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Say you have a L3 switch routing between VLANs 10, 20, and 30, and you need to uplink the switch to a router for the VLANs to get to the rest of the network. The VLANs don't traverse between sites. I know if you had a Cisco switch you would do no switchport on the switch port that connects to the router, essentially making a routed interface. The switches I'm using don't have that option, so I figured I would create an access port on the switch assigned to its own VLAN 100 with a virtual routing interface. I figured this would kind of keep the traffic on the uplink limited to traffic coming in and out of the branch and free from what's bouncing around between VLANs 10, 20, and 30.

Does this make sense? Is there a name for this? Or is there a better practice?

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  • 1
    What is the switch model?
    – Ron Maupin
    Oct 17 '18 at 1:56
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin
    Dec 31 '20 at 4:45
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Let us know what model of cisco layer3 your are using in your configuration . Basically cisco catalyst layer3 switch interface are in "Switchport" mode to make interface in router mode #no Switchport mode " is used

If using catalyst cisco layer3 switch

But in your scenario switch interface is already in Switchport by default . Configure trunk link between router and switch and configure trunk port allowing all Vlans on layer3 . It's will resolve your issue .

If you want to route traffic between router and switch by connecting router to switch L3 interface than in this scenario "no Switchport mode"command is used to make interface port as Router. This command should work on cisco catalyst layer3 switches . If not working means check with vendor.

If Cisco Nexus layer3 switch is used

Basically all interface ports are in "routed mode " to make them Switchport mode "Switchport mode" command is used .

It entirely depends upon what model switch your are using in setup.

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You can very well use a layer-3 switch as inter-VLAN router and still run a VLAN trunk to an external router to connect the router directly to each VLAN for e.g. Internet access or perhaps route into some more subnets.

Whether the L3 switch or the router is used depends on the routing table. When the default gateway of each attached hosts points to the L3 switch it'll always be used unless there's a "better" = more specific route to the external router.

However, using the L3 switch as default gateway for all hosts and setting up more specific routes only there shouldn't have any negative impact as most switches route with the same speed as they switch and you'd need to switch anyway.

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  • In this case we are not trunking anything between the switch and router since we don't manage the router. The question is whether the access port on its own vlan is the best way to simulate a layer 3 uplink.
    – micro
    Oct 17 '18 at 10:57
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    From the outside, there's no difference between an L3 "routed" port and a switch port (group) with a VLAN and a routed SVI interface.
    – Zac67
    Oct 17 '18 at 11:15
  • Using a "external router VLAN" keeps the inter-VLAN routing decision on the L3 switch; as opposed to "trunk to router" which potential gives the choice to router. Which is better depends on your other factors.
    – jonathanjo
    Oct 17 '18 at 12:43

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