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mbruno

join:2003-07-03
Fruitland, MD

[CCNA] CCNA Help for Test question

Greeting everyone,

I have a question about routing (Layer 3) and how it handles a request destin for a remote network. I seem to be a bit confused on the topic and would like for some help with it before i take my test on Friday.

So here is the scenario: You have two pc host A on one network and host b on another network. If host A wants to send a packet to Host B and there are two routers (Router A connected to Router B, Host A connected to Router A and Router B connected to Host B). How does the process work?

I was under the understanding that the source address from Host A would change to Router A but the Destination Address would stay the same going to Host B? I say this because Layer 3 routing does change stuff and just checks the routing table to see if it knows about the route of the destination. Is this correct?

Or better yet Would the source address be Router B since router A is passing the packet to it? When Host B receives the packet and returns the packet would the destination address change to that of Host A and now the source address would be Router A being that Router B passed the packet to it to reach host A?

Any comments would be welcomed.

Thanks

jza80

join:2005-10-29
Sacramento, CA

1 edit
MAC address change, but source and destination IP do not.

As a packet(s) go through a router, the router changes the MAC address. This is because a router decapsulates a packet, examines the destination IP address, looks for a route in its routing table, recapsulates the packet, and sends it on to the next hop router.

Theres probably a few steps that I'm missing, but thats the basics of it. I know I read about it in one of my CCNA classes.

If you need a better explanation, let me know. I'll see if I can find the chapter in the book where it explains the process step by step.

ladino

join:2001-02-24
USA
kudos:1
HostA determines that it needs to sends traffic destined for HostB to RouterA. RouterA looks into its route table & determines that to get to HostB's network, it needs to send the packet to RouterB. RouterA then rewrites the destination MAC address to match that of RouterB. Note the IP address still remain unchanged. When RouterB receives the packet, it too looks into its routing table & determines that HostB's subnet is directly connected on one off its interfaces. RouterB then rewrites the packet's destination MAC address to match that of HostB & routes that packet out of its existing interface towards HostB. Untimately hostB receives the packet.

Note that all along the path the IP address remained unchanged, only the destination 'MAC address' kept changing to match the next hop where the packet was going.
I.E. the MAC layer address changes while the protocol(Network layer) address remains unchanged

mbruno

join:2003-07-03
Fruitland, MD
reply to mbruno
I thought I read some where that the (Layer 3) never changes the Frame information. I.E. destination "MAC address"? or is that the source information?

ladino:

Note that all along the path the IP address remained unchanged, only the destination 'MAC address' kept changing to match the next hop where the packet was going.
I.E. the MAC layer address changes while the protocol(Network layer) address remains unchanged

jza80

join:2005-10-29
Sacramento, CA
said by mbruno:

I thought I read some where that the (Layer 3) never changes the Frame information. I.E. destination "MAC address"? or is that the source information?
You have the layers confused.

IP address (logical address) is layer 3 (network layer).

MAC address (physical address) is layer 2 (data link layer).

.
.

Quoted from the book:

As a packet is forwarded from router to router, the Layer 3 source and destination IP addresses will not change; however, the Layer 2 source and destination data link addresses will change.

ladino

join:2001-02-24
USA
kudos:1

1 edit
reply to mbruno
"I thought I read some where that the (Layer 3) never changes the Frame information"....

You are right L3 address NEVER changes. The destination IP address(HostB) will always be x.x.x.x no matter what, similarly the source IP address(HostA) remains the same.
Only the L2 destination address will change as the packet hops from from router to router or even from switch to switch along the path. For a packet to move from one host to it adjacent host, it needs it neighbour's MAC address. Or simply put it needs to be 'switched' from one host to the next.
As jza80 noted, there seems to be confusion about the L3 & L2 definitions.
Expand your moderator at work


EBel

@eastlink.ca
reply to mbruno

Re: [CCNA] CCNA Help for Test question

I'm probably a little late to cache... in on this one. But I figure I can simplify this one for anyone who reads it later.

If you apply the OSI model to anything it will make things clearer. For example I want to go to Best Buy or Future Shop. Well which one, in what city.

The application layer determines what I want to buy.

The Presentation Layer is what brand.

The session layer is there because I may buy it over and over again. Like a network card. Each time I do it will be a new session.

Transport Layer will determine the first real networking concern. Where am I going to buy it. Future Shop or Best Buy they both sell the same stuff but they have different features and prices so I may choose something different but to me they are both the same. Future Shop and Best Buy are different applications but they both do the same thing so we will say they are port 80...

Network Layer is which store am I going to buy it in. Could be a location in Halifax, or one in New York.

The Data Link Layer is the different roads and sidewalks I'm going to use to get there

The Physical Layer is whether I'm walking, cycling, driving, taking a train, plane or bus.

So what does this mean. I can drive my car on the sidewalk, but it won't work very well.
No matter what I decide where I'm going to buy, based on a lot of information, but once I've committed that isn't going to change, this time. The different ways to get there mean I have to have different means. So even if I'm driving, there's construction (maintenance), accidents (collisions), traffic (bandwidth), poor road planning (lack of resources), one-way streets (protocol mismatches), there has to be many redundant physical links.

If it is too long to drive maybe I'll take a plane. That happens at the data link layer.

Those physical links need a standard to get there. Like road Mustang, sidewalk Nike/Reebok. Even if that changes the logical destination does not change.

Because there is more than one future shop or best buy (network) I need to decide that end goal. How I get there I turn streets all the time change modes of transit. But that goal doesn't change.

Just the links.

That's why the data link address only gets it through one part of the trip. The trip is all about getting to that destination.

I hope this helped. You can change the application over and over and over again and the analogy would change. But I really hope this one example makes it clear.

mbruno

join:2003-07-03
Fruitland, MD
reply to mbruno
Ok, so I took the part 1 of the 2 part test and passed part 1. Now I need to study for part 2 of the test. Thanks for clearing up some of the questions I had.