When GRE networks are created, each network is assigned a unique segmentation ID that is used to encapsulate traffic. As traffic traverses the Open vSwitch tunnel bridge, the segmentation ID is used to populate a field in the encapsulation header of the packet. For GRE packets, the KEY header field is used.

The tunnel_id_ranges configuration option found under [ml2_type_gre] is a comma-separated list of ID ranges that are available for tenant network allocation when tunnel_type is set to gre.

In the following example, segmentation IDs 1 through 1,000 are reserved for allocation to tenant networks upon creation:

tunnel_id_ranges = 1:1000 

The tunnel_id_ranges option supports non-contiguous IDs using a comma-separated list as follows:

tunnel_id_ranges = 1:1000,2000:2500 

GRE networks will not be configured as part of the exercises in this book, so tunnel_id_ranges can remain unconfigured.