Mismatch IP addresses PPP for CCNA?

[llukman.halimi]

Hello,

I am trying to understand how the line protocol and serial 0/0 are both up when the IP address are in different subnets? what I am not understanding is how do these routers communicate with each other without using their IP addresses to find out that both sides are okay for layer 1 and layer 2? I have attached the book page that I am reading? 

And also how does LCP know so start a connection with another side? because I am confused at how encapsulation happens from the upper layer data all the way down to the physical layer and don't know how this is happening?

Please see attached picture of what I am talking about. 

[Richard Burts · Accepted solution]

I am not seeing the picture that you mention but believe that we can answer your question without needing that picture. The first part of the explanation is to be sure that we understand that the layers of the network model operate independently of each other and that operation at any layer depends on successful operation of the layers below it but does not need operation of layers above it..

With that in mind let us think about what happens at the various layers as the serial interface comes up.

First the layer 1 has to work and to determine whether it can successfully send and receive bits on the wire.

Once layer 1 is working OK then layer 2 starts. It checks on connectivity to the other end and checks on whether it is able to encapsulate properly and to receive the correct encapsulation from the remote side. If one side is encapsulating in ppp and the other side is encapsulating in HDLC then the link will not come up and nothing at this layer or higher layers can work. If the encapsulation is correct (in this case both sides are using ppp) then it begins the layer 2 negotiation of ppp (LCP etc). If the layer 2 negotiation is successful then both sides will be able to process ppp  keepalive packets and at this point the interface will show as up/up.

Note that it does not matter what IP address is configured until the device gets to layer 3 operation and the interface status (up/up, up/down, down/down, etc) is already established.

HTH

Rick

[llukman.halimi]

Thank you Rick for you well explained answer!

[Richard Burts]

I am glad that my explanation was helpful. Thank you for using the rating system to mark this question as answered. This will help other readers of the forum to identify threads that have helpful information.

HTH

Rick