Why can't I use the same encapsulation tag on different subinterfaces?

Unanswered Question
Feb 28th, 2007

This is on a 6509/Sup720 IOS 12.2(18)SXF5 and 12.2(18)SXE, and a WS-X6548-GE-TX and a WS-X6724-SFP blade.

I'm trying to create two sub-interfaces on each port, tag one for a data vlan and one for a voice vlan. Each port will then connect to a 3750 switch where IP phones and PC's will be connected to the same port. This is the typical IP phone/PC architecture. I was hoping to use the same vlan numbers on each switch so we don't have to keep track of which vlan is which switch - this works in the layer 2 world because the vlan stops at the layer 3 router port.

Well on the 6509 port this is what I have for the first port:

interface GigabitEthernet2/14

description to SW-232.15

no ip address

no ip route-cache

speed 1000

duplex full

mls qos trust dscp

spanning-tree portfast

!

interface GigabitEthernet2/14.10

encapsulation dot1Q 10

ip address 162.*.*.1 255.255.255.128 (the data vlan)

no ip route-cache

!

interface GigabitEthernet2/14.910 (the voice vlan)

encapsulation dot1Q 910

ip address 172.*.*.1 255.255.255.128

no ip route-cache

Now when I go to the next interface and do the same thing this is the error msg I get:

ENGLAB-6513-Native(config-if)#int g2/16.10

ENGLAB-6513-Native(config-subif)#encapsulation dot1Q 10

Command rejected: VLAN 10 not available

I can put some other dot1q tag on here but not 10 which I've already used.

Aren't these dot1q tags used only between the interface and the layer 2 switch connected to it? Aren't the tags stripped off when packets switch from one interface to another?

If so shouldn't I be able to use the same dot1q tag on different sub-interfaces?

Or am I not understanding things correctly?

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Loading.
swyka Wed, 02/28/2007 - 13:52

I have set up 3750s connecting to 6509s with IPT before & usually Dot1q trunk to the 3750 with the 6509 as a VTP server & the 3750s are VTP clients. Then on the 3750 ports, use the following:

switchport access vlan 10

switchport voice vlan 910

Here's a sample from one of my old production configs:

6509 Vlans:

!

interface Vlan103

description 3rd floor

ip address 10.0.x.x 255.255.248.0

ip helper-address 10.x.x.x

ip helper-address 10.x.x.x

no ip redirects

ip route-cache flow

end

!

!

interface Vlan186

description 3rd Floor VOICE

ip address 10.x.x.2 255.255.254.0

ip helper-address 10.x.x.x

ip helper-address 10.x.x.x

no ip redirects

ip route-cache flow

no ip route-cache cef

end

6509 to 3750:

(6509 port)

!

interface GigabitEthernet3/7

description UPLINK.TO.CLOSET.3C - G1/0/1

switchport

switchport access vlan 999

switchport trunk encapsulation dot1q

switchport trunk native vlan 999

switchport mode trunk

spanning-tree link-type point-to-point

spanning-tree port-priority 32

end

(3750 port)

!

interface GigabitEthernet1/0/1

description UPLINK.TO.6509-1 - G3/7

switchport trunk encapsulation dot1q

switchport trunk native vlan 999

switchport mode trunk

srr-queue bandwidth share 10 10 60 20

srr-queue bandwidth shape 10 0 0 0

queue-set 2

mls qos trust cos

auto qos voip trust

end

All other 3750 ports:

!

interface FastEthernetx/x/x

switchport access vlan 103

switchport voice vlan 186

srr-queue bandwidth share 10 10 60 20

srr-queue bandwidth shape 10 0 0 0

mls qos trust cos

auto qos voip trust

spanning-tree portfast

!

LMK if this helps.

jkeeffe Thu, 03/01/2007 - 07:15

Thanks for the information - I'll give it a try and let you know how it works for me.

Actions

This Discussion