White Paper

Technical white paper | HPN SDN Controller Link Discovery
Using the example topology above, we will illustrate a few of the issues that may arise with link discovery. Assume that
switches A, B, C OpenFlow instances are configured to connect to the same controller and that each inter-switch physical
connection is tagged with the vlans shown. The notation “OF Inst” indicates an OpenFlow instance running on the given
switch.
The following will be the discovered topology:
Ax Bx
Cx Cy
Direct link
Multihop link
Undiscovered link (ERROR)
All of the undiscovered links have switch C as their destination, so we may be tempted to assume that this is a
misconfiguration on switch C. Actually, these links would be discovered with configuration changes on either switch C or the
other controlled switches (A, B).
• The multi-hop link from Ax to Cy is not discovered because the controller injects an untagged link discovery packet from
Ax port 1. The untagged packet is associated with vlan 3 by the uncontrolled switch, so the link discovery packet is never
received by switch C.
• The direct links from Bx to Cx and Cy are not discovered because the controller injects an untagged link discovery packet
from Bx on port 2. The untagged packet is received by switch C and associated with the untagged vlan on switch C port 2.
– If the untagged vlan on switch C port 2 is vlan 1, only the Bx to Cx link will be discovered.
– If the untagged vlan on switch C port 2 is vlan 2, only the Bx to Cy link will be discovered.
– If the untagged vlan on switch C port 2 is neither vlan 1 or 2, no Bx to Cx/y links will be discovered.
Device-specific recommendations
In general, HP recommends that OpenFlow instance-to-vlan mappings remain consistent throughout the controlled
network topology. If an OpenFlow instance contains a set of vlans on one switch, then neighboring switches should also
have an OpenFlow instance with the same set of vlans.
ProVision: Virtualization Mode
There are no known issues when using ProVision products in virtualization mode. Since virtualization mode limits each
OpenFlow instance to a single vlan, the vlan can be derived from the OpenFlow instance. Any packets which the controller
injects into an OpenFlow instance in virtualization mode are tagged correctly by the OpenFlow instance - an 802.1Q header
is added, if the egress port is configured to be tagged. No known issues exist under these circumstances.
Comware: PVID in OpenFlow instance
There are no known issues with link discovery (limited testing) when using Comware devices when the port’s pvid is set to
an OpenFlow vlan. If any link discovery packets arrive untagged on a port, they will be assigned to the pvid. If the pvid is set
to an OpenFlow vlan, then all link discovery packets will be received by the controller from the OpenFlow instance which
contains that vlan.
If multiple OpenFlow instances are configured on a single Comware device, then at least one port must be used per
OpenFlow instance per neighboring switch. This is required because only one pvid can be assigned per port, and no single
vlan can be associated with multiple OpenFlow instances.
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