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I'm usually against tagging a VLAN across a router, as to not incur issues with spanning tree.

My company is asking the following

PLC --Vlan72-- SwitchA --RouterA---CoreRouter---RouterB---DataCenterSwitch--Vlan72--FailoverServer

I'm asking to add an IP to this vlan and have it defined at RouterA, and use a different Vlan and subnet for the failover server in the datacenter. However, these PLC's don't have a mechanic for failover, and they want to Re-IP the failover server incase the primary goes down, which is located on SwitchA.

as a tidbit, i'm using ospf on my routers.

Is there something else I can provide to help them reach a solution? I don't want to stonewall them with not tagging a vlan across our cores.

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    Dec 23, 2021 at 16:26

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You cannot "tag a VLAN across a router" because a VLAN is a layer-2 construct and a router only forwards layer-3 packets.

If you use a specific VLAN ID on different sides of a router, those are different VLANs still. Of course, you can use them like that (I'd advise against it though, for human transparency), but don't expect any layer-2 connectivity between those VLANs.

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  • Sorry I guess I should specify, that they are routing switches, so they can perform both the operation of a router and a switch(specifically cisco 9600). The previous admins have vlans defined on the routing switches and tagged on various ports, including interfaces to other routing switches. I'm trying to move away from that.
    – user3524785
    Feb 16, 2021 at 12:56
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Is there something else I can provide to help them reach a solution? I don't want to stonewall them with not tagging a vlan across our cores.

There are other technologies, such as Pseudo-Wires (PWs). This allows you to extend Ethernet as if it is (mostly) a point-to-point circuit through an MPLS core, but without having to configure related VLANs on any of the core devices.

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