Mismatched IP addresses PPP?

Llukman1

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 know everything starts from the application layer and down so how does LCP know that it needs to start communicating with the other side router?
Please see link picture of what I am talking about. Pages 234-235https://books.google.com/books?id=hP6zDQAAQBAJ&pg=PA234&lpg=PA234&dq=mismatch+IP+address+lammle&source=bl&ots=uY2L4y4QKW&sig=0xGqnuzdxAf2Pej32yLv9z83F4A&hl=en&sa=X&ved=0ahUKEwjlkLDa9sfRAhVLw2MKHfDOBYYQ6AEIJzAD#v=onepage&q=mismatch IP address lammle&f=false

clarson

well ppp is a layer 2 protocal it isn't routed so ip address is irrelevant.

http://www.tcpipguide.com/free/t_PPPLinkControlProtocolLCP.htm

Llukman1

I know that but how are the two routers determining their layer 2 status and layer 1 does this happen before they begin sending data to each other? Is this the LCP's job to establish this connection and once it is established then data can begin transmission.

FullofBit

Llukman1 wrote: »

I know that but how are the two routers determining their layer 2 status and layer 1 does this happen before they begin sending data to each other? Is this the LCP's job to establish this connection and once it is established then data can begin transmission.

To answer your question, Yes one of LCP's jobs is to establish a connection. The routers are determining their Layer 2 status with PPP/LCP. LCP "data" is sent using PPP frames. So technically they are sending "data" to each other while determining their Layer 2 status. Unless you meant application layer data? In which case, they would still need to bring up layer 1/layer 2 before being able to forward application layer data to each other. http://ecomputernotes.com/images/LCP-Packet.jpg

Also, based on your original post, I think that there's a bit of a misunderstanding:

Because I know everything starts from the application layer and down

This statement is true once the link is established. When the link is established and application layer data is being sent, it is sent down the stack where each lower layer adds its headers onto the data. However, when the link is coming up, you are working up the stack starting from the physical layer on up.

so how does LCP know that it needs to start communicating with the other side router?

My guess is when the router sees that there is electrical connectivity on the wire. Maybe from the clocking signals. Sorry, but I don't exactly know what the actual mechanism routers use on serial links to determine if there is any physical layer activity. But once the router sees these electrical signals, it knows that it is physically connected to something and can start LCP negotiations.

I am trying to understand how the line protocol and serial 0/0 are both up when the IP address are in different subnets?

Clarson is correct that the IP addresses are irrelevant for PPP. In the output of show int S0/0, "Serial 0/0 is up" generally refers to the Layer 1 status, and "Line protocol is down" generally refers to the Layer 2 status. *Typically* if the link is "up/up" and you are are having connectivity issues, it's most likely a Layer 3 problem. As far as PPP goes ... PPP is weird. It allows both sides of the link to have IP addresses in different subnets. A router will actually create a host route for the other router's IP address. You should see a route in the routing table for the other router's IP address in the output of show ip route.

P.S. I hope that you're not just reading the sample book and that you actually have the physical book.