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Traditional networking terms and features in the world of SDN

As a network professional, you will have been involved with day to day networking tasks. Tasks such as creating and managing VLANs and ports, trunking (802.1q), managing spanning trees, link aggregation, routing, accessing lists, troubleshooting, and logging.

Let's have a look at what happens to these fundamentals in SDN:

Spanning tree: Spanning tree has been always a painful protocol to manage for all network professionals. Spanning tree is a complex (when it comes to per VLAN, MSTP, RSTAP, compatibility, and so on) mechanism to create a loop-free layer 2 network. Spanning tree is not efficient as it disabled the links that may create a loop.

In recent years, many organizations have tried to eliminate the spanning tree by migrating to a full layer 3 routed fabric or using proprietary multi-chassis link aggregation technologies.

Anyhow, the good news is that in SDN, there is no need for spanning trees. The SDN controller, as the brain of the whole network knows how each switch in the network should send and receive packets in order to have a complete loop-free network.

Also remember that BUM is managed and handled by the SDN controller, which reduces the risk of loops.

The required features of spanning trees are also included in the L2 switching application of ODL.

VLANs: Let's review our traditional understanding of VLANs. A virtual LAN (VLAN) is a method to divide a basic layer 2 switch into multiple standalone switches. Ports in different VLANs will not be able to communicate with each other. Technically the VLAN concept is implemented in the switch's silicon, and doesn't allow any entry in the TCAM table where the source and destination ports belong to different VLANs (that's a simple implementation in silicon).

In the world of SDN, as all forwarding is controlled by the SDN controller, the concept of VLAN is managed by the SDN controller. In ODL it is managed by the L2 application.

Trunking (802.1q): Trunking and the concept of tagged and untagged frames exists in SDN very similar to the traditional world. In traditional layer 2, a switch was able to send a packet untagged (access-port) or with a tag that can only be the VLAN ID of that frame.

In the world of SDN, a switch will send a frame with any 802.1q tag, which the SDN controller decides. For example, a switch might receive a frame with VLAN tag of 100, and then sends it out with the VLAN ID of 200. This is something that is beyond the concepts of traditional 802.1q and VLANs.

Link aggregation: Link aggregation uses standard protocols such as LACP. Link aggregation exists in SDN similar to the traditional networking world. Link aggregation is supported by most SDN controllers, as well as ODL. ODL includes specific modules to support link aggregation.

Technically, when you configure two ports as a link aggregation using the ODL interface, the ODL SDN controller sends the required configuration to the related switch using a southbound protocol (for example, NETCONF) and tells that switch hardware to configure the ports in link aggregation mode.

Routing: Routing between the switches within the SDN domain is not required. It is managed by the SDN controller. However, the SDN controller supports routing protocols such as BGP and OSPF to communicate outside of the world of SDN.