EIGRP DUAL Variance

CCNP ROUTE 642-902 Official Certification Guide

"Routes that are neither successor nor feasible successor can never be
added to the IP routing table, regardless of the variance setting"

I did a little lab

   --- r2 ---
r1 ----r3 ---- r5 loopback 5.5.5.5
    ----r4 ----

no variance

all-links
P 5.5.5.5/32, 1 successors, FD is 158720, serno 55
        via 10.0.0.1 (158720/156160), FastEthernet0/0
        via 20.0.0.1 (161536/158976), FastEthernet1/1
        via 30.0.0.1 (1790976/156160), FastEthernet1/0

topology
P 5.5.5.5/32, 1 successors, FD is 158720
        via 10.0.0.1 (158720/156160), FastEthernet0/0
        via 30.0.0.1 (1790976/156160), FastEthernet1/0

global-rib
D 5.5.5.5 [90/158720] via 10.0.0.1, 00:00:44, FastEthernet0/0

the reason we choose f1/0 path as FS over f1/1 is due to the AD<FD of
successor holding true even if it has a worse FD - expected behaviour and
end result no load-balancing as no equal cost paths (max 4 equal cost paths
by default)

if I apply a variance of x2

all-links
P 5.5.5.5/32, 2 successors, FD is 158720, serno 52
        via 10.0.0.1 (158720/156160), FastEthernet0/0
        via 20.0.0.1 (161536/158976), FastEthernet1/1
        via 30.0.0.1 (1790976/156160), FastEthernet1/0

topology
P 5.5.5.5/32, 2 successors, FD is 158720
        via 10.0.0.1 (158720/156160), FastEthernet0/0
        via 30.0.0.1 (1790976/156160), FastEthernet1/0

global-rib
D 5.5.5.5 [90/158720] via 10.0.0.1, 00:00:53, FastEthernet0/0
                  [90/161536] via 20.0.0.1, 00:00:53, FastEthernet1/1

ok so we're now permitting the f1/1 path being installed as a viable
load-balancing destination, this shows me two things, first that dual will
permit a non-successor or non-feasible successor into global rib as viable
load-balancing paths and second dual compares the end-to-end FD for making
a decision on which path it would prefer to load-balance looking at the
all-links topology as f1/1 has a lower FD than f1/0

this does make the above argument invalid - im wondering if EIGRP has since
been enhanced for this DUAL behaviour? if so then I guess the whole point
for the statement in the first place was to avoid a loop i.e if you're not
a successor or feasible successor then you a loop, what has changed?

--
BR
Tony
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