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  • 1.  Why is there a VLAN field in show ip route command ?

    Posted Mar 22, 2025 08:21 AM

    Consider the output below. It might look like a silly question, but why is there a VLAN field in the output of "show ip route" command ? 
    Unless I'm mistaken, VLANs are purely layer 2, so why show that field here ? VLAN 4001 is our management VLAN, why is it cited for all routes ?

    Switch# sh ip route                                                                                   
      Destination                 Gateway                VLAN    Type     Sub-Type      Metric     Dist.
      ------------------ --------------- ---- --------- ---------- ---------- ----------------
      0.0.0.0/0                     10.255.255.254       4001     ospf      External2      10           110
      10.100.0.0/24             Management           4001     connected                   1            0
      10.100.2.0/24             10.255.255.254       4001     ospf      IntraArea      101         110
      10.100.10.0/24           10.255.255.200       4001     ospf      IntraArea      101         110
      10.100.10.0/24           10.255.255.195       4001     ospf      IntraArea      101         110
      10.200.0.0/16              10.255.255.254      4001     ospf      External2      10           110
      10.111.100.0/24          10.255.255.200      4001     ospf      IntraArea      101         110    



  • 2.  RE: Why is there a VLAN field in show ip route command ?
    Best Answer

    Posted Mar 23, 2025 12:37 AM
    Edited by Alan Mar 24, 2025 10:21 AM

    Usually on Layer 3 switches, VLANs are used as SVI (Switch Virtual Interface). Those interfaces are logical interfaces tied to a VLAN which have an IP Address assigned. Those interfaces are acting as routed interfaces on the switch. In your case, the next hop for that route is the SVI Interface for VLAN 4001 and all OSPF communications and routing decisions are happening over that SVI. 

    According to your routing table, VLAN 4001 is the only Layer 3 interface participating in routing on that certain switch, or at least it is the interface used to reach all next hops in that table. So that means all the OSPF routes are learned through VLAN 4001 in that case. The SVI management which you have configured is doing the routing and not any other Layer-3 interface.  So in your case, even though you are calling it the Management VLAN, you are using it for actual routing, which is fine if configured intentionally, but it does mean that any Layer 3 routing traffic will be seen on that VLAN as well.



    ------------------------------
    Shpat | ACEP | ACMP | ACCP | ACDP
    Just an Aruba enthusiast and contributor by cases
    If you find my comment helpful, KUDOS are appreciated.
    ------------------------------

    Original Message


  • 3.  RE: Why is there a VLAN field in show ip route command ?

    Posted Mar 24, 2025 03:13 AM

    Hi,

    please do a " show ip "command and show the output here. That will make it clearer, as shpat already wrote. To me, it looks like the switch only has an IP address for the MGMT VLAN.

    But IP routing and OSPF are enabled here.

    Regarding the question: All displayed IP ranges are accessed via the GW 10.255.255.254 in VLAN 4001.

    Every Layer 3 needs a Layer 2 (and Layer 1)

    tom


    Original Message


  • 4.  RE: Why is there a VLAN field in show ip route command ?

    Posted Mar 24, 2025 10:28 AM

    Thank you for breaking it down clearly. On a side note, I do not believe it is intended design and it would likely need to be modified.


    Original Message


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