VLAN on Subinterfaces on a 2621 Router

Unanswered Question
Oct 30th, 2007

I have a 100Mb leased line with a 2621 router at each end. I want to span vlan's across this line and have created subinterfaces on the fa0/0 interface on each as follows:

interface FastEthernet0/0

no ip address

speed 100



interface FastEthernet0/0.50

encapsulation dot1Q 50

ip address


interface FastEthernet0/0.254

encapsulation dot1Q 254 native

ip address


interface FastEthernet0/1

ip address

speed 100



The config is the same at both side, but with a different IP address in the same subnet range.

I can now ping between the ip addresses associated with the subinterfaces on the native vlan, but I cannot ping between those non-native vlans.

Any ideas?

Thanks in advance.


I have this problem too.
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Paolo Bevilacqua Tue, 10/30/2007 - 14:12


you need to ask your metro SP to carry 802.1q. They need to support what is called "Q-in-Q".

Hope this helps, please rate post if it does!

campbell.thomps... Wed, 10/31/2007 - 03:25

I got excited about that, but alas no, I spoke to the SP and it's fibre with little in the way of hardware in between.

I'm at a loss...

Paolo Bevilacqua Wed, 10/31/2007 - 04:58

Do they only have media converters to present you the circuit over copper? Positive not even a switch in between ?

Easy check to confirm switch presence is that if you unplug one side the other stays up.

campbell.thomps... Wed, 10/31/2007 - 05:12

Yes, just media converters.

Is there anyway I can debug at packet level to make sure the reference is the same at both sides?

Paolo Bevilacqua Wed, 10/31/2007 - 07:02


this is supposed to work as configured. You can attached a to PC with analyzer to confirm the router is sending the correct encap. Note you need a NIC driver with support for 802.1q to see tagged frames on the PC (not all NICs are like that).

campbell.thomps... Wed, 10/31/2007 - 07:05

Okay, managed to get it working, but I needed to change the encapsulation on the outside port on each router from dot1q to isl... Very weird...

Any thoughts on why this would work?



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