[ovs-discuss] Topology discovery in ovs

[Robin Wang]

Thanks very much for the detailed info, Peter！ And the part of involving
virtual entities into topology is even more interesting. Thanks！

Best,
Bibo
在 2012-6-5 中午12:20，"Peter Phaal" <peter.phaal at inmon.com>写道：

> Hi All,
>
> The original question was about L2 topology discovery. One of the
> differences between sFlow and NetFlow is that sFlow operates at layer 2
> (reporting layer 2 interfaces and MAC addresses) and NetFlow typically
> operates at layer 3 (reporting router (VLAN) interfaces and IP addresses).
> Consequently, inferring L2 topology from sFlow data is much easier than
> using NetFlow.
>
> As Robin pointed out, topology inference is a function of the
> sFlow/NetFlow collector. Inferring L2 topology from sFlow involves
> examining the sampled packet headers and forwarding paths reported by all
> the switches in the network, see:
>
> http://blog.sflow.com/2009/05/packet-paths.html
>
> Installing Host sFlow on the hypervisor (
> http://host-sflow.sourceforge.net/) helps topology discovery by exporting
> virtual machine information (UUID, hostname, MAC addresses) that can be
> joined with sFlow data from OVS to identify which vNICs are associated with
> each VM.
>
> Linking the virtual and physical topology topology gets interesting if you
> use tunneling (GRE, VxLAN etc), but it is possible for an sFlow analyzer to
> tie the layers together based on sFlow data:
>
> http://blog.sflow.com/2012/05/tunnels.html
>
> There is an inherent limitation in using traffic data to map topology -
> you can only see links that carry traffic. However, even backup links carry
> some traffic (STP, LLDP etc.) so it generally isn't much of a problem.
>
> Regards,
> Peter
>
> On Jun 4, 2012, at 7:50 PM, Motonori Shindo wrote:
>
> Kim and Robin,
>
> NetFlow and sFlow can give you some idea how your network topology looks
> like, but it is difficult to draw a complete picture of the network with
> NetFlow/sFlow because they are not designed to do such a thing.
>
> Theoretically NetFlow and sFlow can tell on which interface a particular
> flow traverses in the network and hence allows us to deduce the network
> topology from there. In order to do so, however, NetFlow/sFlow collector
> would have to correlate one flow record exported by one device to another
> flow record exported by another device. This is extremely difficult to do
> when there's a massive amount of flows in the network (and it is often the
> case). In some case (but it is not the case for OVS though), flow is
> "sampled" (meaning that not all packet are collected to generate flow
> records) and it makes topology discovery by NetFlow/sFlow even more
> difficult.
>
> Regards,
>
> ---
> Motonori Shindo
>
> On 2012/06/05, at 2:41, Robin Wang wrote:
>
> Hi Kim,
>
>
> I found an email that indicates OVS doesn't have built-in support. But it
> was a year ago, not sure whether it's supported now.
>
> http://openvswitch.org/pipermail/discuss/2011-July/005400.html
>
>
> For sFlow, to me, it is similar to NetFlow, and usually used to track
> dynamic traffic in a network with the help of flow collector. There're lots
> of different collectors. What you can get through sFlow depends on what
> features the collector provides, not OVS. Not sure if there's a collector
> provides topology things. Maybe someone who is familiar with collector
> could give us some advice. Thanks.
>
>
> Best,
>
> Robin
>
>
> 2012/6/5 Kim, Hyojoon <joonk at gatech.edu>
>
> Hi all,
>
>
>
>
> What is the most common way to do layer-2 network topology discovery in
> OVS? Some searching gives me a hint that people install lldpd, and query
> that table once it’s populated. Is this the most common way?
>
>
>
>
> I figured out there is no SNMP support in OVS, but there is sFlow. Is
> sFlow in OVS capable of doing topology discovery? It gives me a feeling
> that it is more about traffic monitoring.
>
>
>
>
> Thanks,
>
>
> Joon
>
>
>
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