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VLAN Question

Posted on 2004-10-25
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Last Modified: 2010-03-17
Please Correct My Statements.

IN A Network With Two Different Vlans
A,C in a Same Collission Domain.
B,D in a Same Collission Domain.
A,B in The  Vlan 1 ,C,D in the Vlan  2
"A" Host send an address Resoulation Protocol Frame with Host B s IP adress
and Broadcast Mac address.
The layer 2 Siwtch 1(Enabled For Vlan Supporting) ADD Vlan ID Tag to it and progpagate
on Vlan Trunks,Every Switchs on Vlan Trunk that Enabled For That Vlan ID Forward This Frame.
Frame P Was BRoadcasted on Vlan  1 For Reciveing Mac Response.
After Recieving Mac Address  in valn 1.
Host A sends its packets with Mac Address of a Switch 1,
and swith1 send its packets with Mac adress Switch2 on Vlan Trunk
and Switch 2 Send recieved packets to Host B.


thanks in advance for your helps
Hamid Reza




 
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Question by:123456
    23 Comments
     
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    Assisted Solution

    by:lrmoore
    You cannot have two devices in the same collision domain if they are in different VLAN's.
    VLAN's define Broadcast domains. All devices within the same vlan are in the same broadcast domain.
    Collision domains are entirely different. Each and every switchport is its own collision domain.
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    Author Comment

    by:123456
    >>VLAN's define Broadcast domains.
    VLAN Segmented Broad cast domain.
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    Author Comment

    by:123456
    >>All devices within the same vlan are in the same broadcast domain.
    ok
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    Author Comment

    by:123456
    You cannot have two devices in the same collision domain if they are in different VLAN's.
    PLease See Figure 5 From this link.
    http://net2.ucadavis.edu/newvlan.htm
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    Expert Comment

    by:lrmoore
    I cannot access that link

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    by:123456
    >>You cannot have two devices in the same collision domain if they are in different VLAN's.
    You Suppose it is located on differented Vlan Based broadcast Domain.
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    Author Comment

    by:123456
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    Author Comment

    by:123456
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    Expert Comment

    by:lrmoore
    I think you are confusing broadcast domains with collision domains.

    Given that the campus network is all switched, then each and every switch port is its own collision domain. It is physically impossible to have multiple devices on the same port/same collision domain.

    Each VLAN defines a different broadcast domain. A broadcast packet from VLAN 5 will never be seen by hosts in VLAN 4.


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    Accepted Solution

    by:
    I'm gonna add a comment here which is probably unwelcome but should be added from the perspective of completeness.  It is completely possible to have two vlans in one collision domain.  It's just a silly idea, in almost all cases.  Consider the following:



                                                      -----------------------------------------------------------------
                                                      |           BIG Switch supporting VLANS                          |
                                                      -----------------------------------------------------------------
                                                                     |                                             |
                                                                     | VLAN A                                  | VLAN B
                                                                     |                                             |
                                                      -----------------------------------------------------------------
                                                      |                 MULTIPORT REPEATER (HUB)                   |-------Sniffer
                                                      -----------------------------------------------------------------
                                                           |                 |                       |                    |
                                                           |                 |                       |                    |
                                                    NIC-ON-A      NIC-ON-B          NIC-ON-C        Crosover
                                                                                                                     Connection
                                                                                                                     to yet another
                                                                                                                     switch providing
                                                                                                                      VLAN C

    (Now I wait for the walls to come crashing down :))

    Jon


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    Expert Comment

    by:lrmoore
    Nice illustration, Jon!
    That's thinking outside the box!
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    Author Comment

    by:123456

    >>I think you are confusing broadcast domains with collision domains.
    From That Article:
    Virtual LANs (VLANs) can be viewed as a group of devices on different physical LAN segments which can
    communicate with each other as if they were all on the same physical LAN segment.
    Switches using VLANs create the same division of the network into separate broadcast domains but do not
    have the latency problems of a router.

    Broadcast Domain
    The set of all devices that will receive broadcast frames originating from any device within the set. Broadcast
    domains can be bounded by VLANs in a stand-alone environment. In an internetworking environment, they are
    typically bounded by routers because routers do not forward broadcast frames.

    Collision Domain
    In Ethernet, the network area within which frames that have collided are propagated. Repeaters and hubs
    propagate collisions; LAN switches, bridges and routers do not

    >>Each VLAN defines a different broadcast domain. A broadcast packet from VLAN 5 will never be seen by hosts in VLAN 4.
    it is right.


    My purpose Is this Figure:

                                                         Router  
                                                    /                    \                                              
                                                  /                        \          
                                                /                            \    
                                               
                          Switch Layer 2      --------------------    Switch Layer 2    
                                                                                                       
                     /                      \                                    /                     \
                    /                        \                                 /                         \
                  /                            \                             /                             \
       ----------------             --------------------       ----------------             ------------
       NIC A(VLAN 1)            NIC C ( VLAN 2)            NIC B(VLAN 1)           NIC D( VLAN 2)    
                     

    After Attention to My Upper Figure PLease Correct My stateMents in First Comment of my Question

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    Expert Comment

    by:lrmoore
    Then:
    >A,C in a Same Collission Domain.
    >B,D in a Same Collission Domain.
    >A,B in The  Vlan 1 ,C,D in the Vlan  2

    Would be:

      A,B in a Same broadcast Domain.
      C,D in a Same broadcast Domain.
       A,C cannot see each other at layer 2 - a broadcast from A will not reach C
       B,D cannot see each other at layer 2 - a broadcast from B will not reach D
      A,B in The  Vlan 1 ,C,D in the Vlan  2
      If A sends ARP broadast for B, yes, the vlan tagging is handled within the switches only

    >Host A sends its packets with Mac Address of a Switch 1,
    >and swith1 send its packets with Mac adress Switch2 on Vlan Trunk
    >and Switch 2 Send recieved packets to Host B.

    No. Once Host A learns MAC address of Host B, then Host A sends packets directly to Host B MAC. The switches' job is to keep track of which MAC address is associated on which physical port.

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    by:123456
    >>No. Once Host A learns MAC address of Host B, then Host A sends packets directly to Host B MAC
    I dont know How Learn Mac address of B and D?
    Can You Explain me ?
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    Author Comment

    by:123456
    >>A,B in a Same broadcast Domain.
    OK, but Dont you think these stations located on
    Various Collission Domains.
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    Assisted Solution

    by:lrmoore
    Host A will not learn MAC address of C or D - arp broadcasts are not propagated across VLAN's
    Host A learns MAC address of B through ARP
    Host A cannot communicate with C or D, unless there is a router somewhere to route between the VLAN's

    YES, A and B are in the same broadcast domain, separate collision domains.
    YES, C and D are in the same broadcast domain, separate collision domains.
    A, B, C and D are all in separate collision domains.

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    Expert Comment

    by:JonSh
    Hamid, it might help you if I state the general rules for what you are asking:

    Within the same VLAN (whether you traverse many switches or the same switch), ALL traffic is handled at layer 2 (switched).

    Across differing VLANS (whether you traverse many switches or the same switch), ALL traffic is handled at layer 3 (routed).

    Now, once you get this straight, throw it away because we also have layer 2 routing :)  And as far as your statements, lrmoore has corrected them beautifully.  It is important to note (as he said) that A, B, C, & D are all in separate collision domains.

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    Expert Comment

    by:lrmoore
    Does this all make sense to you yet?

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    by:JonSh
    I'm waiting for Hamid to include STP, at which point all the switches crash Vlan 0 :)

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    Author Comment

    by:123456
    Sorry For My Delay.
    I was ill and i Forgotten this Question.
    >>A, B, C and D are all in separate collision domains.
    Can you Explain it bit more?
    IS A,C  on a different collision domains?
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    Assisted Solution

    by:JonSh
    each port on a switch is it's own collision domain.  It isn't like a hub, in which everything is a shared colission domain.  On a switch, every connected device shares a collision domain with the switchport it is connected to.  This is one of the reasons a switch can provide more bandwidth, because each of it's ports is unshared with any other.

    Yes, A,C are on different collision domains.


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    Expert Comment

    by:lrmoore
    >>>A, B, C and D are all in separate collision domains.
    >Can you Explain it bit more?

    As we have explained numerous times above, each and every physical port on a switch is an independent collision domain. Therefore, each and every host that is connected directly to this switch is in its own collision domain. No two hosts will ever be in the same collision domain.
    Ergo:
      > IS A,C  on a different collision domains?
    YES - A, B, C and D are all in different collision domains..

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    Author Comment

    by:123456
    thanks for your good assistance.
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