Subinterfaces

I don't know why, but I swear I have seen subinterfaces used with not only frame-relay but VLANs as well. Am I delusional, or am I missing something?

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EdTheLad

You would have seen subinterfaces used with router on a stick configuration.
The idea is on the switch you have multiple vlans domains, for these domains to communicate with each other you need a layer 3 device, a router.The router is connected to a trunk port on the switch.The router interface is divided into subinterfaces, the number depends on how many vlans you have.The subinterface will be encapsulated with the dot1q to have a vlan associcated with it.The reason is because the switch is sending tagged frames
out towards the router and the router needs the vlan id so it can read the frame.Also each subinterface will have an ip address which should be part of the same subnet as the hosts in the corresponding vlan.
So with this setup you can have hosts in different vlan domains routing between each other.

nuglobe

Then what is the point of VTP?

EdTheLad

VTP is a protocol to send vlan configuration between switches.If you have multiple switches connected together that share common vlans,instead of going to every switch and creating the vlans, you just need to configure one switch as the vtp server and the other switches as vtp clients.When you config a vlan on the server,this vlan info will be sent to the other clients.So it just saves you the effort of administrating the vlans.You will still need to assign specific ports to the vlans on the different switches.

nuglobe

Doh, I ment to say what is the point of trunking? Why do you need to set up subinterfaces for each vlan if you could just set up the physical interface as a trunk and skip logical interfaces all together?

Humper

If you want to communicate from one vlan to another (known as inter-vlan routing) you need a router to do this. trunking's job is to carry traffic for multiple vlans. A trunk cannot route, you need a layer 3 switch or router to perform this task....

You setup subinterfaces on the router, so it can pass traffic from one vlan to another through the routing table.

Cauthon

And each subinterface gets an IP that's part of the VLAN's subnet so that it can act as the default gateway for the VLAN.

Humper

Cauthon wrote:

And each subinterface gets an IP that's part of the VLAN's subnet so that it can act as the default gateway for the VLAN.

Yup :P

nuglobe

LoL yeah thanks. I figured it out on my way home today.

nuglobe

Alright, one last question.... Trunking has to be active on the physical link that these subinterfaces are created under correct?

wizarddeath

You set the port on the switch to trunk mode, if thats what you mean by active.(if not im way off base, sorry!)

nuglobe