VLAN question

notgoing2failnotgoing2fail Posts: 1,138Member
Hey guys, I'm confused about something.

To make it simple:

Say you have one switch with 12 ports. That would be a possibility of 12 collision domains correct?

If that switch connects to a router. The link between that switch and router is NOT considered a collision domain.

1) Can the link between the switch and router be half-duplex?

I believe that if the hosts connected to the switchports are at full-duplex, then you DONT have a collision domain....only if you're connected at half?


I've seen questions like this before where they have hosts connected to a switch and you're suppose be able to tell how many CD's there are. I don't recall if they tell you if the hosts are connected at half or full...

This is why I'm a horrible test taker.... LOL....
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Comments

  • mikej412mikej412 Posts: 10,090Member
    Say you have one switch with 12 ports. That would be a possibility of 12 collision domains correct?
    If you hook up 12 devices -- yes.

    If you hook up one device and have 11 empty switch ports you have 1 collision domain.
    If that switch connects to a router. The link between that switch and router is NOT considered a collision domain.
    I count it as a collision domain.... I must have missed the CCNA Collision Domain Change memo icon_scratch.gif
    1) Can the link between the switch and router be half-duplex?

    I believe that if the hosts connected to the switchports are at full-duplex, then you DONT have a collision domain....only if you're connected at half?
    Anything connected to a switch port is still a collision domain -- but if you're running full duplex you should never get any collisions.

    You can configure a router interface to be half-duplex.... but you'd probably only do it if you were connecting to an old obsolete hub or switch with auto-negotiation issues.

    Um -- so where's the VLAN question?
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  • notgoing2failnotgoing2fail Posts: 1,138Member
    mikej412 wrote: »
    If you hook up 12 devices -- yes.

    If you hook up one device and have 11 empty switch ports you have 1 collision domain.


    I count it as a collision domain.... I must have missed the CCNA Collision Domain Change memo icon_scratch.gif


    Anything connected to a switch port is still a collision domain -- but if you're running full duplex you should never get any collisions.

    You can configure a router interface to be half-duplex.... but you'd probably only do it if you were connecting to an old obsolete hub or switch with auto-negotiation issues.

    Um -- so where's the VLAN question?


    Thanks Mike,

    I'm trying not to overthink the approach. I remember either in Odoms book or Jeremy from CBT nuggets mentioning that if you are connected at full duplex it's no longer a collision domain. I understand this from a technical standpoint, but I think if I get a question like this, I shouldn't go that deep and just consider it a collision domain...this is why I'm not good at test taking.

    As for the VLAN, LOL wow, it must have been so late I didn't even realize it!!!
  • danb83danb83 Posts: 22Member ■□□□□□□□□□
    Thanks Mike,

    I remember either in Odoms book or Jeremy from CBT nuggets mentioning that if you are connected at full duplex it's no longer a collision domain.

    I dont get that can you explain please?

    If it is no longer a collision domain then what is it?

    As far as I know, each switchport is its own collision domain?
  • hexemhexem Posts: 177Member
    It's not correct.

    every port is still a collision domain, the full duplex nature of the port just illiminates collisions from happening on that collision domain.
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  • notgoing2failnotgoing2fail Posts: 1,138Member
    boswd1983 wrote: »
    I dont get that can you explain please?

    If it is no longer a collision domain then what is it?

    As far as I know, each switchport is its own collision domain?


    Read what Hexem said, it's basically what he just described.

    But this confused me as well. Not what Hexem said but from what I read. I'll have to track down who said it.

    But I am 99% sure they said because of the nature of full duplex, you can't have any collisions, there for technically it isn't a collision domain.

    This is where it gets me, I don't want to get an answer on the exam wrong because I reading too deep into the question...
  • rwwest7rwwest7 Posts: 300Member
    Hey guys, I'm confused about something.

    To make it simple:

    Say you have one switch with 12 ports. That would be a possibility of 12 collision domains correct?

    If that switch connects to a router. The link between that switch and router is NOT considered a collision domain.

    1) Can the link between the switch and router be half-duplex?

    I believe that if the hosts connected to the switchports are at full-duplex, then you DONT have a collision domain....only if you're connected at half?


    I've seen questions like this before where they have hosts connected to a switch and you're suppose be able to tell how many CD's there are. I don't recall if they tell you if the hosts are connected at half or full...

    This is why I'm a horrible test taker.... LOL....
    I think where ever you read this they may have refering to the fact that a switch to router connection is considered two differant broadcast domains. A switch makes seperate collision domains, a router makes seperat broadcast domains.
  • hexemhexem Posts: 177Member
    Actually the switch to router connection is considered part of the same broadcast domain, or could be part of multiple broadcast domains if the connection is a trunk, you just think of routers as containing (not fowarding) broadcast traffic. p.s (by default) ;)
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  • rwwest7rwwest7 Posts: 300Member
    hexem wrote: »
    Actually the switch to router connection is considered part of the same broadcast domain, or could be part of multiple broadcast domains if the connection is a trunk, you just think of routers as containing (not fowarding) broadcast traffic. p.s (by default) ;)
    For the CCNA I'm pretty sure they want you to consider routers as creating seperate broadcast domains and switches as creating seperate collision domains. You can put a bunch of "ifs" in there, but that's what they want you to understand at this level. Or am I thinking of the CCENT exam? It all blends together at this point.
  • notgoing2failnotgoing2fail Posts: 1,138Member
    rwwest7 wrote: »
    For the CCNA I'm pretty sure they want you to consider routers as creating seperate broadcast domains and switches as creating seperate collision domains. You can put a bunch of "ifs" in there, but that's what they want you to understand at this level. Or am I thinking of the CCENT exam? It all blends together at this point.


    No I think you're right. At the CCNA level, I don't think they want you to over think the question.

    If you have 12 port switch, then you have to count those 12 ports as possible collision domains. One can argue the technical side if they are set to full duplex but that's how someone like me would start failing the questions. LOL....
  • hexemhexem Posts: 177Member
    The question's on the exam aren't like the questions people are finding on exam sims, most of the stuff is harder and worded more ambigious than what's going to be found on the exam, it's straight foward to the point where you know enough about a topic you can easily identifiy the correct answer.
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  • kalebkspkalebksp Posts: 1,033Member ■■■■■□□□□□
    To really understand what's happening you may want to consider why there are no collisions with full duplex. When two devices are directly connected via Ethernet (ie no hubs in between) the transmit wires of device 1 are directly connect to the receive wires of device 2 and vice versa. Therefor it is not physically possible for a collision to occur because the two devices will never attempt to transmit on the same wire.

    Full duplex doesn't eliminate collisions but rather the physical impossibility of collisions allows for full duplex. In the example above it wouldn't matter if the devices were set to full or half duplex, there still can't be a collision.

    In terms of collision domains, each switch port is a collision domain whether or not collisions are possible. It's just trivia, not applicable to the real world in most cases.
  • mikej412mikej412 Posts: 10,090Member
    If you have 12 port switch, then you have to count those 12 ports as possible collision domains.
    I've never seen a Cisco question that asks for "possible" collision domain.

    The collision domain is the link between the switch port and the device plugged into it.

    With this topology
    PC <--> 48 port 3500XL-EN Layer 2 Switch <--> PC
    There are 2 collision domains, not 48.

    A hub and all the devices plugged into it is 1 collision domain
    PC\
    PC->Hub
    PC/


    A hub and all the devices plugged into it plugged into a switch is 1 collision domain.
    PC\
    PC->Hub <-->Switch
    PC/
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  • notgoing2failnotgoing2fail Posts: 1,138Member
    mikej412 wrote: »
    I've never seen a Cisco question that asks for "possible" collision domain.

    The collision domain is the link between the switch port and the device plugged into it.

    With this topology
    PC <--> 48 port 3500XL-EN Layer 2 Switch <--> PC
    There are 2 collision domains, not 48.

    A hub and all the devices plugged into it is 1 collision domain
    PC\
    PC->Hub
    PC/


    A hub and all the devices plugged into it plugged into a switch is 1 collision domain.
    PC\
    PC->Hub <-->Switch
    PC/


    Thanks for clarifying it, that's what I mean initially, that the 12 ports would be occupied by hosts connected to it....


    I really wish I can remember where I got it from. It was either a CBT nuggets video by Jeremy or the Odom video....or it could just be another source, I can't keep track....
  • rage_hograge_hog Posts: 42Banned ■■□□□□□□□□
    In a full duplex communication data flow is bi-directional, data is sent and received at the same time. Thus, full duplex does not use CSMA/CD as half duplex does. No collisions. One of the points for using switches is they reduce the broadcast domains. The switch port AND the connected device must be in full duplex to avoid errors.

    "1) Can the link between the switch and router be half-duplex?"

    Yes, but why would you if you could avoid it? If it was then the whole link would be a collision domain.

    Cisco Catalyst switches have three settings:
    Auto, full, and half.
  • DevilWAHDevilWAH Posts: 2,997Member ■■■■■■■■□□
    rage_hog wrote: »
    In a full duplex communication data flow is bi-directional, data is sent and received at the same time. Thus, full duplex does not use CSMA/CD as half duplex does. No collisions. One of the points for using switches is they reduce the broadcast domains. The switch port AND the connected device must be in full duplex to avoid errors.

    "1) Can the link between the switch and router be half-duplex?"

    Yes, but why would you if you could avoid it? If it was then the whole link would be a collision domain.

    Cisco Catalyst switches have three settings:
    Auto, full, and half.

    No no no, A switch does not reduce the number broadcast domains!!!

    only the collision domains. for the reasons people have said.

    And as other people have said the questions in the exams are a mile away from the quests in the books. They don't try to trick you, they don't ask silly questions. Like You I would say I am a horrible test taker, But CISCO exams are different, in my view they are simpler because you are expected to understand the topics, rather than be able to recall lists of facts about the.
    You will do better to have a logical brain that can apply knowledge as it goes, than one that can remember details.
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  • rage_hograge_hog Posts: 42Banned ■■□□□□□□□□
    I don't think I can agree with that. However, nice use of the color red. True, all the answers are not in the book. Captain obvious.
  • mikej412mikej412 Posts: 10,090Member
    rage_hog wrote: »
    One of the points for using switches is they reduce the broadcast domains.
    This is the CCNA Forum -- a layer 3 router reduces the broadcast domain, not a switch.

    A layer 2 switch with VLANs and no router performing a layer 3 routing function is the equivalent of 3 separate networks -- which are 3 independent broadcast domains.

    PC --> Switch

    PC --> Switch

    PC --> Switch

    At some point the future CCNAs here will learn that each VLAN on a switch can be it's own broadcast domain, but that's after they've gotten past figuring out what a broadcast and collision domain are.

    If you connect 12 PCs to one switch with full duplex connections, while you probably won't have any collisions, as far as Cisco and the CCNA Exam is concerned it's 12 Collision domains.

    If you want to put each of those 12 switch ports into 12 separate VLANs -- and STATE THAT AS A QUALIFICATION TO A QUESTION -- then you would have 12 broadcast domains and 12 networks that CANNOT talk to each other. When you add a layer 3 router to route between VLANS/Networks, then you have 1 Network with 12 Broadcast Domains and 12 Collision Domains.
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  • DevilWAHDevilWAH Posts: 2,997Member ■■■■■■■■□□
    rage_hog wrote: »
    I don't think I can agree with that. However, nice use of the color red. True, all the answers are not in the book. Captain obvious.

    If you can't agree with it please explain how a switch limits broadcast traffic. The CCNA clearly states that switch's increase collisions domains and not broadcast domains (routers are the boundaries of a broadcast domain and VLAN's don't counts as a limiter of broadcast traffic, as they require layer 3 device to talk to each other.)

    I also did not say the answers are not in the book... I said the types of questions you will get asked are not in the book. By which I was saying that the format of an official CISCO exam is very different to the lay out of the questions that come with the study materials. To many people go on about getting 1000 out of 1000 in the test questions and then can't understand why they fail the exam.

    (in addition to what Mike said, I would say a switch with 3 separate VLANS, is in effect 3 separate switches as far as the logical network goes. This is why I don't consider vlan's as a true separator of broadcast domains, as like mentioned they require a layer 3 device to communicator together, if for example you linked two vlan's together using access ports, you could rejoin them in to a single broadcast domain again. But as Mike said this goes beyond CCNA level)
    • If you can't explain it simply, you don't understand it well enough. Albert Einstein
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  • rage_hograge_hog Posts: 42Banned ■■□□□□□□□□
    DevilWAH wrote: »
    If you can't agree with it please explain how a switch limits broadcast traffic.
    Are you serious?
  • rwwest7rwwest7 Posts: 300Member
    DevilWAH wrote: »
    If you can't agree with it please explain how a switch limits broadcast traffic. The CCNA clearly states that switch's increase collisions domains and not broadcast domains (routers are the boundaries of a broadcast domain and VLAN's don't counts as a limiter of broadcast traffic, as they require layer 3 device to talk to each other.)

    I also did not say the answers are not in the book... I said the types of questions you will get asked are not in the book. By which I was saying that the format of an official CISCO exam is very different to the lay out of the questions that come with the study materials. To many people go on about getting 1000 out of 1000 in the test questions and then can't understand why they fail the exam.

    (in addition to what Mike said, I would say a switch with 3 separate VLANS, is in effect 3 separate switches as far as the logical network goes. This is why I don't consider vlan's as a true separator of broadcast domains, as like mentioned they require a layer 3 device to communicator together, if for example you linked two vlan's together using access ports, you could rejoin them in to a single broadcast domain again. But as Mike said this goes beyond CCNA level)
    VLANs are a true seperator of broadcast domains. That's why they can't communicate with each other, because an arp request is a broadcast. And with out being able to arp than you need a router of some sort. Being able to communicate is not a requiremant for being seperate broadcast domains. And seperate broadcast domains don't have to be able to reach each other, in fact sometimes is desired that they don't.
  • hexemhexem Posts: 177Member
    Vlan = broadcast domain = logical subnet.
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  • mikej412mikej412 Posts: 10,090Member
    Given one of the standard questions about broadcast domains and collision domains with a diagram similar to the following:

    pc---hub---hub---switch---router---hub---switch

    How many broadcast domains and collision domains are in the above diagram?
    A. 1 Broadcast Domain and 1 Collision Domains
    B. 1 Broadcast Domain and 2 Collision Domains
    C. 1 Broadcast Domain and 3 Collision Domains
    D. 2 Broadcast Domains and 1 Collision Domain
    E. 2 Broadcast Domains and 2 Collision Domains
    F. 2 Broadcast Domains and 3 Collision Domains
    G. None of the above since no IP Addressing, STP information, VLAN information, or cable types are given.

    Note: Answer as if it were a CCNA exam question.
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  • hermeszdatahermeszdata Posts: 225Member
    mikej412 wrote: »
    Given one of the standard questions about broadcast domains and collision domains with a diagram similar to the following:

    pc---hub---hub---switch---router---hub---switch

    How many broadcast domains and collision domains are in the above diagram?
    A. 1 Broadcast Domain and 1 Collision Domains
    B. 1 Broadcast Domain and 2 Collision Domains
    C. 1 Broadcast Domain and 3 Collision Domains
    D. 2 Broadcast Domains and 1 Collision Domain
    E. 2 Broadcast Domains and 2 Collision Domains
    F. 2 Broadcast Domains and 3 Collision Domains
    F. None of the above since no IP Addressing, STP information, VLAN information, or cable types are given.

    Note: Answer as if it were a CCNA exam question.

    The first "F"
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  • hexemhexem Posts: 177Member
    I think the q was meant for the original poster lol
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  • mikej412mikej412 Posts: 10,090Member
    The first "F"
    Fixed it :D
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  • DevilWAHDevilWAH Posts: 2,997Member ■■■■■■■■□□
    rwwest7 wrote: »
    VLANs are a true seperator of broadcast domains. That's why they can't communicate with each other, because an arp request is a broadcast. And with out being able to arp than you need a router of some sort. Being able to communicate is not a requiremant for being seperate broadcast domains. And seperate broadcast domains don't have to be able to reach each other, in fact sometimes is desired that they don't.


    Ahh that was a misstype on my part. I ment to say "I don't consider a SWITCH a true seperate of bradcast traffic". A VLAN is a seperator of traffic, but I think VLANS go beyond what a switch is (at its core purpose) I would say a good defination of a switch, is a device that switchs packets based on the source and destination mac address with in a single bradcast domain.

    But yer cheers for pointing that out ;)

    And VLANS are a bit beyond this thread I think, you can get intervlan communication using nothing more than a standard network cable, wiht out the need for a a layer 3 device. VLANS devide a single switch in to mutiply logical swichs, so although VLANS do create seperate broadcast domains, this is not by a function of layer 2 switching. This is some thing that was origianl added to higher end switchs to reduce cost

    I think what I am trying to say is that a Switch does not have to be able to support VLANS to still be called a switch. VLAN's although a switching technology, are not a defination of what a switch must be able to do to be called a switch. That make sence? ;) So therefor if I a have a non vlan capabable switch, how does that prevent bradcasts between its ports?
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  • Forsaken_GAForsaken_GA Posts: 4,024Member
    But I am 99% sure they said because of the nature of full duplex, you can't have any collisions, there for technically it isn't a collision domain.

    Be careful here, this is a logical fallacy. It's still a collision domain even if only possible device can be transmitting. A full duplex switched environment eliminates collisions as a concern, and essentially turns the concept of a collision domain into a trivia question, but it doesn't rewrite the specs.
  • Forsaken_GAForsaken_GA Posts: 4,024Member
    rage_hog wrote: »
    One of the points for using switches is they reduce the broadcast domains.

    Yes and no.

    The big benefit of switches is that they eliminate collisions. If I have three switches daisy chained together, and all 3 switches have all of their ports in the same vlan, then I have the exact same broadcast domain as if I'd daisy chained 3 hubs together. However, I would have no collisions in the daisy chained switch setup, whereas I'd have mucho collisions in the daisy chained hub setup.

    Switches, just by virtue of being a switch do not give a flip about broadcast domains, until you implement VLAN functionality on those switches. And don't downplay this, because it's important in the real world. I have more than a few rows of access layer switches with fully populated ports and all in the same vlan.

    Busting up the broadcast domain is the province of the layer 3 device. Switches forward broadcast frames, this means they do not reduce the size of the broadcast domain.

    Now, you can go ahead and argue that VLAN's bust up broadcast domains. And you'd be right. But from a logical standpoint, that doesn't change anything. Sure, this may all be contained in one physical device, but logically, if I seperate the ports on a switch into two different vlans, I have 2 different devices.

    It may seem like I'm splitting hairs, but I'm not. I'm sure we've all seen the little skits of the indian guy in his apartment answering the phone as tech support for one company, and then the next phone call is for another company, and so on. It's one guy, but the customers don't know the difference. If 5 people talk to him for 5 different companies, he might as well be 5 different guys.

    So lets think about it logically. Let's say I have 24 devices, and I want to reduce my broadcast traffic because it's killing my throughput. All 24 devices are connected to a switch. Ok, great, so I seperate them into 2 vlans. Now each vlan has less broadcast traffic. However, I need all 24 devices to communicate with each other. So what have I accomplished by my attempt to use VLAN's as broadcast containment? Absolutely nothing. I could have achieved the exact same results by simply turning 12 of those hosts off, as far as the other 12 are concerned.

    How do I fix this? Simple, I add a layer 3 device. Now I have achieved the goal of reducing my broadcast traffic, and all the devices that need to talk to each other still can.

    So yeah, the router is actually what breaks up the broadcast domain into manageable chunks. A switch can be used to do it via vlan's if maintaining interconnectivity is not important, but that's not usually the case.

    You have to remember when thinking in network terms that it's the logical presentation of the network that counts, not the physical one. And in a logical representation, you have to treat each VLAN as if it were an independant switch, because logically, that's what it is, even though that vlan may span many switches.
  • Forsaken_GAForsaken_GA Posts: 4,024Member
    rwwest7 wrote: »
    VLANs are a true seperator of broadcast domains. That's why they can't communicate with each other, because an arp request is a broadcast. And with out being able to arp than you need a router of some sort. Being able to communicate is not a requiremant for being seperate broadcast domains. And seperate broadcast domains don't have to be able to reach each other, in fact sometimes is desired that they don't.

    Ok, if controlling broadcast traffic is a concern, that means you need interconnectivity between all those devices currently in the broadcast domain. This means using vlan's as a means of broadcast control is a bad plan and works at cross purposes with you until you add a layer 3 device into the mix.

    Broadcast containment is not the function of the switch, it's a router function. Everyone here is entirely too used to using managed switches and forget that dumb switches do exist, and those switches know nothing of vlans, they're going to forward a broadcast out all of it's ports no matter how much it pisses you off. The difference between a managed switch and a dumb switch is that the managed switch has the capability of performing some voodoo and making the people talking to it think it's more devices than it really is. If you setup 3 vlans on a switch, I can mimic the function of that setup exactly by using 3 dumb switches.

    Do *not* confuse management function with switching functionality.

    Several people in this thread need to go read Radia Perlman's Interconnections for a better understand of how this stuff actually works.
  • Forsaken_GAForsaken_GA Posts: 4,024Member
    Let me see if I can explain this a little better without the novel:

    The behavior of a switch when dealing with broadcasts is to forward them.

    When you create a vlan on an intelligent switch, what you are essentially doing is adding a dumb switch to the network, and each of those dumb switches you create are going to forward broadcasts.
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