All Cisco documents says it is not possible to create
EIGRP neighborship on secondary addresses.
Let test in our LAB and check if it
really a true statement.
R1:-
interface FastEthernet0/0
ip address
112.112.112.1 255.255.255.0 secondary
ip address
12.12.12.1 255.255.255.0
no shut
end
router eigrp 1
network
112.112.112.1 0.0.0.0
R2:-
interface FastEthernet0/0
ip address
112.112.112.2 255.255.255.0 secondary
ip address
12.12.12.2 255.255.255.0
no shut
router eigrp 1
network
112.112.112.2 0.0.0.0
Verification:-
R1#sh ip eigrp neighbors
EIGRP-IPv4 Neighbors for AS(1)
H Address Interface Hold Uptime SRTT
RTO Q Seq
(sec) (ms) Cnt Num
0 12.12.12.2 Fa0/0 12 00:28:42 80
480 0 7
We saw in our verification section that neighborship has
come up. But how it can be possible?
Neighborship is up because of the below two conditions:-
1. Network command enable Eigrp on the interface and it will not check whether it is of primary address or secondary. As soon as network command subnet under EIGRP process matches with any interface it start sending the hello packets using the primary address.
2. Both the neighbors must have primary address in same subnet.
Let’s have a look at the hello packets by enabling debug
on R1.
It shows that hello packets are generated using primary address as the
source IP of hello packets. It means hello packet will always use Primary
address as source.
EIGRP Packet debugging is on
R1#
*Apr 1
11:46:50.327: EIGRP: Received HELLO on FastEthernet0/0 nbr 12.12.12.2
*Apr 1
11:46:50.327: AS 1, Flags 0x0:(NULL),
Seq 0/0 interfaceQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/0
*Apr 1
11:46:53.439: EIGRP: Sending HELLO on FastEthernet0/0
*Apr 1
11:46:53.439: AS 1, Flags 0x0:(NULL),
Seq 0/0 interfaceQ 0/0 iidbQ un/rely 0/0
*Apr 1
11:46:54.803: EIGRP: Received HELLO on FastEthernet0/0 nbr 12.12.12.2
*Apr 1
11:46:54.807: AS 1, Flags 0x0:(NULL),
Seq 0/0 interfaceQ 0/0 iidbQ un/rely 0/0 peerQ un/rely 0/0
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