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Let’s keep it simple. Every tunnel, whether it’s a real one on a road or a logical one in networking, always has two ends. But what is one tunnel endpoint in networking terms? Think of an IPsec tunnel - how would you describe each end of an IPsec tunnel?

One tunnel endpoint in the Catalyst SD-WAN fabric is called Transport Locator (TLOC). Every data plane tunnel in the SD-WAN fabric is formed between two TLOCs. A TLOC uniquely identifies a particular WAN interface of a specific Edge router and consists of all the information that a remote router needs to establish a tunnel to that WAN link. 

But to really understand what a TLOC is, let’s go through a thought experiment. Suppose you are tasked to write a script that automates the process of building GRE tunnels between routers. Each router must autonomously advertise its tunnel endpoints to a centralized controller and form a tunnel to all remote routers. Spend a few minutes thinking about it. It should not be that complex, right? 

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