ATM sub-if and VLAN-ID

[Luca CQ]

Hi all

Cisco 7600 - Version 15.3(3)S5

i have an ATM configured with subinterface, with internal VLAN assigned by IOS:

interface ATM 1/0/0.500 point-to-point
 ip vrf forwarding VPN_A
 ip address 10.1.1.1  255.255.255.252
 pvc 100/500
 class-vc XDSL
 ...
!
end

# sh vlan internal usage | i .500

VLAN Usage
---- --------------------
1210     ATM 1/0/0.500

Question is:
may i use the vlan id 500 for other subinterface?
as follows:

interface GigabitEthernet 1/1.500
 encapsulation dot1Q 500
 ip vrf forwarding VPN_B
 ip address 10.1.1.1 255.255.255.248
..

end

Thanks anyone who will reply.

[Reza Sharifi]

Hi,

I don't have 7600 to test with, but I think since you are putting them in separate VRFs, it should work just fine.

HTH

[MHM Cisco World]

as I know the VRF is work in L3 Layer 
VLAN tag is work in L2 layer 
so assign two subinterface with same L2 subinterface is not work. 

BUT let me check, I will lab add two vrf and I will config two interface with same tag one for each vrf. 

[Luca CQ]

@Reza Sharifi @MHM Cisco World thanks for your replies (and thank you MHM for try this in your LAB if you can).

Actually I think the the point, besides the 2 different VRFs, could be that the ATM is only L3 and so the vlan tag used is the one assigned automatically by IOS and reported in the "show vlan internal usage" ( =1210) and not the one that match the subif (=500).
And this is why I suspect (hope, really :)) that I can use the vlan tag 500 for another interface (g1/1.500).
Otherwise i will totally agree with this: "assign two subinterface with same L2 subinterface is not work. "

I will appreciate any lab test if you can or any other point of view about that.
Thanks,
LCQ

[mlund]

Yes, you can reuse the subinterface number. Because this is just a number and have nothing to do with vlan numbers.

As you can see in a ethernet interface with subinterface number, the vlan that is used is the number in the encapsulation command. These two (subinterface/encapsulation) doesn't need to be the same. In your example the gig1/1.500 have encap 500, this reserves vlan 500 so it can not be used somewhere else. Same thing if you have g1/1.500 encap 600 would reserv vlan 600.

For atm interfaces there is a difference, because you don't specify encapsulation, therefore ios reserv a vlan for internal use. So in your case vlan 1210 is reserved and can not be used somewhere else.