Communication Processes

rakemrakem Member Posts: 800
Ok so im a week out from doing the CCNA exam there is one type of question that is bothering me... i dont have a specific example but they are the type that have a network setup with two routers directly conected, a switch at each end and a host connect to each switch, the questions are generally like:

hostA is transmitting to hostB, what will the source / destination addresses be in the data header

and they give you answers like

the MAC address of the ethernet interface of the switch
the MAC address of the destination
the IP address of ther source
the IP of the destination
blah blah.

I have been looking through my books for details on this, does anyone have any links or ideas where i can find info of question like this, or can anyone explain it to me?

I have Sybex CCNA 5th edition (todd lammie)
and CCNA ICND self study guide
and Exam cram 2.

if you know of anywhere in those books can you point me to it or somewhere on the cisco site... anything please!!!!
CCIE# 38186


  • david_rdavid_r Member Posts: 112

    Here is how I understand this process.

    First thing to remember: src/dst IP addresses do not change.
    Host A generates a packet for delivery to host B of the format
    src IP src MAC dst IP dst MAC
    The packet will look like this A:A:A:A C:C:C:1
    The router will receive the packet and send it to The packet will look like this. C:C:C:2 D:D:D:1
    The other router will receive the packet and send it to the host as D:D:D:2 F:F:F:F

    A return packet will look like this: F:F:F:F D:D:D:2 D:D:D:1 C:C:C:2 C:C:C:1 A:A:A:A

    To understand why this is happening, we have to look at the OSI model and the encapsulation/decapsulation that happens.
    When host A creates the packet, it checks it's arp cache for the MAC of the default gateway. IOW, layer three hands the packet to layer 2 and says, "Here is the MAC this needs to get to. Get it there!" Layer 2 encapsulates the src/dst MAC addresses and hands the frame to layer one for delivery to the wire.

    When the switch receives the packet, it is not doing any decapsulation/enacapsulation. It receives a stream of bits that make up a frame. It reads the dst MAC address of the frame and moves it to the appropriate outgoing buffer. Thus, the MAC addresses don't change.

    When the router receives the packet, it must deencapsulate the layer 2 packet for layer three to read the destination address. It checks it's arp cache, finds the proper mac for the next hop and reencapsulates the packet. When it does this, it's MAC address becomes the src MAC for the packet and the MAC address of the next device (the next router) becomes the destination address.

    When the second router receives the packet, it must once again encapsulate it, read the layer 3 address and encapsulate it with the new SRC/DST MAC addresses.

    Do you see a common theme here revolving around layer 3 devices? Do you also see that layer 2 devices don't change anything in the packet?

    Anyway, that's how I understand the process to work. If I'm off-base, somebody will be along shortly.
  • rakemrakem Member Posts: 800
    thanks for that!

    i didnt know that the source and destination IP addresses wouldnt change, and that it is just the MAC addresses that are stripped off and then changed.

    thanks alot again!
    CCIE# 38186
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