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Strange question on MTU

Discussion in 'ASUS Wireless' started by Primarch, Jul 20, 2018.

  1. Primarch

    Primarch New Around Here

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    I'm running an RT-AC88U with stock firmware. Everything is fine with the current settings. However I would like to be able to adjust the MTU value on the fly when needed, without changing other settings. It would appear that on the stock firmware that's only possible via WAN mode set to PPPoE which my ISP isn't using for a cable connection.

    I don't need a lot of other fluff or anything fancy, I'm just lost as to what the simplest option for accomplishing my goal is? Do I need Merlin/Tomato/something else? This was super simple with my old linksys e3000, but now seems oddly complicated.
     
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  3. ColinTaylor

    ColinTaylor Part of the Furniture

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    Merlin's:

    WAN > Internet Connection > Account Setting > MTU
     
  4. KevTech

    KevTech Very Senior Member

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    If you want to stay on stock firmware you can still set MTU through telnet or SSH.

    To see what it is currently set to: ip link show eth0

    To set it to a different number (i will use 1470 in this example): ip link set mtu 1470 eth0
     
  5. Primarch

    Primarch New Around Here

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    Thank you both. Networking isn't really my forte and I'm really rusty on some stuff but I'm not a terrible coder. Installed Merlin as it seemed to make things a bit more user friendly, and then used KevTech's advice to fix an otherwise very janky coding solution to a silly problem.
     
    Last edited: Jul 24, 2018
  6. agilani

    agilani Senior Member

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    I've noticed that enabling jumbo frames does not change the MTU settings. Does that need to be changed manually?
     
  7. ColinTaylor

    ColinTaylor Part of the Furniture

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    Just bear in mind that unless you put that command in a startup script it will be lost when the router reboots.
     
  8. RMerlin

    RMerlin Super Moderator

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    MTU setting is routing on the WAN side, while jumbo frames is for switching on the LAN side.
     
  9. agilani

    agilani Senior Member

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    the ethernet ports are all lan right? with jumbo frames enabled they all stills how 1500 mtu on my ac88

    Code:
    eth0      Link encap:Ethernet  HWaddr 2C:4D:54:21:17:F0
              inet addr:23.242.44.106  Bcast:23.242.47.255  Mask:255.255.240.0
              UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
              RX packets:1071470159 errors:4 dropped:0 overruns:0 frame:4
              TX packets:966417455 errors:0 dropped:0 overruns:0 carrier:0
              collisions:0 txqueuelen:1000
              RX bytes:3585336201 (3.3 GiB)  TX bytes:677672959 (646.2 MiB)
              Interrupt:181 Base address:0x6000
    
    eth1      Link encap:Ethernet  HWaddr 2C:4D:54:21:17:F0
              UP BROADCAST RUNNING ALLMULTI MULTICAST  MTU:1500  Metric:1
              RX packets:8050327 errors:0 dropped:0 overruns:0 frame:0
              TX packets:51493610 errors:0 dropped:4296892 overruns:0 carrier:0
              collisions:0 txqueuelen:1000
              RX bytes:2412062338 (2.2 GiB)  TX bytes:3797705460 (3.5 GiB)
    
    eth2      Link encap:Ethernet  HWaddr 2C:4D:54:21:17:F4
              UP BROADCAST RUNNING ALLMULTI MULTICAST  MTU:1500  Metric:1
              RX packets:0 errors:0 dropped:0 overruns:0 frame:0
              TX packets:728052990 errors:0 dropped:8386 overruns:0 carrier:0
              collisions:0 txqueuelen:1000
              RX bytes:0 (0.0 B)  TX bytes:3516150719 (3.2 GiB)
    
    the only interface with a different mtu size with jumbo frames seems to be local loopback
    Code:
    lo        Link encap:Local Loopback
              inet addr:127.0.0.1  Mask:255.0.0.0
              UP LOOPBACK RUNNING MULTICAST  MTU:16436  Metric:1
              RX packets:5211008 errors:0 dropped:0 overruns:0 frame:0
              TX packets:5211008 errors:0 dropped:0 overruns:0 carrier:0
              collisions:0 txqueuelen:0
              RX bytes:1226500361 (1.1 GiB)  TX bytes:1226500361 (1.1 GiB)
     
  10. RMerlin

    RMerlin Super Moderator

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    Again, MTU and Jumbo Frames are separate settings. Enabling Jumbo Frame support at the switch level does not change the MTU.
     
  11. agilani

    agilani Senior Member

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    Sorry if I am being dense Eric, I am looking to use Jumbo frames on my lan. The MTU settings are part of determining what size ethernet frames are supported. Jumbo frames are typically anything larger than 1501 MTU. I guess what i don't understand yet, what is the correlation between the interface ports and if they are lan or wan. Is there something that i can use to read up on how the interfaces are used or allocated?

    Looking at the arp table, eth0 is wan so the wan mtu setting is used for that

    br0 which i am assuming a bridged interface between lan and wireless is set to MTU 1500 as are all of the other intefaces

    My understanding is that the ac88 has two switches as well, the first four ports that are controlled by the broadcom chipset and the remaining 4 ports though a connected realtek switch. In a cisco or juniper switch, i would expect to see MTU size for any interface that supports jumbo frame to be set larger than 1501 MTU - typically 9000 MTU. Each server would then typically also have to have the MTU size set in its interface settings.
     
  12. kfp

    kfp Very Senior Member

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    @agilani In other words, MTU Setting: WAN side, Jumbo Frames: LAN side (which also changes MTU)
     
  13. ColinTaylor

    ColinTaylor Part of the Furniture

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    Except it doesn't change the MTU size as noted in post #9. MTU has to be changed separately.

    FWIW even with Jumbo Frames enabled on the router I wasn't able to increase the MTU of br0 from 1500. Regardless, I was able to configure 9k MTUs on two (wired) LAN clients and send jumbo packets between them.
     
  14. agilani

    agilani Senior Member

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    .

    Thanks Colin, that's the piece that i was missing. These clients were directly wired into the router and not some downstream switch?
     
  15. kfp

    kfp Very Senior Member

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    Eh, good to know and thanks for testing.
     
  16. RMerlin

    RMerlin Super Moderator

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    https://en.wikipedia.org/wiki/Maximum_transmission_unit

    MTU is at the protocol level, while jumbo frames are at the Ethernet (link) layer level. Switched traffic (within your LAN) only occurs at the Ethernet level.
     
  17. RMerlin

    RMerlin Super Moderator

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    Jumbo Frames are generally a bad idea in 2018, as they require every single LAN device to support them. With people connecting all sort of cheap IoT devices these days, these generally don't support them.

    The minimal performance gain you'd get is greatly outweighted by the pain involved in getting it to work - assuming you can get it to work with all of your devices.
     
  18. agilani

    agilani Senior Member

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    I only enable it on devices that do large data transfers or backups. For example QNAP NAS and Server 2016. It considerably reduces the time for backing up one to the other. I have been looking at 5g or 10g for home use, but the number of switches are pretty limited. I used to just attach everything to the netgear switch that supports it, but since the ac88 has 8 ports, i'd rather use the additional ports than add yet another switch to my network.
     
  19. agilani

    agilani Senior Member

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    So first test using jumbo frames through the ac88 was a bust on the lan. I see the packets are 9014 bytes, and they aren't being fragmented and no errors but for some reason the throughput went from 111 MB/s to 50MB/s! I was expecting it to increase to 120MB/s or so. I'll probably play around with it a little more, but with the nominal gains possible, I may have to agree with Merlin. It may not be worth the effort.

    The setup

    workstation <==> netgear <==> ac88 <==> Server

    I don't know if windows calculates the throughput correctly once the frame sizes have increased. I'll try ubuntu with the qnap next or try it with everything connected to the netgear switch.
     
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