On the Internet routing protocol Enhanced Interior Gateway Routing Protocol: is it optimal?

Cisco's proprietary routing protocol, EIGRP (Enhanced Interior Gateway Routing Protocol) is one of the two most widely employed routing protocols in the Internet. The underlying algorithm is reputed to be optimal with respect to the EIGRP metric. We construct a counterexample to illustrate that it is not optimal. We implemented the test network from the counterexample in our Networking Lab and it was confirmed that the Cisco routers did not find optimal routes. We suggest ways in which the EIGRP algorithm can be improved. These suggestions would also improve the operation of the Diffusing Updating Algorithm, the portion of EIGRP used to recover from link/node failures.