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How to Describe the Routing Process, and the Switching Process

Discussion in 'Networks' started by Coupe2T, Apr 3, 2012.

  1. Coupe2T

    Coupe2T Megabyte Poster

    Hi All,

    As a part of my ongoing internal training at work (Basically I have been given a switch, a router and 2 books and left to my own devices), I have been asked to prepare something for our Network team manager.

    This has come somewhat out of the blue, and I have spread my reading etc across quite a space of time so need to make sure i'm getting my details right here and not missing anything out that I should be including.

    I have been asked to describe the routing process and the switching process. I will copy paste what I have below, please ignore grammatical/spelling erros etc, not proof read yet, just wanted to make sure the general info was correct etc. I also wanted to make sure I was capturing the differences between the two correctly.

    Routing Process
    I will assume the routing process starting from a Client PC, which has already learned it's Subnet Mask, DNS and Default Gateway details etc via DHCP. This client PC wishes to transmit information to another networked device outside of it's own Subnet. Therefore the process would essentially be as follows:

    The packet will be encapsulate into a frame with the source and destination MAC addresses for the two devices, using ARP to obtain this if not already in the clients ARP cache.

    The Frame will then be sent to the Default Gateway (ie the Router, all be it via a switch, but I am skipping this point as it will be covered in the Switching Process section.), which will then perform the FCS check to ensure that the frame is good.

    Once the FCS is cleared, The router will check the Frames destination and source datalink addresses (MAC Addresses, level 2), and if the Destination MAC address is that of the Router, it will process further, else discard.

    Assuming the MAC address is that of the router, it will remove the frame header/trailer and look at the IP packet. It will then compare the IP destination to it's routing table to find the next destination/hop. This will be based on the longest prefix that matches the IP address, assuming there are more than one matching subnet routes known to the router. (I include this part about longest prefix as I know we use mostly static routing here, which I understand can often lead to more than one matching entry).

    It will then use the details from it's routing table to re-encapsulate the IP packet into another frame, with the source and destination MAC addresses and forward it out the relevant interface as held in the routing table to the next networked device.

    Process essentially then repeats to destination segment, as Routers essentially connect several smaller LAN sections into a greater WAN topology.

    Switching Process
    The switching process differs to the Routing process in that they generally work at layer 2 of the OSI model only (Excluding multi layer switches). This means that they essentially don't care about IP addresses, and deal only with MAC addresses of hosts.

    So if the same Client as above wanted to send information within the same LAN segment, it would send an ARP request for the MAC address of the respective client, before forwarding the packet out on the line, where it would reach the Switch. The switch would then look at the destination MAC address and compare it to it's MAC address table.

    If the MAC address is known in the MAC address table, it will forward the packet out of the relevant interface listed in that table. If however it is not known in the MAC table, then it will flood it out of all ports Except for the one in which it was received. This is assuming that VLAN's are not in place, in which case it will forward it out of all ports within the same VLAN.

    At the same time as forwarding frames, the Switch also examines the Source mac address of all frames coming into all interfaces, comparing them to it's MAC address table and if an entry does not already exist, or is incorrect, then it will update the table to show the interface attached to this new or update the table to correct the current details for where that MAC address resides. All MAC address learning on a switch is done by checking the Source MAC addresses on frames entering the switch.

    If the destination MAC address is seen to be off of the same Interface in which it arrived, then the Switch will simply filter the frame out and ignore it.


    Is the above about right? This is how it basically works from best I can remember, but not sure if I need to brush up slightly on certain aspects of how the devices communicate and/or if there is any other information I should be adding and clearing up.
    Certifications: ECDL, Does that Count!?!
  2. danielno8

    danielno8 Gigabyte Poster

    In your routing process detail, i would highlight exactly which source and destination MAC will be added to the frame.
    Certifications: CCENT, CCNA
  3. Coupe2T

    Coupe2T Megabyte Poster

    Yep, I see your point. Could be clearer.

    Thanks for the input.

    Other than that, Everything look all ok and correct in your eyes etc?
    Certifications: ECDL, Does that Count!?!

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