Native VLAN trunking

[cdcjim2877]

Hi,

If one assigns the native vlan to vlan100, it is my understanding that the following command is necessary to bring up communication between the switches (trunked ports):

switchport trunk native vlan 100

Is this correct?

I have a few additional questions too...

- Is it considered best practice to use a different VLAN instead of the default VLAN 1 for switch management?

- How can one get the switches to communicate over VLAN100 without the "switchport trunk native vlan 100" command?

Thanks,

J

[andrew.prince]

To answer your questions:-

No - trunks are not dependant on which vlan passes without a tag.

Yes it is good idea not to use the standard vlan 1 for management.

You create an svi interface that you want to use for management, say 42. As long as all other switches have an svi in that vlan, should be fine.

Do not forget - native vlan just means, packets that DO NOT HAVE a tag, with the VLAN ID.

HTH>

HTH>

[Jon Marshall]

Just to add to Andrew's post

"Is it considered best practice to use a different VLAN instead of the default VLAN 1 for switch management"

Yes it is and it also recommended that the vlan you choose should

a) not be the native vlan

b) not have user ports assigned to it.

"How can one get the switches to communicate over VLAN100 without the "switchport trunk native vlan 100" command? "

Without the switchport trunk native vlan 100 command vlan 100 would be sent across the trunk tagged with a vlan ID of 100. They would still communicate.

Also you do not need a L3 SVI for your native vlan. In fact it is better that you don't.

Jon

[cdcjim2877]

Jon,

"Also you do not need a L3 SVI for your native vlan. In fact it is better that you don't."

Why is this? Is it that L3 takes slightly longer to pass packets vs switched links? Your comment is exactly where I am stuck in udnerstanding the difference between L3 and L2; as it pertains to VLAN's.

Thank you for you answers. They are greatly appreciated.

Jim

[Jon Marshall]

Jim

Generally 1 vlan is associated with 1 ip subnet so for the purposes of this discussion we will assume that.

We have 2 vlans -

vlan 10 - 192.168.5.0/24

vlan 11 - 192.168.6.0/24

Now lets say we have a L3 switch and these 2 vlans exist at layer 2 only.

You allocate 3 machines into vlan 10 and give them an IP address out of 192.168.5.0/24

You allocate 3 machines into vlan 11 and you give them an IP address out of 192.168.6.0/24

The 3 machines in vlan 10 can all communicate with each other.

The 3 machines in vlan 11 can all communicate with each other.

But the machines in vlan 10 cannot communicate with the machines in vlan 11 and vice-versa. For that to happen you would need a L3 SVI for each vlan. So on the switch

int vlan 10

ip address 192.168.5.1 255.255.255.0

int vlan 11

ip address 192.168.6.1 255.255.255.0

Each machine in vlan 10 has it default-gateway set to 192.168.5.1 and each machine in vlan 11 to 192.168.6.1. Now these machines should be able to communicate with each other because the L3 switch can route between the vlans. It couldn't do this until you created the SVI's.

The reason you don't need a L3 SVI for the native vlan is because there should be no devices in that vlan that need to route. The native vlan is used purely across trunk links.

Hope that makes sense.

Jon

[cdcjim2877]

Jon,

Yes it makes very good sense. If more than one switch is present and communication is needed between them (the switches), aren't they in a sense, devices?

I think I am seeing the top of the hill...just need another push... :-)

So, in the case where two switches are connected (now trunking is introduced), both containing hosts in both vlans, how does one assign communication between the switches to manage the switch information?

I believe I am getting it mixed up between switch to switch information (and communication) and communication across the trunks for the hosts..are they one in the same?

thank you for your patience,

Jim

[Jon Marshall · Accepted solution]

Jim

Switch to switch communication would be the likes of

VTP - a way to automate the update of the vlan database on each switch

DTP - used for dynamically forming trunks

CDP - Cisco Discovery protocol, a way for Cisco devices to discover each other across links

PaGP/LACP - used for dynamically forming etherchannels

STP - Spannning Tree - used to avoid loops in the L2 network.

UDLD - used to avoid uni-directional links

All of the above are used by switches to build and maintain the L2 topology. They are nothing to do with user data as such. This switch communcation does cross trunks although the links do not necessarily need to be trunks for all of them, they could just be a connection that is in the same vlan at both ends.

Generally speaking switches are interconnected via trunks because you usually have more than one vlan you want to exist across switches.

User data also goes across the switch interconnects but as you say it is from an end host to another end host/server.

No problem if you have further questions.

Jon

[cdcjim2877]

Jon,

Eureka! I see it...

thanks,

Jim

[Jon Marshall]

Jim

Glad to be of help and appreciate the rating.

Jon