1. Recursive infinite Loop
Even from the time immemorial, the chicken and egg question has been a hard nut to crack.
As interesting and puzzling as it is, it constantly finds its way to all facet of life. Today we
are going to see its occurrence in the world of routing cisco BGP technology especially when
we are constrained from using floating route and are required to neighbor using the loopback
interface of the routers.
From the figure below,
Our attention is on router 1 and 8 as we will be using both as EBGP neighbors, and in
adhering to the rule of KISS(Keep It Simply Stupid),we will be making use of AS 18 and 81
respectively.
Our constraint:
 Pair using only loopback interfaces
 Don’t use any static route
 Advertise the loopbacks into BGP
With this in mind, we know from our CCNP/CCIE classes that this neighbor relationship is
going to be EBGP.
Here is the configuration figure 1.

2. We can easily glean from the above(Figure 2) config that the EBGP state is idle.
Now going back to the fundamentals, BGP needs transport on port 179 to form neighbor.so
lets verify that transport is okay, ie router 1 can reach router 8 loopback and vice-versa.

3. From the ping above(Figure 3) it is obvious that transport is broken though we can ping the
directly connected links but not loopbacks, but we need the neighbor to form on
loopbacks,that is loopback must be reachable from both routers.
Solution 1:
 Create a static route to loopbacks #not allowed by the constraint
 Run routing protocols # OSPF,EIGRP,RIP,ODR I will chose EIGRP
EIGRP configuration # here I will use the AFI/Named format
Figure 4.

4. Figure 5 below

5. From above we can see that EIGRP neighbor is formed,hence lets verify if our eBGP
neighbor is established by running show ip bgp summary as below Figure 6.
Here our eBGP session between R8 and R1 is established.So far we are solid and haven’t
gone against any rule.Now lets advertise the loopbacks to BGP figure 7 below
Now lets see the routing table as below figure 8

6. From the table above (figure 8),hope we can envisage a disaster here in the BGP routes?As
the prefix 8.8.8.8/32 is now learned as a BGP route as opposed to EIGRP in figure 6 above.
Here we see the eBGP neighbor go down and immediately come up(flap).this behavior
continues ie the neighbor makes and breaks.This is phenomenon is known as recursive
infinite loop.
Now I will let you think of it and come up it a solution, though there are multiple solutions to
it.