When making posts here or on any other forums, you have to be more specific when presenting problems like this. Do you have a specific network topology that you’re working with? What is your current EIGRP configuration, what specific adjacency problem do you have? Are the peers not exchanging routes or not forming neighborships at all?
Make sure to provide the information mentioned above so we can further help you resolve your issue.
My problem is that even if I issued router eigrp as no. On both the routers no hello messages are sent as described in the neighbour Adjacency part by you. The router start sharing hello messages as soon as I inject the routes. Why is that??
Issuing the router eigrp x command creates the EIGRP process but it does not enable EIGRP on any interfaces so no hello messages are sent thus no neighborships are formed. If you want routers to become neighbors and exchange routes, you must activate EIGRP on the interfaces that connect the routers together by using the network command.
The network command does the following two things:
Advertises the networks that fall within this range in EIGRP. If an IP address configured on an interface falls under the network command, EIGRP will be activated on that interface and the network on that interface will be advertised.
Activating EIGRP on the interface(s) means that the router will start sending hello packets out of them and form peerings with the neighboring routers if the conditions are right (each routing protocol has different parameters that need to match). After an adjacency is formed, the routes are exchanged.
I’ve moved the conversation to the thread corresponding to the lesson you’re talking about. @davidilles has done a good job of responding to your initial questions and I concur. Concerning your final question, you may not see this behavior with the loopback depending on the IOS version and/or emulator you are using.
Strictly speaking, any interface that has a network which is included in the network command of EIGRP should begin to send hello messages to attempt to create EIGRP adjacencies with any existing EIGRP neighbors.
As we know, loopback interfaces are virtual and will never have an EIGRP router connected to them. The IOS version used by Rene in this lesson, chooses to strictly stick to this behavior of EIGRP even though no adjacency can ever be created over a loopback interface. Some other operating systems opt to eliminate the sending of hellos on loopbacks, so it may be that your setup behaves in this way.
Ultimately, for the operation of EIGRP, the result is the same. So it’s important to understand the “why” for this behavior. But either way, EIGRP still functions correctly. Does that make sense?
