?