is an ip address needed on an encapsulated interface

[mmercaldieze]

I am configuring a point to point switch for a client that is not really technical so I am trying to keep it simple and setup a few static routes rather than ospf and eigrp.  

The setup is the following:

4300<--P2P-->4300<--Trunk-->3850

I am configuring a trunk containing the only 2 vlans on the 3850 to the 4300 router.  When creating the sub interfaces on the 4300, will I need to setup an ip address on the encapsulated interface or can I just do enc dot1q 4 and not assign an ip address?  I just want to make sure the 4300 knows about vlan 4  when it receives requests from the other 4300?

[Masoud Pourshabanian]

Hello,

Int fa0/0

no ip address [IP address for VLAN1 if used]

int fa0/0.10

encapsulation dot1q 10

ip address

int fa0/0.20

encapsulation dot1q 20

ip address

In this example I supposed you have VLAN 10 and 20.

In you have VLAN1 which is native VLAN, you need to set IP address on main interface

Masoud

[mmercaldieze]

Thanks

So in your example, if I setup an ip on fa0/0.10 but not on fa0/0.20, will .20 use the native vlan ip?

[Masoud Pourshabanian]

Let me explain a little more.

By default, native vlan is 1. Switch does not add any tag to native VLAN. If a frame from VLAN 1 goes on the trunk interface, switch does not add any tag so router receive a frame with no tag. Router redirects it to main interface if main interface has IP address.

If you change native VLAN on trunk for example to VLAN 20, a frame generated from VLAN 20 goes on trunk. Switch does not add any tag since native is set to 20. Router receive a frame with no tag, so it redirects it to the main interface with IP.

I hope I understood your question

Masoud

[Masoud Pourshabanian]

Just adding to my previous comment.

int fa0/0.20

encapsulation dot1q 20

fa0/0.20. 20 here is not important. You can choose any numbers. 20 in encapsulation dot1q 20 command is important

Masoud

[Masoud Pourshabanian]

" if I setup an ip on fa0/0.10 but not on fa0/0.20, will .20 use the native vlan ip"

I am sorry, I did not understand the bold part of your question. But generally, you do not have to set IP address on your main interface. SVIs on switch and subinterfaces on router will still communticate.

Masoud

[Jon Marshall]

You need an IP for each subinterface even if the subinterface is using the native vlan.

This is assuming you are routing the vlans on the 4300 although why you are doing that is not clear.

A more common scenario would be to use SVIs on the 3850 for routing between vlans and then have a L3 routed P2P link from the 3850 to the 4300.

Then just configure statics on all your L3 devices if you don't want to run an IGP although again I'm not sure why statics make it simpler ie. configuring EIGRP for example requires very little configuration.

If the customer then wants to add another vlan to the 3850 they only have to update the 3850 configuration whereas with your solution they need to upate the 3850 with the vlan information, add a subinterface to the 4300 and add a static to the other 4300.

Jon