EIGRP delay summarization?

Question

Could you please explain why when sending EIGR updates only delay of incomig interfaces are summarized and delay of outgoing interfaces is not considered. Please see picture below:

Answer 1

What your drawing has is one, or both, of the routers overriding the interface delay. Under normal circumstances, the delay of a link is exactly the same on both ends. One of the routers may be doing that on purpose to influence routing. Changing the delay is a common way to influence routing. The other way would be to manually change the bandwidth, but that could affect things like QoS. Changing the delay is a safer way to influence the routing.

The reason is that you would be double counting the delay for a link. If, for example, the default delay for the link is 100, and R2 added the outgoing delay, of 100 and R1 added the incoming delay of 100, the link delay would be counted as 200, but it is really only 100.