[CentOS] VLAN issue

Mon Jan 26 00:05:49 UTC 2015
Boris Epstein <borepstein at gmail.com>

Thank you everyone.

OK, the mystery deepens, I guess. The machine does need to support several
VLAN's, it is currently on a trunkport (8021q encapsulated), it made it
into the ARP table - which I specifically tested for by physically
unplugging the table, clearing the ARP table and plugging it back in.

The ARP table currently looks like this:

hq#show arp
Protocol  Address          Age (min)  Hardware Addr   Type   Interface
Internet          0   0025.6440.0301  ARPA   Vlan48
Internet          -   001b.906a.bcc4  ARPA   Vlan48
Internet            0   0025.6440.063f  ARPA   Vlan48
Internet            0   0025.6440.0547  ARPA   Vlan2
Internet             -   001b.906a.bcc2  ARPA   Vlan3
Internet             -   001b.906a.bcc1  ARPA   Vlan2
Internet             -   001b.906a.bcc3  ARPA   Vlan7

The network config on the machine currently looks like this: it has nothing
assigned to eth0, eth0.48 =, eth0.49 =,
eth0.50 =

And - even though the ARP table seems to be OK - there is no connectivity!


On Sun, Jan 25, 2015 at 11:42 AM, 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
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