Specific OSI questions help..

I've run into some problems trying to figure out the OSI model. I'm sure I'll run into more, but I'd like to use this thread to asks the ones I have on the top of my head for now so...

Questions Regarding the OSI model

In my Sybex CCNA book it says

"For a host to send packets to individual hosts on a local network as well as transmit packets between routers, the Data Link Layer uses hardware addressing. Each time a packet is sent between routers, it's framed with control information at the Data Link layer, but that information is stripped off at the recieving router and only the original packet is left completely intact. This framing of the packet continues for each hop until the packet is finally delivered to the correct receiving hots. I'ts really important to understand that the packet itself is never altered along the route; it's only encapsulated with the type of control information required for it to be properly passed the different media types.

From reading that I get the impression that when a Frame gets to a router, the frame is stripped off and a new one is added that tell it it's next hop...

Am I interpreting this correctly? And it doesn't mean the whole frame as added by the the Data Link layer does it? I mean the (MAC address) for the final destination I would think would stay on there the whole time yet it says "that frame information is stripped off" so...

Edit: figured out my LLC question, looking at it the LLC is just like the Transport layer

Would anyone disagree with that analogy?

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Comments

wat08

Disarray wrote:

looking at it the LLC is just like the Transport layer

dynamik

Maybe this will help you out:

iworms

MAC address is for local connections, so there is no MAC address for the final destination. When you send a packet to a remote address, your destination IP address is the remote IP address, but your destination MAC address is the next hop node.

MAC information changes during every hop between routers. A frame is roughly frame header + frame content (upper layer packet) + frame footer, or you can call the content "payload" and the header and footer "overhead." What gets "stripped" and replaced by a router is the overhead. But then it's no longer the same frame, so you can say the old frame ends at the router and the payload gets put into a new frame.

Disarray

Thanks Iworms, brain fart on my part :P

But, you clearing that up brings up another question

The LLC sublayer adds it's own header that tells the recipient what Network Layer Protocols
Service access point to send the packet through.

Why would that part of the frame be removed through all the hoping, right now I'm under the impression that part at least is for the final destination.

Dynamik that was all already pretty clear to me, but thanks anyway it's reaffirming and nice to have a diagram of. Self education always carries the risk of misinterpretation which is why I plan to ask a bunch of questions

Wat08 what I meant by the LLC and Transport layer being similar is.

The LLC can do

flow control
sequencing
uses one of two methods.. Connection or less
acts as an interface between Mac and Network (as opposed to upper and lower layers)

and so on, of course theirs differences I said it though because they seem to have more in common then any other two layers in the OSI...

iworms

The book and I were referring to something far simpler than what you had in mind:

The MAC addresses and the checksum definitely get changed. I'm not sure how LLC gets handled. I'm not even sure whether common LAN Ethernet frames today have LLC information. To me, LLC is one of those things that we just need to know what it stands for and a vague idea of what it is. The details are more engineering oriented. Of course, it won't hurt to dig deep.