How to check your Diagram for potential Routing Loops

One of the most challenging tasks in the CCIE Lab is how to deal with routing loops, specially when dealing with redistribution. I still working my way through it and I still missing alot (any comments, thoughts, suggestions will be welcome!).

Anyway... I will not deal here (in this specific post of course) with the details about redistribution, and anything else. I will just check the topology for potential routing loops, and redistribution points! (and please, correct me if I´m wrong, ok?!).

Let´s check this diagram from IPExpert Workbook Vol. 3 - Lab. 1 (this Lab 1 is available for download at IPExpert Website, go get yours).

To keep things simple, forget that R4 Serial connection is also connected to R5 through Frame-Relay network, just pretend it is connected direct through Serial Interfaces, ok?!

So we have EIGRP, OSPF and RIP all over our topology, and guess which protocol was choosen to be in the Frame-Relay network?! Yes, you´re right! OSPF!

Anyway, checking it a little deeper, we can see that routers BB2, R4, R5 and R2 have direct connections and those routers are using RIP. Also, R1 and SW1 are using RIP between them! That should not take too long to be configured (depending on what is being asked), but probably not very challenge.

In the EIGRP "world" we have BB1, R6, R7 and R9 routers. Note that there are two interfaces between R6 and R9. Again, that also should not take too long to be configured depending on what the task is asking and the restrictions applied!

Now... OSPF! It´s being used between R2, R5 and R6 in the OSPF Area 256, and between R1 and R2 in OSPF Area 12.

Now that we checked the IPv4 IGP Routing Protocols in this diagram, we also need to verify it for potential Routing Loops. Check the bellow drawing:

Starting from BB2, to the top of the topology, we can see two loops in our network. One represented by the "ORANGE" arrows and the other by the "RED" arrows.

First, let´s analyze the loop in "ORANGE" between R6 and R9. It´s inside EIGRP AS679. So, this loop is "contained" inside one routing protocol. The routing protocol will deal with it. No worries!

Now... the loop between R2 and R5 (represented by the "RED" arrows). This one involves two distinct routing protocols! So we must take care when doing redistribution between those routing domains, either by using different Administrative Distance (AD), route-maps, access-list or any tool that we´re allowed to use to avoid it!

Regarding the redistribution points... we´ll need to redistribute routes in R2, R6 and "maybe" in R1, it´ll all depend on the connections!

Just keep in mind that, when dealing with routing loops we can have situations like:

Routing Loop with a single IGP: That kind of situation will not create a routing loop when redistribution is performed, because, it´ll be filtered by the IGP in use.

Routing Loops  with two or more IGPs:  In  this situation we´ll need to take care off "manually", using filters, maps, access-lists, and other tools when dealing with redistribution, avoiding for example that RIP domain learn OSPF routes, and them send those routes back to the OSPF domain!

This analysis seens simple, but try to do it in some different diagrams on your own to get more practice, I´ll do the same!

Later we´ll discuss the details involving the redistribution and the IGPs!

Cheers!