That's a great description. Thanks! --Justin
On Aug 3, 2012, at 4:34 PM, Ben Pfaff <b...@nicira.com> wrote: > Signed-off-by: Ben Pfaff <b...@nicira.com> > --- > FAQ | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 files changed, 54 insertions(+), 0 deletions(-) > > diff --git a/FAQ b/FAQ > index bdd96ce..4658bb9 100644 > --- a/FAQ > +++ b/FAQ > @@ -302,6 +302,60 @@ A: Yes. ovs-vswitchd.conf.db(5) is a comprehensive > reference. > VLANs > ----- > > +Q: What's a VLAN? > + > +A: At the simplest level, a VLAN (short for "virtual LAN") is a way to > + partition a single switch into multiple switches. Suppose, for > + example, that you have two groups of machines, group A and group B. > + You want the machines in group A to be able to talk to each other, > + and you want the machine in group B to be able to talk to each > + other, but you don't want the machines in group A to be able to > + talk to the machines in group B. You can do this with two > + switches, by plugging the machines in group A into one switch and > + the machines in group B into the other switch. > + > + If you only have one switch, then you can use VLANs to do the same > + thing, by configuring the ports for machines in group A as VLAN > + "access ports" for one VLAN and the ports for group B as "access > + ports" for a different VLAN. The switch will only forward packets > + between ports that are assigned to the same VLAN, so this > + effectively subdivides your single switch into two independent > + switches, one for each group of machines. > + > + So far we haven't said anything about VLAN headers. With access > + ports, like we've described so far, no VLAN header is present in > + the Ethernet frame. This means that the machines (or switches) > + connected to access ports need not be aware that VLANs are > + involved, just like in the case where we use two different physical > + switches. > + > + Now suppose that you have a whole bunch of switches in your > + network, instead of just one, and that some machines in group A are > + connected directly to both switches 1 and 2. To allow these > + machines to talk to each other, you could add an access port for > + group A's VLAN to switch 1 and another to switch 2, and then > + connect an Ethernet cable between those ports. That works fine, > + but it doesn't scale well as the number of switches and the number > + of VLANs increases, because you use up a lot of valuable switch > + ports just connecting together your VLANs. > + > + This is where VLAN headers come in. Instead of using one cable and > + two ports per VLAN to connect a pair of switches, we configure a > + port on each switch as a VLAN "trunk port". Packets sent and > + received on a trunk port carry a VLAN header that says what VLAN > + the packet belongs to, so that only two ports total are required to > + connect the switches, regardless of the number of VLANs in use. > + Normally, only switches (either physical or virtual) are connected > + to a trunk port, not individual hosts, because individual hosts > + don't expect to see a VLAN header in the traffic that they receive. > + > + None of the above discussion says anything about particular VLAN > + numbers. This is because VLAN numbers are completely arbitrary. > + One must only ensure that a given VLAN is numbered consistently > + throughout a network and that different VLANs are given different > + numbers. (That said, VLAN 0 is usually synonymous with a packet > + that has no VLAN header, and VLAN 4095 is reserved.) > + > Q: VLANs don't work. > > A: Many drivers in Linux kernels before version 3.3 had VLAN-related > -- > 1.7.2.5 > > _______________________________________________ > dev mailing list > dev@openvswitch.org > http://openvswitch.org/mailman/listinfo/dev _______________________________________________ dev mailing list dev@openvswitch.org http://openvswitch.org/mailman/listinfo/dev
