In this article, I will focus on an illustration that I turned into an actual lab. Here is the scenario:
You have 3 routers that connect via a frame-relay connection. One of them is using a single sub-interface that terminate multiple connections. This multipoint connection has connectivity to the other two routers.
When EIGRP gets deployed in this situation, what SHOULD happen according to the illustration is R1 will not be able to learn routes attached to R2 for the reason that split-horizon is enabled by default.
The chapter goes into detail as to why this issue occurs in EIGRP. My thought was to first create the frame relay that matches closely the illustration. For posterity, I have screen captured the lab and have it below if you would like to learn how to create a multipoint sub-interface on a router and configure frame-relay on serial interfaces.
When that is completed, this lab is ready to experiment with the focus of the topic which is how to get R1 to learn the connected route (126.96.36.199) from R2. Please see GIF below:
Some comments about above GIF:
I typically start the labeling of the DLCI with 102, so that on the other end, the DLCI would have 201. This number convention is used LOTS of times to help keep which connections straight.
Also in this lab, you would be actually creating a sub-interface that is a multi-point.
That’s kind of important since the other two routers are to send out updates on a regular interface (S1/0) but router 3 will have an issue based on the fact that both frame relay interfaces will ‘share’ the same serial link going into Router 3.
Let me rephrase that…it’s not that router 3 itself will have an issue…but when an engineer deploys EIGRP…because split-horizon is set by default…we should start to see the issues presented in Chapter 5 of the CCNP Routing and Switching ROUTE 300-101 Official Cert Guide.
We are now ready to experiment with EIGRP.