I want to known what is native vlan, and when do i using it ???
Native VLAN is relevant only when there is 802.1q trunking between switches or a switch and a router ( in case of router on stick). Native VLAN has no significance when ISL trunking is used.
In 802.1q trunking each frame is being tagged by 4 bytes field in the ethernet header and before IP header (assume IP traffic). The 4 bytes tag field contains vlan number (12 bit), 3 bit field for layer 2 QoS, TPID (16 bits) field in order to identified the tagged frame(this is set to 0x8100 for 802.1q) etc.
The native VLAN does not have the 4 bytes tag in the frame. When one switch sends native vlan traffic to the other switch over the 802.1q trunk, the receving switch must also have same native vlan configure in order to accept the native vlan from other switch. If the receiving switch has different native vlan than what it is receiving in that case this switch may leak the received native vlan traffic in to its own native valn traffic. Switches have protection mechanisms in order to prevent this to happen by throughing error messages saying "native vlan mismatch" these error came out because negotiation of native vlan between the switches is a part of DTP and CDP. If these two protocols are disabled this error message will not appear but traffic can still leak.
By default VLAN 1 is the native VLAN in CISCO switches and can be configured to any vlan as native. It is normally used for management traffic.
Hope this may help
It is Good Explanation by Deepak Sharma. So in summary, Native VLAN frames are sent on a 802.1Q trunk as untagged, and when untagged frames are received on the trunk port they are assumed to belong to the Native VLAN. Native VLAN can be used in the following cases:-
On the other hand, if a device on the path does not understand untagged frames, Native VLAN should be disabled. To do so, use the VLAN dot1q tag native global command which will affect all trunk ports and cause Native VLAN frames to be tagged.
Another point to note is that this command causes all native VLAN traffic to be dot1q tagged at egress with the appropriate VLAN ID in the dot1q tag. However the switch can receive the native VLAN either dot1q tagged or untagged.
This command is all or nothing on the Catalyst 3560 - however on some other platforms (Cat 6500) you can enable globally and then disable on a per port basis with no switchport trunk dot1q native vlan tag
Important point to note though a carefully designed working network won't face this situation.