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

 

 

 

 

 

 

 

 

 

 

 

 

CxCy

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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