Hello again, now that we know what is segment routing, time has come to uncover secrets of segments in Segment Routing(Source Routing) . Source Routing is about controlling routing by the source who initiates traffic, hence the routers within the path don’t know about the path to be taken. Data packet is forwarded step by step based on the path. At each step, the packet has a destination label. Router, based on this label directs the packet to its outgoing link. These labels help router decide the instruction to be followed. Thus, label with an instruction is a segment. Ultimately, the path is a collection of segments. Take a look at the network below, to help yourself easy to visualize.
Segments in Segment Routing
Consider your city as an interconnection of OSPF/ISIS areas. Each router(station), as I mentioned is identified by a global label like 104, 105 etc. This is the IGP prefix segment. This label is globally unique, so with 104 as the destination, it is guaranteed that the data packet will definitely reach to 104. Routers are connected by links and IGP Adjacency segment, identifies these links. The value of the label is such that, the two connecting routers can be easily recognized, for example, 145 label is the link between stations 104 and 105. This segment is local to the router but based on the label value, it knows where the packet should be guided.
Now, Tom’s city is a larger one, which can be assumed as made of EGP connections, like BGP network. Comparing with nominations of your city, each router in BGP is identified by a global BGP prefix segment, as 103 is above. And 158 is the BGP Adjacency prefix segment, a local value, which is pairing between two BGP routers. Features of these two segments are same like IGP segments.
The path Tom is taking to reach Jerry’s place is, 103-104-145-158. That’s it, these 4 segments complete the route. Hence, the Segment Routing…!!!!
You may wonder that, for the purpose of explanation these labels are quite easy, but actually, labels must be huge numbers. You are wrong. The designers of SR have tried to follow simplicity, so actual label values will be somewhat same.
As you saw, these segments are enough for the router to make a decision and thus, do routing. Segment Routing with such simplicity is definitely on its way to create a revolution. Till then let us just keep our ears open to the more upcoming buzz of SR…
Happy segment routing..!!!