EIGRP on secondary IP address?

[fjcardenas-1]

Hello,

According to the book.... One of the requirements to establish a neighborship is that the Primary IP address on the interface must be on the same subnet (not a secondary IP).

However, as a test, I configured an the interfaces with a primary IP address in subnet 200.0.0.0/24 and a secondary IP address in subnet 10.0.0.0/24. Then I applied the network command only for 10.0.0.0 0.0.0.255 (in both ends of the link of course), and I do see the routers become neighbors. So, I do not clearly understand why the book says it is a requirement that the the primary IP address must be on the same subnet..... can someone explain if this is an inaccurate statement or clarify the point? Thank you.

interface FastEthernet0/1

ip address 10.0.0.1 255.255.255.0 secondary

ip address 200.0.0.1 255.255.255.0

duplex auto

speed auto

!

router eigrp 10

network 10.0.0.0 0.0.0.255

auto-summary

Router#show ip eigrp neighbors

IP-EIGRP neighbors for process 10

H   Address                 Interface       Hold Uptime   SRTT   RTO  Q  Seq

                                            (sec)         (ms)       Cnt Num

0   200.0.0.2               Fa0/1             13 00:08:22 1274  5000  0  3

[Alessio Andreoli]

The book is right. The author does not say that you can't form a neibourship between secondary subnets... If you would enable BOTH primary and secondary subnets you would observe that the primary takes over and form an EIGRP adjacency. In your config you did not give a chance to the primary subnet to exchanges information with the other router.


Hope this helps
Alessio

Sent from Cisco Technical Support iPad App

[jesusmeza192168]

When you configure the network statement with the secondary IP, EIGRP looks for any interface with that network. It finds f0/1 and actives EIGRP on the interface. If you look at the hello packets being sent you will see that the source address comes from the primary IP address and not the secondary. It is not possible to establish EIGRP adjacency between routers with the secondary IP but you can turn on the EIGRP process on an interface with the secondary IP. Remember in EIGRP process is turned on in a per interface basis not per network basis.