[CentOS] VLAN issue

Dennis Jacobfeuerborn

dennisml at conversis.de
Sat Jan 24 14:11:41 UTC 2015


Hi Boris,
what I'd like to know is the actual VLAN configuration of the switch
port (link-type and tagged and untagged VLANs). When I look at the
switchport coniguration here I get (among other things):

...
 Port link-type: trunk
  Tagged   VLAN ID : 8, 1624
  Untagged VLAN ID : 10
...

Here is my suspicion:
Your ports have an access link-type with an untagged VLAN ID of 48. That
would explain why the moment you configure an IP from that VLAN on eth0
you get connectivity because then the packets the Linux box sends are
untagged as the switch would expect them to be. If you only put an
address on eth0.48 then the packets get tagged by Linux but if the
switch port is not configured to receive the packets for VLAN 48 as
tagged then it will simply drop these packets and you will not get
connectivity.

So getting the actual VLAN config of the switch port would help to
determine if the switch actually expects to receive the packets the way
you send them from the Linux box.

Regards,
  Dennis

So if you
On 24.01.2015 13:35, Boris Epstein wrote:
> Do you need the whole configuration? On the switch end, we have the
> relevant VLAN (VLAN 48) with the assigned IP address of 192.168.48.101 and
> the range of ports (Gi1/0/1 - Gi1/0/8) assigned to that VLAN.
> 
> Seems - and acts - like a legitimate setup and works fine, except for this
> particular instance.
> 
> Thanks.
> 
> Boris.
> 
> On Fri, Jan 23, 2015 at 8:54 PM, Dennis Jacobfeuerborn <
> dennisml at conversis.de> wrote:
> 
>> We have lots of servers with a similar setup (i.e. tagged vlans and no
>> ip on eth0) and this works just fine.
>>
>> What is the actual vlan configuration on your switchport?
>>
>> Regards,
>>   Dennis
>>
>> On 24.01.2015 01:34, Boris Epstein wrote:
>>> Steve,
>>>
>>> Thanks, makes sense.
>>>
>>> I just don't see why I have to effectively waste an extra IP address to
>> get
>>> my connection established.
>>>
>>> Boris.
>>>
>>>
>>> On Fri, Jan 23, 2015 at 7:16 PM, Stephen Harris <lists at spuddy.org>
>> wrote:
>>>
>>>> On Fri, Jan 23, 2015 at 07:10:57PM -0500, Boris Epstein wrote:
>>>>
>>>>> This makes two of us. I've done everything as you have described and it
>>>>> simply does not work.
>>>>
>>>> Are you actually seeing VLAN tagged traffic, or is the cisco switch
>>>> just providing a normal stream?
>>>>
>>>> At work we have hundreds of VLANs, but the servers don't get configured
>>>> for this; we just configure them as normal; ie eth0.  The network
>>>> infrastructure does the VLAN decoding, the server doesn't have to.
>>>>
>>>> Try configuring the machine as if it was a real LAN and forget about
>>>> the VLAN.
>>>>
>>>> If that doesn't work then what does 'tcpdump -i eth0' show you?
>>>>
>>>> --
>>>>
>>>> rgds
>>>> Stephen
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