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What is Optimal Routing and Suboptimal Routing in Networking

What is Optimal Routing and Suboptimal Routing in Networking? This may be seen very easy for some of you but let’s make a philosophy a little bit, means let’s design around optimal routing. 

Network engineers know that one of the tradeoff in network design is Optimal Routing. We want our application traffic to follow Optimal Routing right?

Not exactly right. I will explain why it is not always right but let’s understand what exactly we mean with ‘ Optimal Routing’.

Based on which parameter optimality we are looking for?

Answer of this question is IGP Metric. From source of the traffic to the destination, between the network nodes, cumulative/total cost. It can be Layer 2 or Layer 3 protocol, we prefer shortest path. Shortest path, most of the time is calculated based on ‘Bandwidth’.  Thus, overall which path provides more bandwidth, that path is considered shortest path, thus Optimal Routing.

This is not necessarily always the case, for example BGP uses shortest AS-Path length, not the bandwidth for Inter-domain routing but OSPF, IS-IS for example look at the total bandwidth for the cost calculation inside the domain/AS.

Some of you might think that, other than shortest IGP cost, maybe lowest physical distance (mostly called as fiber mile distance) or cheapest monetary cost we might use for optimality purpose, you are right. You may have a valid business case for it and in fact we have technical solutions to do that.

RSVP TE and Segment Routing based Traffic Engineering provides a functionality for you have different policies in the network.

As we understand that Optimal Routing is mostly considered as shortest cumulative IGP cost between source and the destination network nodes, let’s talk about why we may not want to send the traffic through the path which provides Optimal Routing.

Above picture is famously called as ‘ Fish Diagram/Topology’

Reason is; it looks like a fish but for us it is important to understand that with this very basic topology, we can understand why we may need traffic engineering.

If every interface between the routers has same IGP Metric, between source and the destination routers in the above topology, always top path would be chosen due to shortest cumulative/total IGP Metric.

Bottom path wouldn’t be used at all if we would rely on IGP routing protocols for path selection.

If we would like to utilize bottom path for some of our application traffic, we need more than IGP routing functionality. RSVP and Segment Routing provides traffic engineering capability but those are not the topics of this post.

When we start doing Traffic Engineering, we are in fact accepting to do ‘ Sub Optimal Routing ‘.

We send only some of our traffic over the shortest IGP cost (Remember our Optimal Routing Definition) , and intentionally we send some other traffic over longer IGP cost path which is Sub Optimal Routing path.

By the way, shortest IGP cost path doesn’t have to be monetary wise cheaper one, or it may not be the lowest delay path etc.

You may have satellite link with the GEO orbit, which comes with 500+ msec latency but you may have more bandwidth on that compare to other path.

Let me summarize.

  1. We use Optimal Routing to define shortest IGP cost path.
  2. This path may not be Optimal path as per your policy
  3. Optimal Routing may not be desired for every of your application or service traffic
  4. Technologies such as RSVP and Segment Routing are used for Traffic Engineering which intentionally violates the Optimal Routing Path
  5. We intentionally send some of our Application traffic over Sub Optimal Routing path, but that path satisfy the constrain of the applications overlay that path
  6. There is no one correct answer for all networking requirements!

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