[c-nsp] multiple vlans on a port

[Matthew Huff]

Yes, the machine will need to speak 802.1q. Most modern OS have no trouble with that. Windows, Linux, Solaris, etc.. work fine with 802.1Q.

One thing more, unless Linux has started speaking Cisco DTP (which I doubt), you want to disable DTP messages from sending to the host. Dynamic Trunking Protocol (or DTP) is used to negotiate trunking protocols (ISL or 802.1q), etc... Since you know you want to do 802.1Q and you want to always trunk, you will want to add "switchport nonegotiate" to the interface. This keep cisco from sending a DTP frame every 30 seconds. Those frames won't hurt anything, but can show up on port statistics as bad packets on the host.

Also, with 802.1q framing, you might run into fragmentation on the non-native VLANs. You may want to adjust the MTU on the virtual machines if Linux doesn't do it automatically.


interface GigabitEthernet0/15
   switchport access vlan 120
   switchport trunk native vlan 120
   switchport trunk allowed vlan 100,120,231,321
   switchport mode trunk
   switchport nonegotiate
end


----
Matthew Huff       | One Manhattanville Rd
OTA Management LLC | Purchase, NY 10577
http://www.ox.com  | Phone: 914-460-4039
aim: matthewbhuff  | Fax:   914-460-4139

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of A.L.M.Buxey at lboro.ac.uk
Sent: Monday, July 13, 2009 6:15 PM
To: Cord MacLeod
Cc: cisco-nsp at puck.nether.net
Subject: Re: [c-nsp] multiple vlans on a port

Hi,

> I realize this is impossible, at least I have read it is on an access  
> port.  So if I sent up a trunk port with the machine, does the machine  
> need to speak 802.1q as well?
>
> interface GigabitEthernet0/15
>  switchport access vlan 120
>  switchport trunk native vlan 120
>  switchport trunk allowed vlan 100,120,231,321
>  switchport mode trunk
> end
>
> The purpose of this is that the machine in a Linux machine running Xen, 
> so the cloud will decide what machines and vlans it needs to spin up at 
> what time.  Meaning this port will need access to these vlans.  This 
> being the case, will I need to configure the Linux machine for 802.1q 
> trunking as well?  I found this article that seemed to suggest, yes, but 
> I wanted a second opinion.  http://www.linuxjournal.com/article/7268

Linux very happily talks 802.1q.  yes, if you want to feed multiple
networks to the Xen host you need to send it a trunk feed... or invest
in multiple NICs and assign NICs to virtual hosts. our Xen boxes
get trunk feeds and /sbin/ifconfig lists all the pvlanXXX and xenbrXXXX
and xenbrtrunk etc.  VMWare has the virtual switch technology so currently
is _slightly_ ahead of Xen on that point...

alan
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