VLAN trunking and fragmentation

[Sean Chittenden]

>>>> interface ethernet 1/g1
>>>> switchport mode trunk
>>>> switchport trunk allowed vlan add 10
>>>> exit
>>> I think this is an issue with default VLAN membership.
>>
>> I don't think it's where the problem lies, otherwise even a simple  
>> ping
>> wouldn't work. The problem here explicitly arises when packets are
>> fragmented...
>
> fragmentation?
> I think, you use ethernet links, all have 1500byte max packet size,
> you have 1500byte packet size also comfigured at the vlan interface,  
> so..
> why do you think anything should start fragmenting?
> (and tagged vlan frames are bigger than 1500byte at the "raw"  
> ethernet link).
>
> check for maximum mtu that can travel over the link.
> (ping -s 1472  for 1500byte packet)


It's the MTU size, not the fragmentation or VLAN membership.  The use  
of a default VLAN can cause problems potentially because the size of  
each frame can vary and, IMHO, is the cause of the problem.  By  
default VLAN membership, I don't mean to suggest you've got your port  
tagged in the wrong VLAN.  Switch all traffic to 802.1q tagged and you  
should see everything work.  Use of VLANs reduces the MTU size  
automatically, but cycling between the two?  Likely a no-go and the  
source of your problems.

> Even nicer is using <interface>.<number> autoconfig, the following is
> equivalent to above:
>
> cloned_interfaces="re0.10 re0.11"
> ifconfig_re0="media 100baseTX mediaopt full-duplex"
> ifconfig_re0.10="inet 192.168.60.1 netmask 255.255.255.0"
> ifconfig_re0.11="inet 192.168.100.1 netmask 255.255.255.0"


*boggles*  That's hawt!  'ya learn something new every day,  
thanks!  :~]  -sc

--
Sean Chittenden
sean at chittenden.org
http://sean.chittenden.org/