EIGRP Secondary Address interesting behaviour

We all know that EIGRP can't form adjacencies over secondary addresses, as the primary ones will take over . However, what will happen when you have a config like this:

R1

interface FastEthernet0/0
 ip address 192.168.1.5 255.255.255.252
 duplex auto
 speed auto


router eigrp 1
 network 192.168.1.4 0.0.0.3
 no auto-summary

interface FastEthernet0/0
 ip address 192.168.1.6 255.255.255.252 secondary
 ip address 172.16.0.1 255.255.255.0
 duplex auto
 speed auto


router eigrp 1
 network 192.168.1.4 0.0.0.3
 no auto-summary

The result of show ip eigrp nei:

R1#show ip eigrp nei
IP-EIGRP neighbors for process 1
R1#

R2#show ip eigrp nei
IP-EIGRP neighbors for process 1
H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq
                                            (sec)         (ms)       Cnt Num
0   192.168.1.5             Fa0/0             12 00:01:07    1  5000  2  0

So, R1 has no active adjacencies but R2 thinks it has. Cool huh? Came across this one while preparing for my TSHOOT and just thought I'd share.

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Comments

TechGuru80

Quite interesting...a bunch of error messages showing 172 is not on the same subnet on R1 after trying it out.

fredrikjj

The key thing to look for is the "Q count" in the eigrp neighbor output. It basically says that EIGRP has messages that it hasn't received acknowledgement for. Some kind of underlying transport problem. Since it's 2 in this case it should give you a clue at least that something isn't right.