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Thread: GOT MTUs?

  1. #1
    Junior Member
    Join Date
    Sep 2016
    Posts
    15

    Thumbs up GOT MTUs?

    Physical Interface (Currently TRUNKED interface to my (L2)Switch<Cisco 3750G>):
    FGT-90D (internal1) # sh full | grep "set mtu"
    set mtu-override enable
    set mtu 9208 <-WHOOHOO JUMBOS!

    Virtual VLAN interface:
    FGT-90D (LANNET_VLAN9) # set mtu
    <integer> Maximum transmission unit(68-1500)

    My manual settings:
    FGT-90D (LANNET_VLAN9) # sh full | grep "set mtu"
    set mtu-override enable
    set mtu 9028 <-WTF??? Yes Jumbos/No Jumbos??

    Ping results using 8000 bytes:
    C:\Users\amnesia>ping -l 8000 (com'on big packets!) 10.10.9.254 VLAN9 Sub interface on FG90D

    Pinging 10.10.9.254 with 8000 bytes of data:
    Request timed out. <-LIES
    Request timed out. <- LIES
    Request timed out. <- LIES
    Request timed out.
    <- LIES

    Ping statistics for 10.10.9.254:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

    Ping results using 1390 bytes: >1500Bytes
    C:\Users\amnesia>ping -l 1390 10.10.9.254 VLAN9 Sub interface on FG90D

    Pinging 10.10.9.254 with 1390 bytes of data:
    Reply from 10.10.9.254: bytes=1390 time<1ms TTL=255 <- NO JUMBO FOR YOU!
    Reply from 10.10.9.254: bytes=1390 time<1ms TTL=255 <- NO JUMBO FOR YOU!
    Reply from 10.10.9.254: bytes=1390 time<1ms TTL=255 <- NO JUMBO FOR YOU!
    Reply from 10.10.9.254: bytes=1390 time<1ms TTL=255 <- NO JUMBO FOR YOU!

    Ping statistics for 10.10.9.254:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
    Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 0ms, Average = 0ms

    Moral of Story:
    Only Physical interfaces ( this has only been verfied on my FW90D v.5.4.1 have not verified on anything else) will support Jumbo frames, virtual/sub interfaces only support standard 1500 MTU packets.. SAD DAY.

  2. #2
    Sorry dude. I would have told you had you asked haha. Would have saved you a little time. I probably wouldn't have remembered until I read this though.

  3. #3
    Junior Member
    Join Date
    Sep 2016
    Posts
    15
    Yea, looks like I will be moving away from the VLAN interfaces and using physical ones. I wonder, just how much overhead on the FGT using vlan interfaces actually causes.

  4. #4
    Junior Member
    Join Date
    Sep 2016
    Posts
    15
    Well, changed all my sub-interfaces to physical interfaces and now all my Jumbos belong to me!! Woot!
    Home Prod: FGT-90D / FortiOS 5.4.1
    Supported FGTs: x6 800Cs /FortiOS 5.2.9 & 5.4.1 | x2 FGT-1500D Clusters / FortiOS 5.2.9

    "Find your passion and go for it!"

  5. #5
    Nice fix. It is annoying to have to do that but it is what it is.

  6. #6
    My understanding is MTU on a virtual or sub interfaces must match the MTU of the physical interface.

    I ran into this with a P2P circuit that required a slightly smaller MTU because of an IPSec tunnel. We had to set MTU on each physical interface and virtual interface as well as each SSID.

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