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inter vlan routing

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  • 1.  inter vlan routing

    Posted Oct 05, 2021 10:56 AM
    Hello everyone,

    I joined a team where the network is set up by HP 1920s, 2920 and 5406Rzl2

    I have read that once you add a vlan to 1920s an SVI is added automaticaly and routing between vlans is on. If that the case why HP is offering a dedicated menu for routing on the GUI ?

    on the other hand we have got routing on 2920 but how 1920 get the packet destined for another vlan to the uplink

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    otman ibrak
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  • 2.  RE: inter vlan routing

    Posted Oct 05, 2021 04:35 PM
    Hello Otman,

    "I have read that once you add a vlan to 1920s an SVI is added automaticaly and routing between vlans is on."

    It's not correct: on HPE OfficeConnect 1920S you first need to define a VLAN and then (eventually, it is neither mandatory nor necessary if you don't need it) you can define a VLAN Interface (SVI) assigning to that VLAN Id an IP Address and IP Routing then need to be enabled globally IIRC (see manual here). Once a VLAN Id has an IP and IP Routing is globally enabled, that SVI is routed by the Switch.

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    Davide Poletto
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  • 3.  RE: inter vlan routing

    Posted Oct 06, 2021 09:42 AM
    Hello Davide,

    Below, the config we have on all 1920s. Routing is disabled but VLANs are there and the 1920s are uplinked to 2920 where I can see the SVI via command line. Thus, what baffles me is how the 1920S route a packet destined to a different vlan than the PVID.

    Thanks 



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    otman ibrak
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  • 4.  RE: inter vlan routing

    Posted Oct 11, 2021 05:29 AM
    If you have multiple switches, in most cases you have access switches that don't do any routing, are just L2. Then you have aggregation/distribution/core switches that handle the L3. If your 1920 provides L2 connectivity, it can be that the actual routing happens on the 2920 or 5400. It's not so common to do routing on each of your switches, but instead, just use selected switches.

    ------------------------------
    Herman Robers
    ------------------------
    If you have urgent issues, always contact your Aruba partner, distributor, or Aruba TAC Support. Check https://www.arubanetworks.com/support-services/contact-support/ for how to contact Aruba TAC. Any opinions expressed here are solely my own and not necessarily that of Hewlett Packard Enterprise or Aruba Networks.

    In case your problem is solved, please invest the time to post a follow-up with the information on how you solved it. Others can benefit from that.
    ------------------------------



  • 5.  RE: inter vlan routing

    Posted Oct 11, 2021 07:03 AM
    Hello Herman,

    Thanks a lot. IF a 1920 has a port1 assigned to vlan X and port 2 to vlan Y and a packet from port1 destined for port 2, the packet has to be forwarded to 2920 where SVI resides in order to be routed to VLAN Y. Is that right ?

    Thanks

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    otman ibrak
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  • 6.  RE: inter vlan routing

    Posted Oct 11, 2021 07:34 AM
    If the 2920 is the device that has the IP interfaces, is enabled for ip routing, and the clients are using those interface IPs for routing/as their default gateway, then yes. And what you need to do on the 1920 is, as you mentioned, have the client ports assigned to the correct VLANs, and have an uplink with both VLANs to the 2920, then packets are switched within the VLAN from client to the uplink port, then routed on the 2920 and the routed packet is then switched again back to the 1920 where it is switched to the client.

    This is pretty basic networking principles, with L2 and L3.

    ------------------------------
    Herman Robers
    ------------------------
    If you have urgent issues, always contact your Aruba partner, distributor, or Aruba TAC Support. Check https://www.arubanetworks.com/support-services/contact-support/ for how to contact Aruba TAC. Any opinions expressed here are solely my own and not necessarily that of Hewlett Packard Enterprise or Aruba Networks.

    In case your problem is solved, please invest the time to post a follow-up with the information on how you solved it. Others can benefit from that.
    ------------------------------



  • 7.  RE: inter vlan routing

    Posted Oct 11, 2021 08:02 AM
    Yes , but what pushed me to ask such question is why not tap into 1920s L3 capability and lower the latency or why not implement Purely L2 switches at access layer and lower the cost ? both questions in fact are not more for you than the team who implemented it.
    My question for you, please, as I know SVI has its own forwarding table too, if I enable the routing on 1920s, SVI will be checking its table to match the MAC and IP in the header but it is not found, will it be flooding the network with ARP request ?

    Thanks

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    otman ibrak
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  • 8.  RE: inter vlan routing

    Posted Oct 11, 2021 09:16 AM
    The switch will only route the packet if the destination MAC address is it's own SVI. If your 2920/5400 is your L3 device, and clients are configured to use that as default gateway, the 1920 will not perform routing/L3, just switching L2 as the destination MAC address is the SVI/L3 of the 2920/5400.

    There are very few cases where an additional L2 hop adds a significant amount of latency to experience performance degradation, assumed there is no bottleneck on the uplink port, but that is more a design excercise.

    ------------------------------
    Herman Robers
    ------------------------
    If you have urgent issues, always contact your Aruba partner, distributor, or Aruba TAC Support. Check https://www.arubanetworks.com/support-services/contact-support/ for how to contact Aruba TAC. Any opinions expressed here are solely my own and not necessarily that of Hewlett Packard Enterprise or Aruba Networks.

    In case your problem is solved, please invest the time to post a follow-up with the information on how you solved it. Others can benefit from that.
    ------------------------------