Confused over the "vlan dot1q tag native" command

From: YourPal (dearprudence28@gmail.com)
Date: Wed Jan 23 2008 - 07:10:12 ARST

Dear Group,

I have a scenario relating to the implication of "vlan dot1q tag native"
command. My simple setup is as follows:

SW3 (Gi0/19) -------------802.1Q------------- (Gi0/19) SW4

1. The native VLAN of the 802.1Q trunk between SW3 and SW4 is VLAN 1.
2. SW3 has dot1q native vlan tagging disabled by default. SW4 has it
explicitly enabled.
3. Question is, why SW3's Vlan1 SVI can still ping to SW4's Vlan1 SVI?

Please see outputs below:

SW3 

---
SW3#sh int tru

Port        Mode             Encapsulation  Status        Native vlan
Gi0/19      on               802.1q         trunking      1

SW3#sh vlan dot1q tag native
dot1q native vlan tagging is disabled

SW3#sh run int vl1
Building configuration...

Current configuration : 53 bytes
!
interface Vlan1
 ip address 1.1.1.3 255.0.0.0
end

SW3#pi 1.1.1.4

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.4, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

SW4
---
SW4#sh int tru

Port        Mode             Encapsulation  Status        Native vlan
Gi0/19      on               802.1q         trunking      1

SW4#sh vla dot tag native
dot1q native vlan tagging is enabled

SW4#sh run int vl1
Building configuration...

Current configuration : 53 bytes
!
interface Vlan1
 ip address 1.1.1.4 255.0.0.0
end

SW4#pi 1.1.1.3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 1.1.1.3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms

I was under the impression that SW4, with native vlan tagging enabled, will
drop any untagged frames that it receives on a 802.1Q trunk. In this case,
SW3 sends untagged frames (belonging to VLAN 1) to SW4. SW4 should be
expecting ingress VLAN 1 frames to be tagged. Hmm...I must be missing some
obvious points.

Kindly enlighten.

Thank you.

BR,
Emil

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