CentOS - Jun 2012 - kvm CLI :: meaning of "vlan=" and "name=" options in -net nic and -net tap

Did not get any response in the CentOS Virt list.

Posting in the CentOS General list hoping that some one here can
provide clarification.

Thx,
-- Arun Khan


---------- Forwarded message ----------
From: Arun Khan
Date: Tue, Jun 12, 2012 at 8:54 PM
Subject: Meaning of "vlan=" and "name=" in Linux KVM
To: CentOS Virt <centos-virt at centos.org>


Greetings KVM gurus,

I am trying to understand a "use case" scenario for the
"vlan=n"
option and "name=name" for the "-net nic" and "-net
tap" directives.

I have done some search but I have not come up with anything that
sheds any light on the above.

<quote from kvm man>
Network options:

-net nic[,vlan=n][,macaddr=mac][,model=type]
[,name=name][,addr=addr][,vectors=v]
? ? ? ? ? Create a new Network Interface Card and connect it to VLAN
n (n = 0 is the default). The NIC is
? ? ? ? ? an e1000 by default on the PC target. Optionally, the MAC
address can be changed to mac, the
? ? ? ? ? device address set to addr (PCI cards only), and a name can
be assigned for use in monitor
? ? ? ? ? commands. ?Optionally, for PCI cards, you can specify the
number v of MSI-X vectors that the card
? ? ? ? ? should have; this option currently only affects virtio
cards; set v = 0 to disable MSI-X. If no
? ? ? ? ? -net option is specified, a single NIC is created. ?Qemu
can emulate several different models of
? ? ? ? ? network card. ?Valid values for type are "virtio",
"i82551", "i82557b", "i82559er",
"ne2k_pci",
? ? ? ? ? "ne2k_isa", "pcnet", "rtl8139",
"e1000", "smc91c111",
"lance" and "mcf_fec". ?Not all devices are
? ? ? ? ? supported on all targets.


-net tap[,vlan=n][,name=name][,fd=h][,ifname=name]
[,script=file][,downscript=dfile]
? ? ? ? ? Connect the host TAP network interface name to VLAN n, use
the network script file to configure
? ? ? ? ? it and the network script dfile to deconfigure it. If name
is not provided, the OS automatically
? ? ? ? ? provides one. fd=h can be used to specify the handle of an
already opened host TAP interface. The
? ? ? ? ? default network configure script is /etc/qemu-ifup and the
default network deconfigure script is
? ? ? ? ? /etc/qemu-ifdown. Use script=no or downscript=no to disable
script execution.

</quote from kvm man>

My objective is to create a small "virtual" network using bridges/tap
interfaces on the Host OS and running the Guest OSs (the network
segment separated by vlan=somenumber)

My hypothesis is that with "vlan=X" and "name=somename" the
VMs can be
separated into different segments i.e. a set of tap interfaces on
vlan=10 and another set of tap interfaces on vlan=20 connected to the
same bridge br0.

I have two VMs started with ?"kvm ?-net tap,vlan=10" and "kvm
?-net
tap,vlan=20" respectively but with IP numbers in the same subnet
172.16.0.0/24; they are able to ping each other with different "vlan"
numbers.

I would appreciate any clarification on the "vlan=" and
"name=" options.
Sample script for my KVM VMs

<script>
#!/bin/bash

kvm \
-vga std \
-m 1024 \
-cpu core2duo \
-smp 2,cores=2 \
-drive file=/home/kvmusr/KVM/vdisks/centos62.img,index=0 \
-net nic,vlan=1,model=e1000,macaddr=${nic_mac_addr0} \
-net tap,vlan=1,ifname=tap0,script=no,downscript=no \
</script>

Thanks,
--
Arun Khan