In the previous post, we have discussed, how does PCEP can be used as south bound protocol to discover the active RSVP LSP in the network. In this post, I will be discussing how we can use any IGP protocol to discovery the existing network topology in the SDN controller. This mechanism is also known as inband. topology discovery by using existing IGP.
We have the various types of SDN Controller connectivity design available and need to understand which one is the best of the existing network.
Design 1: Single area or Small IGP
If you have single area or small IGP design, in that case controller can directly participate in the same domain and discover the existing network topology. The controller connects to the network only to obtain information from it, no data traffic should ever be sent to the controller. Within single area, the TED is the same in all nodes, that’s why we need single connection to the controller.
Design 2: IGP Domain With Multiple Areas
If you have large IGP domain with multiple areas configured, in that case traffic engineering or TED information is only propagated within single area. However the routing among the areas is taken care by border routers. In this case, we might require multiple links to the controller. In large IGP domain, TED information is only available within area. So if we need to get the entire TED information, we may need to connect the controller with different ABRs. If the controller is connected to multiple paths in the network, special care has to be taken so that controller should not come in the forwarding plane.
The main disadvantage of this approach is that, you might require hierarchical PCEP controller because it has to correlate the TED information from different areas. Second disadvantage of this design is that you might require multiple connectivity from different areas or place different controllers near to different areas.
Design 3: SDN Controller With GRE Tunnel
With the known limitations as per design 2, we can use the GRE tunnels to get rid multiple physical connectivity and extract the TED information. The main advantage of this design is that you need single physical connectivity by creating logical interfaces on the top of it.
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