Home > CCIE, EIGRP > EIGRP on secondary interfaces – quiz

EIGRP on secondary interfaces – quiz


When we configure EIGRP on interfaces that are not on the same subnet we will get a message from EIGRP telling us this. What happens if we configure EIGRP on secondary interfaces? Try this setup with two routers:

R1:

Primary IP: 163.19.1.1/24
Secondary IP: 163.19.11.1/24

R2:

Primary IP: 163.19.1.2/24
Secondary IP: 163.19.22.2/24

Then you enable EIGRP and run network 163.19.11.1 0.0.0.0 and 163.19.22.2 0.0.0.0 respectively on each router. Will they be able to form an adjacency? Post your answer in the comments section.

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Categories: CCIE, EIGRP Tags: ,
  1. adham
    July 19, 2011 at 8:48 pm

    Yes they will be adjacent ..
    to form adjacency u need two conditions:
    1- same subnet …Eigrp run from primary addresses and they are in same subnet 163.19.1.0
    2- enable process in interfaces…seconadry address under eigrp process will enable it.

    so will be adjacent.

  2. Gabriel
    July 19, 2011 at 9:52 pm

    I admin I thought “No, they won’t” and even found a page on cisco.com (kinda) supporting this answer. But I also remembered that I read somewhere that there’s a quirk. And then I found it. 🙂

    http://routing-bits.com/2010/11/27/eigrp-adjacency-using-a-secondary/

    • July 19, 2011 at 9:55 pm

      Well done guys.

      Like Adham said the network statement is only used for enabling EIGRP on an interface, it does not matter if IP is primary or secondary. Also EIGRP packets are always sent from primary IP so adjacency will form. Kind of a corner case but we never know what might be tested at the lab 🙂

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