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  1. Senior Member notgoing2fail's Avatar
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    #51
    Quote Originally Posted by fly351 View Post
    They might be loosely thrown around, but at the same time... if someone is working on their CCNP I hope they can understand the difference I can see where someone would get confused at the CCENT/CCNA level, but if you can grasp the different encapsulations then it shouldn't be to confusing.

    Absolutely. I think that is actually why CCIE's/video/book authors loosely throw the terms around because THEY know what they are really talking about, and if you're a CCNP wanna-be reading their books, you should too.

    The up and coming engineers may get slightly confused.
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  3. wino burbankmarc's Avatar
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    #52
    Routing involves a RIB lookup where switching is cache based. There seems to be a fair amount of over thinking going on here.
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  4. The Bringer of Light DevilWAH's Avatar
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    #53
    [QUOTE=notgoing2fail;425878]Absolutely. I think that is actually why CCIE's/video/book authors loosely throw the terms around because THEY know what they are really talking about, and if you're a CCNP wanna-be reading their books, you should too.
    [QUOTE]

    I think the real main reason, is that many of the terms such as a Switch and a Router are so out of date now that it becomes inpossible to use them.

    Things like the OSI model as well are really just guides and there are so many exception to the rules.

    I mean surely a router should also be called a fire wall as it can filter packets based on policies, and what about fire walls that can do routing?

    What I mean is at what point does a router become termed as a firewall or a fire wall termed as a router?
    • If you can't explain it simply, you don't understand it well enough. Albert Einstein
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  5. Senior Member notgoing2fail's Avatar
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    #54
    Quote Originally Posted by burbankmarc View Post
    Routing involves a RIB lookup where switching is cache based. There seems to be a fair amount of over thinking going on here.
    Just trying to have a healthy debate....

    Quote Originally Posted by DevilWAH View Post


    I think the real main reason, is that many of the terms such as a Switch and a Router are so out of date now that it becomes inpossible to use them.

    Things like the OSI model as well are really just guides and there are so many exception to the rules.

    I mean surely a router should also be called a fire wall as it can filter packets based on policies, and what about fire walls that can do routing?

    What I mean is at what point does a router become termed as a firewall or a fire wall termed as a router?

    Yeah this is true. The ISR's have really changed what is a router, what is a firewall and what is a switch....oh, and what is an IPS!!

    I do know that it also comes down to how the device deals with the packets/frames.

    For example an old school router will forward packets done at the software level so this is true forwarding and more CPU intensive.

    But since "ip route cache/IP CEF", now you've got FIB tables that are clone copies of routing tables. So the first packet goes to the route processor, and all ensuing packets are now switched at the hardware layer.

    I suppose this thread should really go into the CCNP section but I think it's healthy to talk about for new CCNA's.

    Is it overthinking/nitpicking with some OCD mixed in? Sure!!

    But why not discuss and see what other people think? That's why I created this thread....I would love to get CCIE point of views on this....
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  6. Senior Member alan2308's Avatar
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    #55
    Quote Originally Posted by notgoing2fail View Post
    Is it overthinking/nitpicking with some OCD mixed in? Sure!!
    That's what we do here.
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  7. Senior Member notgoing2fail's Avatar
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    #56
    Quote Originally Posted by alan2308 View Post
    That's what we do here.

    There was a VLAN lab that I did recently, haven't finished it yet, but boy, it totally wakes you up as to your knowledge of VLAN's....

    Just when you thought you knew....
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  8. Senior Member alan2308's Avatar
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    #57
    Quote Originally Posted by notgoing2fail View Post
    There was a VLAN lab that I did recently, haven't finished it yet, but boy, it totally wakes you up as to your knowledge of VLAN's....

    Just when you thought you knew....
    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.
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  9. Cisco Guru mgeorge's Avatar
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    #58
    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.

    When thinking of stuff like this you really have to reference the OSI model (yeah that annoying ass 7 layer piece of paper that you'd thought you'd never use in real life) haha...

    First off we all know layer 3 uses packets as the pdu, layer 2 is frames and layer 1 is the bits... (keep in mind 8 brownie bits equal a brownie byte) haha. I could totally go for a double chocolate fudge brownie right now, anywho...

    You have to keep in mind that the OSI model is like the harmony in the matrix, one layer dependent upon another layer and shall one fail the entire matrix will fail haha.

    Any PC sending traffic to another PC on the same network is going to be in some form or fashion switched (unless you use a hub). If its the same subnet then the PC will ARP for the mac address of that destination IP and place it in the DST field of the frame and send it on the wire to the switch. However if the PC knows that the destination IP address is not in its local subnet it sends the packet out the wire with the mac address of its default gateway.

    This is all common knowledge. 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)

    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.

    Layer 2 switches switch the frames based in src and dst mac addresses. Layer 3 switches basically combine layer 2 and layer 3 functions in a single box obviously, in this case any PC sending traffic to a PC on a different subnet forwards the frame out the PC's NIC through layer 1 with the dst mac address of the switches RP. Once the RP does its thing, it hands it back down to the SP then gets switched out the specific interface where the dst mac address is located at

    As far as the terms go, if you understand the operation of the OSI model and the technology that Cisco uses to accomplish the requirements of the OSI model put forth by the International Standards Organization then you should not have a problem with understanding this stuff ^_^

    Spanning tree in and of its self is used to prevent multiple active physical/logical links between two devices on a common layer 2 network segment by blocking the forwarding path of one link to break the loop.

    Switching - when a frame is switched from one interface to another on a switch based on source and destination mac addresses.

    Forwarding - This term is loosely used to describe an action taken by a device to forward the traffic it receives along the transit path from the source to the destination regardless of how the traffic is processed.

    CEF Switched - This differs from Layer 2 switching. The router keeps a mirrored copy of the RIB (routing information base; aka: routing table) and calls it the FIB (forwarding information base; aka: cef table) and uses the CEF table as a layer 3 switching table so when traffic enters the router, if it is not process switched it is cef switched. The router will look up the destination ip in the cef table and forward it out the attached interface.

    As an instructor it amazes me to see how many people just memorize the OSI model and what happens at each layer and goes on with their happy network engineering life without understanding how each layer inter-operates with each other. The comprehension of the OSI models lower layers if critical to a fundamental understanding of basic network operations.

    Sorry for the long reply, just felt the need to rant...
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  10. Cisco Moderator mikej412's Avatar
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    #59
    Quote Originally Posted by mgeorge View Post
    double chocolate fudge brownie
    This is all I saw in your post.....













    Just kidding -- nice post
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  11. Cisco Guru mgeorge's Avatar
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    #60
    Haha!!! I knew that was coming from someone, I was waiting for that reply...

    The thought alone is mouth watering isn't it?

    Although I forgot to add the sprinkles... Gotta have the sprinkles ^_^

    There is a great book in the Google Books library that discusses the different types of switching; check it out if you want an in-depth analysis of all the crazy operations of switching types.

    CCIE practical studies, Volume 2 - Page 308
    Last edited by mgeorge; 07-02-2010 at 07:41 PM.
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  12. wino burbankmarc's Avatar
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    #61
    Quote Originally Posted by mgeorge View Post

    There is a great book in the Google Books library that discusses the different types of switching; check it out if you want an in-depth analysis of all the crazy operations of switching types.

    CCIE practical studies, Volume 2 - Page 308
    This link has info on the different switching types too, except for flow based switching since it isn't used anymore:

    Cisco IOS Switching Services Configuration*Guide, Release*12.2 - Cisco IOS Switching Paths Overview [Cisco IOS Software Releases 12.2 Mainline] - Cisco Systems
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  13. Senior Member notgoing2fail's Avatar
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    #62
    Quote Originally Posted by alan2308 View Post
    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!!!!


    Quote Originally Posted by mgeorge View Post
    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?


    Quote Originally Posted by mgeorge View Post
    (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...


    Quote Originally Posted by mgeorge View Post
    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! =)


    Quote Originally Posted by mgeorge View Post
    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?



    Quote Originally Posted by mgeorge View Post
    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....

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  14. Cisco Guru mgeorge's Avatar
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    #63
    Quote Originally Posted by notgoing2fail View Post
    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!
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  15. Senior Member notgoing2fail's Avatar
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    #64
    Quote Originally Posted by mgeorge View Post

    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.
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  16. Senior Member alan2308's Avatar
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    #65
    Quote Originally Posted by notgoing2fail View Post
    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
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  17. Cisco Guru mgeorge's Avatar
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    #66
    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...
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  18. Senior Member notgoing2fail's Avatar
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    #67
    Quote Originally Posted by alan2308 View Post
    I'm pretty sure this is it:

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


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



    Quote Originally Posted by mgeorge View Post
    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.

    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....
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  19. wino burbankmarc's Avatar
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    #68
    Quote Originally Posted by alan2308 View Post
    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.
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  20. The Bringer of Light DevilWAH's Avatar
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    #69
    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.
    Last edited by DevilWAH; 07-02-2010 at 11:00 PM.
    • If you can't explain it simply, you don't understand it well enough. Albert Einstein
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    #70
    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.
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  22. Senior Member notgoing2fail's Avatar
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    #71
    Quote Originally Posted by Selfmade View Post
    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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