Native VLANs and subinterfaces

[dbarasch]

What does a native VLAN mean? Cisco documentation states that if you have VLAN1, then that is the native VLAN. What constitutes something as being native. Suppose I didn't have VLAN1, but have VLANs 4,5,6. What would make one native as opposed to the other VLANs.

Second question... If I have two IPs on a single NIC on a router and I want to create subinterfaces on it, am I assigning new IPs on the subinterfaces or using the IPs I already assigned to the NIC.

Example:
Interface Fa0/1 has 192.168.13.2 and 192.168.11.2 (secondary).

Now I am creating two subinterfaces on Fa0/1. Should Fa0/1.1 have 192.168.13.2 or a different IP on the same subnet. Likewise with Fa0/1.2 should it have 192.168.11.2 or 192.168.11.5.

 

[burtsbees]

On int fa0/1...
router(config-if)#no ip add
router(config-if)#no shut
The IP add goes on the subinterfaces...
The native VLAN is VLAN1, or the default vlan, the vlan that controls everything, or the management vlan. It is the one you assign an IP address to, and then you can reach the switch via browser or telnet (considering there is a password set on the vty lines...).

Burt

 

[ADB100]

The Native VLAN on a 802.1q trunk is the VLAN that traffic arriving without a VLAN Tag is handled by. For example if you have a trunk that allows VLAN's 4-8 and the Native VLAN is the default (1). Traffic arriving on the trunk without a VLAN Tag is handled as if it arrived with a Tag of VLAN 1. In this case since VLAN 1 wasn't in the list of allowed VLAN's then it should be dropped. However Cisco ALWAYS handle certain traffic with a VLAN 1 Tag (control-plane traffic - CDP, DTP, VTP etc).

There are certain security issues with not tuning Trunk interfaces Native VLANs so you should always ensure you use unique Native VLANs on each point-to-point trunk.

Using VLAN 1 for data and leaving this as the default on Trunks is a major sin in Cisco's eyes........

HTH

Andy

 

[ADB100]

With the subinterfaces you must tell the router what the VLAN Tag is:

interface FastEthernet0/1.4
 encapsulation dot1q 4
 ip address x.x.x.x x.x.x.x
!
interface FastEthernet0/1.5
 encapsulation dot1q 5
 ip address x.x.x.x x.x.x.x
!
interface FastEthernet0/1.6
 encapsulation dot1q 6
 ip address x.x.x.x x.x.x.x
!

The switch this connects to should be hard-coded as a 802.1q trunk and allow VLANs 4-6. By default the Native VLAN will be 1, however you should change this to be some unused VLAN and define this at either end:

switch:

interface FastEthernet0/1
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 999
 switchport trunk allowed vlan 4-5
 switchport mode trunk
 switchport nonegotiate 
!

router:

interface FastEthernet0/1.99
 encapsulation dot1q 999 native

HTH

Andy