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So when the switches are connected and if they are configured as of Segment Router type, the Segment Router driver would populate at startup during the OF handshake procedures the SPRING-OPEN pipeline in those switches for which this ONOS instance is performing MASTER role.

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In order to reduce the latencies during the Segment Routing flow creation, the Segment Routing driver at startup pre-populates the OF 1.3 groups in all the Segment Routers for which this ONOS instance is performing MASTER role. There are two types of groups that Segment Routing driver creates in the switches:

• OF Indirect group (Single bucket group) – It’s main advantage is that when many, many routes (IP dst prefixes) have a Next-Hop that requires the router to send the packet out of the same port with or without same label, it is easier to envelop that port and/or label in an indirect group. Note that these Groups are only created on ports that are connected to other routers (and not an L2 domain). Thus the Dst-MAC is always known to the controller, since it is the router-MAC of another router in the Segment Routing domain. Also Note that this group does NOT push or set VLAN tags, as these groups are meant to be used between routers within the Segment Routing cloud. Since the Segment Routing cloud does not use VLANs, such actions are not needed. This group will contain a single bucket, with the following possible actions:
  • Set Dst MAC address (next hop router-mac address)
  • Set Src MAC address (this router’s router-mac address)
  • At ingress router
    • Push MPLS header with ethtype as IP and mpls label 
    • Copy TTL out
    • Decrement MPLS TTL
  • Output to port (connected to another Router)

• OF Select group (ECMP Group) – This is an OF 'Select' Group. It will have one or more buckets, that each point to actions of an Indirect group or point to another group (incase of group chaining). Note that this group definition does NOT distinguish between hashes made on IPv4 packets, and hashes made on packets with an MPLS label stack. It is understood that the switch makes the best hash decision possible with the given information.