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support for "jumbo" frames in Aruba CX

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  • 1.  support for "jumbo" frames in Aruba CX

    Posted Dec 31, 2020 03:21 AM
    in Aruba OS, it was enough to configure "jumbo" command under vlan to enable jumbo frames for all traffic in that vlan. I am not able to find equivalent command in Aruba CX as there are multiple places where MTU can be configured:

    1- under "interface vlan x" there is "ip mtu" command. 

    2- under "interface x/x/x" there is "mtu" and "ip mtu" commands.

    to enable jumbo frames in any vlan, is it required to configure mtu and ip mtu on all interfaces part of this vlan + L3 vlan interface?

    no single command for this?

    ------------------------------
    Ahmad Enaya
    ------------------------------


  • 2.  RE: support for "jumbo" frames in Aruba CX
    Best Answer

    MVP GURU
    Posted Jan 03, 2021 06:09 AM
    Hi,

    No, there is not single command for enable jumbo every where

    you need to enable mtu on all physical interface and ip mtu on all L3 interface (vlan or physical)

    ------------------------------
    PowerArubaSW : Powershell Module to use Aruba Switch API for Vlan, VlanPorts, LACP, LLDP...

    PowerArubaCP: Powershell Module to use ClearPass API (create NAD, Guest...)

    PowerArubaCX: Powershell Module to use ArubaCX API (get interface/vlan/ports info)..

    ACEP / ACMX #107 / ACDX #1281
    ------------------------------



  • 3.  RE: support for "jumbo" frames in Aruba CX

    MVP GURU
    Posted Jan 03, 2021 10:57 AM
    Hi Alexis,

    "you need to enable mtu on all physical interface and ip mtu on all L3 interface (vlan or physical)"

    that "and" is confusing me a bit...did you mean that - considering a simple scenario (let me say: a physical interface, non routed, operating in access or trunk mode) - just setting an MTU value at physical interface level is not enough?

    ------------------------------
    Davide Poletto
    ------------------------------



  • 4.  RE: support for "jumbo" frames in Aruba CX

    EMPLOYEE
    Posted Jan 04, 2021 04:10 AM
    Hi Davide,
    "IP MTU" command is only required for routed traffic. "MTU" command is enough for pure L2 switch.

    ------------------------------
    Vincent Giles
    ------------------------------



  • 5.  RE: support for "jumbo" frames in Aruba CX

    MVP GURU
    Posted Jan 04, 2021 04:19 AM
    +1 with vincent about IP MTU

    ------------------------------
    PowerArubaSW : Powershell Module to use Aruba Switch API for Vlan, VlanPorts, LACP, LLDP...

    PowerArubaCP: Powershell Module to use ClearPass API (create NAD, Guest...)

    PowerArubaCX: Powershell Module to use ArubaCX API (get interface/vlan/ports info)..

    ACEP / ACMX #107 / ACDX #1281
    ------------------------------



  • 6.  RE: support for "jumbo" frames in Aruba CX

    MVP GURU
    Posted Jan 06, 2021 09:44 AM
    Hi Vincent,

    Thanks for clarifying.

    One more question just to be sure enough, let me consider a simple scenario: having one or more physical interfaces, non routed, operating in access (or trunk) mode on various VLANs AND having the IP Routing enabled (with some SVIs defined); does it mean - as I believe (I did it that way) - that MTU value should be specified (via the mtu command) both at each physical interface context level AND also (via the ip mtu command) at each VLAN interface context level OR specifying it at VLAN interface context level is enough?

    Thanks!


    ------------------------------
    Davide Poletto
    ------------------------------



  • 7.  RE: support for "jumbo" frames in Aruba CX

    EMPLOYEE
    Posted Jan 06, 2021 04:04 PM
    Hi Davide,
    Both MTU on physical interface context and IP MTU on SVI must be specified to guarantee that packet will get routed for the received MTU size.
    If SVI MTU is not specified, default MTU (1500 byte) applies and if packet is received with IP packet > 1500 byte, it won't get routed.
    Hope this clarifies.

    ------------------------------
    Vincent Giles
    ------------------------------



  • 8.  RE: support for "jumbo" frames in Aruba CX

    MVP GURU
    Posted Jan 07, 2021 06:08 PM
    Hi Vincent, Thanks a lot. Perfectly clear. Davide.

    ------------------------------
    Davide Poletto
    ------------------------------



  • 9.  RE: support for "jumbo" frames in Aruba CX

    MVP
    Posted Jan 24, 2021 09:32 AM
    Hey gang,

    Just to further clarify..  

    Let's say I have 8325 with VLAN 200 for voice, 210 for WiFi, and 220 for Cameras.
    200 is 10.200.0.1/24
    210 is 10.210.0.1/24
    220 is 10.220.0.1/24

    I have SVI MTU for 210 at ~2000 (or whatever it should be for the APs to encapsulate the stuff).
    I have port 1-10 all set for trunks, MTU at ~2000, and VLANs 200,210,220 are allowed.

    Now let's say there are 3 6200 connected to port 1/2/3.
    Ports 1-5 are access vlan 210, and each port is set with mtu 2000
    Ports 6-10 are set for 802.1x for clearpass auth and are set with default mtu
    Ports 11-48 are going to be 'colorless' ports.  Do I set the MTU on all those ports to 2000?  I'm guessing yes, since the local vlan on the switch might change depending on what device is connected.  But what happens if a camera device gets vlan 220 at 2000 MTU on the access switch, and then the larger packet gets sent up to the core where it's only set to 1500?  Does the packet get dropped only if it's supposed to be routed outside of that vlan (220)?  Since the port is set for 2000MTU but the VLAN isn't, I just wonder what might happen?

    Keeping straight the SVI vs port MTU is going to be a real treat in our environment.  We're accustomed to the 2930/3100 vlan jumbo and done command to keep the MTU consistent across campus.  I can imagine that it would get very tedious for a larger campus.

    ------------------------------
    Phillip Horn
    ------------------------------



  • 10.  RE: support for "jumbo" frames in Aruba CX

    MVP GURU
    Posted Jan 26, 2021 07:05 AM
    Yes, the packet will be dropped only if routed oustide
    and if you have only one SVI with MTU 2000 is not really needed... (to configure the MTU on the SVI...)

    ------------------------------
    PowerArubaSW : Powershell Module to use Aruba Switch API for Vlan, VlanPorts, LACP, LLDP...

    PowerArubaCP: Powershell Module to use ClearPass API (create NAD, Guest...)

    PowerArubaCX: Powershell Module to use ArubaCX API (get interface/vlan/ports info)..

    ACEP / ACMX #107 / ACDX #1281
    ------------------------------



  • 11.  RE: support for "jumbo" frames in Aruba CX

    Posted Jan 04, 2021 01:46 PM
    Thank you

    ------------------------------
    Ahmad Enaya
    ------------------------------



  • 12.  RE: support for "jumbo" frames in Aruba CX

    Posted Jul 24, 2022 06:42 AM

    If we were to follow the analogy above,

    should we also differentiate the MTU vs. IP MTU ?,  in CX.

    In the AOS,(eg.2930), the sh jumbos states:

    • sh jumbos
    • Jumbos Global Values
    • Configured : Max Frame Size :  9216      IP-MTU :  9198
    • In Use     :      Max Frame Size :  9216      IP-MTU :  9198

    Indicate a smaller difference of 18 bytes.

    Hence, the fact there are no such command/setting in CX,

    and settings is related to the physical interface[L2] and SVI[L3] ?

    The CX states on a Physical interface:

    SW(config-if)# mtu

    • BYTES The MTU value in bytes in the range <46-9198> (Default: 1500)

     

    The CX states on a SVI:

    SW(config-if-vlan)# ip mtu

    • <68-9198> The IP payload MTU value in bytes (Default: 1500)

     

    Anyone, please elaborate in the variance in the MTU/IP MTU

    handling in the two OS.

    (Seeing that there is no Max Frame Size setting in CX,

    Although and the two "settings" MTU vs. IP MTU states same max-values,

    but a minimum value at a smaller difference of 22 bytes)




  • 13.  RE: support for "jumbo" frames in Aruba CX

    Posted Apr 04, 2023 02:35 PM

    The ethernet frame header is 18 bytes, or 22 if single-tagged.

    46 is actually defined in the ethernet standard as the minimum size of the payload of an ethernet frame, with the minimum size of the whole frame including a minimal non-tagged ethernet header being 46 + 18 = 64.

    These days without old stuff like DECNet and AppleTalk running around, SVIs are being used only for IP.  My bet is that the 68 represents the minimum MTU needed for IP (60 byte maximum IP header size must be tolerated, and minimum 8 bytes in a fragment, per RFC 791)  You'll never use an MTU that low, really, so I would not let it bother you.

    FWIW I did some tests on an 8360, between two L2 connected clients on the same switch.

    With the MTU set to 2000, it let packets up to 2004 bytes (including the ICMP header) through if the client was on an access/native VLAN and 2002 through if the client was on a tagged VLAN.  It would seem that there is some discrepancy there probably due to some hardware handling of the tagging, or perhaps to provide wiggle room for something.

    Really, properly handling MTUs is mostly the clients' OSes' responsibility, one that not all OSes take seriously.  The network gear can help by forging fragmentation-needed ICMP responses back to the clients to aid in path MTU discovery when it drops a packet, but that's all it can do.  If the network gear cannot crank back ICMP responses, the best bet is to just let it forward the largest packets it can.  If it can send back ICMP, then use it to fence in any lower-MTU gear that cannot.

    If path MTU discovery (and MSS for TCP) is broken, the only fix is to set all clients to the lowest common denominator... which should be done in the first place, to prevent performance impacts of fruitless MTU discovery.  This CAN in fact be done in the routing table on some clients so the MTU can be set differently for different destination nets.




  • 14.  RE: support for "jumbo" frames in Aruba CX

    Posted Apr 09, 2023 11:56 AM

    When you have a no routed interface: no routing, the ip mtu xxxx, is obsolite,
    and the command return: Interface 1/1/1 is not L3.
    Only when routing is enabled, the command work.



    ------------------------------
    Steinar
    ------------------------------