Hi,
There had been some discussion on MSTP support in OpenFlow Spec. But I
don't see yet any MSTP support extension in Spec.
Even, if some non-standard STP evolves for OF controller. but it still has
serious issue with interop with legacy switches.
For example: Traditional switch will form loop with OF Switch if standard
STP is not running in OF Switches. And If STPs are to be run on OF Switches
instead of controller, then we are loosing the centralize control
advantages of OF Switches.
Is there any thought of extending MSTP support in OpenFlow controller?
I mean running control protocol in controller environment (not necessarily
tightly coupled with controller) to get interop with traditional switches.
Loop Overview:
----------------------
Traffic flow is decided by controller and it is the flood message
(broadcast, multicast and unknown unicast messages) that causes loop in
OpenFlow Switch (assuming basic Learning switch implemented by controller).
Solution:
-------------
Now, once message hits controller, (after checking ingress port state)
default flood should change to forward data on forwarding ports of the
frame Vlan (classified Vlan of the frame).
And, for bidirectional traffic, once Macs are learnt for both direction
install a flow. therefore, flow will be installed only on forwarding ports
dynamically.
- This should be done by controller not devices, OF switch port
should remain unblocked for all Vlan.
- This will give benefit to allow static flow creation even on
blocked Vlan ports.
Running MSTP in centralized environment:
-------------------------------------------------------------
Controller and Control Protocol need not be tightly coupled. Consider, MSTP
as separate module interacting with controller to - gets switchId,
SwitchMac, Ports capability from controller and sets port state in
controller.
This control plane module can get their own CLI and Management plane, just
like traditional switch.
Static Flow:
------------------
Static Flow provision need not consult [Vlan, Port] state to provision
flow. This will allow specific traffic to be provisioned by admin even on
blocked ports. Since nothing is actually blocked in OF Switches, static
flow will work, even on STP blocked ports.
Advantage:
-----------------
1) Interop with legacy swiches.
2) Utilization of blocked link for static flows.
Disadvantage:
---------------------
1) Packet received on blocked port will still hit controller, instead of
getting dropped in OF Switch itself. (But this also can be fixed)
I can propose following MSTP extension in OpenFlow spec:
1) - Table for Vlan to Instance mapping
- [Instance, Port] PortState
2) Static Flow need not consult these table install flows.
This model is a step to allow other control protocol to make there way in
centralized environment of OF Network and at the same time interop with
exisitng devices.
If there is already some work in progress on this, please ignore this mail.
Regards
Sudeep
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