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I have a theoretical question about the loopback implementation on a router.

I understand that the idea of a loopback interface is that it is a virtual interface which never goes down due to a physical line or network problem. Loopback interfaces can be used for testing. I also understand that Loopbacks are often used as the source address for various services (NTP, TFTP, FTP, SSH, TACACS, domain lookup, etc.). These services would, by default, use the IP address of the interface by which they exit the router.

What happens if the routers in the networks are trunked and vlan tagged? Is it then still possible to use the loopback interface to perform domain lookup, ftp or curl?

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  • Remember that routers really do not understand VLANs or trunks, and they will have a virtual interface for each VLAN. Those virtual interfaces are treated like physical interfaces from the perspective of the router, so there is no difference.
    – Ron Maupin ♦
    Oct 3 '20 at 14:08
  • No, the packets simply follow the routing table, either through a physical or virtual interface. There is no difference in how a router operates because a router does not know about VLANs (a layer-2 construct), it only knows networks (layer-3).
    – Ron Maupin ♦
    Oct 3 '20 at 14:13
  • Routers route packets based on the routing table. They have no idea bout VLANs. Each VLAN will have a virtual interface with an assigned address and mask that the router uses to determine the directly attached network for that interface. Routers inherently know about directly attached networks, but must be told (either static configuration or through a routing protocol) about other networks.
    – Ron Maupin ♦
    Oct 3 '20 at 14:21
  • That depends on if the router has knowledge of how to reach the network where the DNS server is located. That has nothing to do with VLANs or not. Either the router knows how to reach the destination network and sends the traffic that way, or it does not know how to reach the destination network and it drops the packets.
    – Ron Maupin ♦
    Oct 3 '20 at 14:29
  • You completely changed the question to mean something different than the original question for which you already got an answer. That is not how this is supposed to work. For a new question, simply start a new question. Do not keep changing the original question to ask different questions.
    – Ron Maupin ♦
    Oct 11 '20 at 14:41
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What happens if the routers in the networks are trunked and vlan tagged? Is it then still possible to use the loopback interface to perform domain lookup, ftp or curl?

VLAN tagging or LAG trunking are options for physical interfaces. You can't tag or LAG a router itself (or any other device).

You need to use an interface address to send anything to a device. Since any interface can be down in case of failure, a virtual loopback interface can be created and used. The loopback is always up and can always be sent to as long as there's a path to the device. Also, it's address doesn't need to change when you renumber networks and interfaces, so it can be used as a constant reference to the device.

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What happens if the routers in the networks are trunked and vlan tagged? Is it then still possible to use the loopback interface to perform domain lookup, ftp or curl

Loopback interface does not have any concern with physical interface loopback interface is virtual interface created in layer2 and devices for management access of switch , for taking backup , restoration of backup,etc

But in router or layer3 devices SVI are configured switch virtual interface each svi are virtual interface and tagged with vlan . But for trunking physical interface is considered.

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