[CentOS] VLAN issue

Sun Jan 25 16:42:54 UTC 2015
Les Mikesell <lesmikesell at gmail.com>

On Sun, Jan 25, 2015 at 8:38 AM, Andrew Holway <andrew.holway at gmail.com> wrote:
> On 25 January 2015 at 15:12, Boris Epstein <borepstein at gmail.com> wrote:
>> OK... but why does it need to be a trunk port?
> Because a trunk port will "trunk" the vlan.
> A VLAN is basically a 4 byte "tag" that gets injected into the packet
> header when the packet enters the VLAN network. When we trunk a VLAN we say
> to the switch "pass packets on VLAN x but do not strip the tag out".
> You can either terminate the VLAN at the switch port (untagged) which will
> strip out the VLAN tag or you can pass the packet containing the VLAN tag
> to the computer or other device(tagged/trunk). This device can then pull
> out the tag. On linux this mechanism is done by an 8021q VLAN interface.
> Hope this is useful.

Just to add to that - normally if a host only needs to be on one
subnet you would use an access port on the switch to select a single
vlan and deliver those packets untagged so the host does not need to
care about tags or vlan numbers.   And to that end, switches default
to treating everything as access ports on native/untagged vlan 0
unless configured otherwise.   However, if the host needs interfaces
on multiple subnets, you can do it on a single network connection by
giving it a trunk connection from the switch and letting it split out
the vlan interfaces internally.

   Les Mikesell
      lesmikesell at gmail.com