Packets or Frames

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  • notgoing2failnotgoing2fail Member Posts: 1,138
    alan2308 wrote: »
    One of the free labs Narbik Kocharians gave away was a lab on RIP that is supposed to take someone with CCIE level knowledge 8 hours to complete. Just looking through it is quite a humbling experience.

    Wow are you serious??? Is this workbook still available for free? I've got to see why it would take 8 hours for RIP!!!!

    mgeorge wrote: »
    Notgoing2fail; you're definitely on the right track with understanding switching/forwarding/cef and you're right although many engineers and authors loosely throw around the terms. Either because of their lack of intelligence or lack of motivation to explain the stuff in detail lol.

    Thank you. It's not that I want people to think I'm making a big deal about this, I know it comes off that way, and I guess I am a little, but it's kinda the same thing as if an engineer is describing the transport layer and saying how it adds port info to its frames.

    Then someone says, "oh don't you mean segment?"

    And the engineer says, "oh yeah segment, packets, frames whatever, same thing you know what I mean right?"

    Does that really change the way the engineer performs at work? I highly doubt it. I don't doubt for a second that he won't be able to configure a router our troubleshoot networking issues.

    But why not strive to use the right terms?

    mgeorge wrote: »
    (keep in mind 8 brownie bits equal a brownie byte)

    I have never heard that one before, LOL, I'm going to have to remember that one...

    mgeorge wrote: »
    When the frame is received by the router, because in order for a device to process incoming traffic it has to go back UP the OSI model, not just start at layer 3 like some people magically believe. It then examines the packet contained in the frame (kinda like a picture in a picture in a picture frame) and then checks out layer 4 and 5 for other ip services related stuff but ultimately determined rather or not that particular packet needs to be CEF switched or process switched. If it is CEF then the router references the CEF table (aka: forwarding information base) and then rp recalculates the CRC and then forwards the packet (yes forwards it as its switched via SW or HW ASIC's in the router) out a particular interface with the matching route prefix. If it is process switched for whatever reason such as ACL logging or policy based routing, NAT, etc... then the route processor process any policies configured for the traffic then look up the routing table and forward the traffic accordingly out a particular interface tied to the route.

    Keep in mind Cisco is becoming good at offloading process based functions via ASIC's, there are a lot of Cisco devices that do NAT, PBR and ACL processing using hardware. In this case the same functions still occur, its just done by a different processor (ASIC's) so to speak. In some cases where all HW based processing is used, the RP may not be used at all (which is Cisco's goal), traffic comes into a layer 3 switch, goes up and does its little dance with the ACL/PBR/NAT/CEF ACISs and gets shoved off the stage at the local county fair ho down dance. (meaning forwarded out the correct interface)



    <-- shorten for brevity ---> .....


    Beautifully written! Have we read the same CEF book? I'm not done yet, trying to juggle it along with my SWITCH studies.....Heck I don't even have to finish the book just reread your post! This is the gray area that gets blurry when you're introducing different types of devices that can perform the same procedures but differently. At the end of the day, no one really needs to know these details other than engineers.

    Marketing doesn't care (well a little), most clients/consumers don't care, they just want it to work. They don't care if routing is done at RP or ASIC. If they can keep surfing the Internet, they won't complain! =)

    mgeorge wrote: »
    A router performs functions of a layer 2 device even though its main purposes is layer 3/4. How else does traffic get from layer 3 back down to layer 2 then forwarded over a layer 1 medium? It's not magic and you cant blame it on the matrix either haha.


    Now you've got me thinking. Does the router encapsulate the packet with a frame before it hands it off to the switch? Or does it send it as a packet, the switch then encapsulates it into a frame?


    mgeorge wrote: »
    Sorry for the long reply, just felt the need to rant...


    Absolutely not. This is what I live for. This is why I put in so many hours. You've done a great job positioning your POV and I'm going to have to reread your post a couple more time but I do get the jist of it. It was very well thought out and written....

    I didn't know you are an instructor, that's definitely great to have one around here on the forum!!!


    BTW: Growing up, I loved getting Double Doozies from The Great American Cookie company. They were sooooo good! Now at my age, if I eat one, I'd have to avoid sugar and run 100 miles to make myself even again....

    bigbitedoubledooziemm.jpg

  • mgeorgemgeorge Member Posts: 777
    Now you've got me thinking. Does the router encapsulate the packet with a frame before it hands it off to the switch? Or does it send it as a packet, the switch then encapsulates it into a frame?

    Hmmm Maybe i should play Morpheus and answer your question with a question...

    ~sticks his arms behind his back wearing his long brown leather jacket looking at notgoing2fail~

    If a router does not encapsulate a packet back into a frame before sending it on the wire to a switch then how does the switch know how to switch it to its correct port/destination if it lacks a destination mac address?

    Hmmmm....

    By the way, that cookie looks great!
    There is no place like 127.0.0.1
  • notgoing2failnotgoing2fail Member Posts: 1,138
    mgeorge wrote: »

    If a router does not encapsulate a packet back into a frame before sending it on the wire to a switch then how does the switch know how to switch it to its correct port/destination if it lacks a destination mac address?


    Ok. So let's see, the router will then encapsulate the packet with a frame. It already knows the source and destination IP's based on some source "host" trying to communicate with another "host" on another segment. Assuming that ARP entries are accurate, it will encapsulate the frame with it's own source mac address of it's outgoing interface and insert the destination mac address for the destination host.

    The switch will now take that frame, with no modifications and switch the FRAME to the destination mac address/port based on what it's found in it's CAM table.

  • alan2308alan2308 CISSP, MCSA 2008, MCSA 2012, CCNA R&S, CCNA Security Ann Arbor, MIMember Posts: 1,854 ■■■■■■■■□□
    Wow are you serious??? Is this workbook still available for free? I've got to see why it would take 8 hours for RIP!!!!

    I'm pretty sure this is it:

    New Years News From Narbik!!!!! : 65403
  • mgeorgemgeorge Member Posts: 777
    In a nutshell your right.

    If the destination IP address is on a directly connected interface of the router it will replace the src mac with its egress interface mac address and destination ip stays the same. In this case the destination mac address is changed to the mac address of the destination ip address node.

    However if the router needs to forward the packet to another router, it places the dst mac address of the next hop in the dst field of the layer2 header.

    This occurs till the frame gets to a router that has the destination ip address as a directly connected interface. in which case it places the dst mac address of that dst ip address in the egress frame and forwards it out the egress port to a switch when in turn switches the packet according from the ingress port to the egress port matching the mac address destination.

    Viola...
    There is no place like 127.0.0.1
  • notgoing2failnotgoing2fail Member Posts: 1,138
    alan2308 wrote: »
    I'm pretty sure this is it:

    New Years News From Narbik!!!!! : 65403



    Excellent!! I'm gonna have to download it before it disappears....


    mgeorge wrote: »
    In a nutshell your right.

    If the destination IP address is on a directly connected interface of the router it will replace the src mac with its egress interface mac address and destination ip stays the same. In this case the destination mac address is changed to the mac address of the destination ip address node.

    However if the router needs to forward the packet to another router, it places the dst mac address of the next hop in the dst field of the layer2 header.

    This occurs till the frame gets to a router that has the destination ip address as a directly connected interface. in which case it places the dst mac address of that dst ip address in the egress frame and forwards it out the egress port to a switch when in turn switches the packet according from the ingress port to the egress port matching the mac address destination.

    Viola...


    Thanks for the confirmation. I was going to include the hop counts as well but then I got lazy. icon_rolleyes.gif

    I'm going to have to start using egress and ingress as well. It would completely clear up any possible confusion one may have.

    Check your PM....

  • burbankmarcburbankmarc Member Posts: 460
    alan2308 wrote: »
    I'm pretty sure this is it:

    New Years News From Narbik!!!!! : 65403


    Good stuff. I'm on the rip portion the TCP/IP vol 1 so this will be a good lab for the weekend.
  • DevilWAHDevilWAH Member Posts: 2,997 ■■■■■■■■□□
    surely the difference between switching and routing is.

    A Switch, switches individual items, each different destination address is independent from any other, and only if the switch has an exact match in its tables does it "switch" the packet other wise it floods.

    The logic of the process is, do have have an exact match for the destination address if yes what it the exit interface, if not flood.

    A router on the other hand "switches" packets based on matched criteria. Does it belong to this group, or this group or this group, if so what is the next hope. Even if it does not have a match then it still group all the unmatched packets together and deals with them in a set way (default route).

    Routing groups packets and makes is decisions at the group level, not on full exact address matches.

    I would say it has nothing to do with what hardware software carries out the process.

    up to a level switching is more efficient than routing if you can keep the switch table to a reasonable size, that is efficient to search at wire speed, and as most switch blocks are at most about 500 devices, then a switch's MAC (CAM) table is still small enough for it to work.

    On the other hand a organisation may have 20,000+ ip address, and to deal with this in the switch fashion would require every device have a complete table of all these, the core devices could never switch packets at wire speed. So the concept of routing was introduced, now this huge table can be summarised, and also split between device.

    In my mind that's the differenced between switching and routing, it my not be what books say, They are two different things, and have two different purposes in networking.

    The term Switch and Router these days do not nesseraly reflect which one of these processes a device carries out, in fact most do both. Neither does the layer of the OSI determine what process is being used, the old layer 2 switching and layer 3 routing idea is falling away.
    • If you can't explain it simply, you don't understand it well enough. Albert Einstein
    • An arrow can only be shot by pulling it backward. So when life is dragging you back with difficulties. It means that its going to launch you into something great. So just focus and keep aiming.
  • SelfmadeSelfmade Member Posts: 268
    NG2F, you know, you can think of it this way, you know you know your stuff when you can dissect something as minute as that and you can explain the difference.

    Don't you think you're overthinking it though lol.
    It's not important to add reptutation points to others, but to be nice and spread good karma everywhere you go.
  • notgoing2failnotgoing2fail Member Posts: 1,138
    Selfmade wrote: »
    NG2F, you know, you can think of it this way, you know you know your stuff when you can dissect something as minute as that and you can explain the difference.

    Don't you think you're overthinking it though lol.

    Not so much overthinking, just a way to get people's thoughts on how what seems to be a loosely way of wording L3/L2 terms.

    I'm not trying to create a RFC here to order engineers to ONLY use "frames" when speaking about layer 2 switching.

    I know a couple people that would dissect this topic alive, and I haven't brought it up to them yet because they have been terribly busy.

    But I'm really enjoying the back and forth comments on this topic.....

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