Is Vlan Tag added within one Switch?

Here is the situation.

One Switch.

Int fas 0/1-12 belongs to VLAN_A.

0/13-24 VLAN_B.

 

I was wondering if the host PC which is connected to fas 0/1

would like to send frame to another host belongs to fas 0/24,

the Ethernet frame would be add VLAN_A tag?

 

I thought tag is available only in trunk.

I mean, between the Switches.

Comments

  • <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> They can't send to each other while they are in different vlans, there should be a L3 device to route between the two vlans

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    From: Castor <[email protected]>
    Date: 28 Jul 2010 20:09:29 -0700
    Subject: [CCIE R&S] Is Vlan Tag added within one Switch?

    Here is the situation.

    One Switch.

    Int fas 0/1-12 belongs to VLAN_A.

    0/13-24 VLAN_B.

     

    I was wondering if the host PC which is connected to fas 0/1

    would like to send frame to another host belongs to fas 0/24,

    the Ethernet frame would be add VLAN_A tag?

     

    I thought tag is available only in trunk.

    I mean, between the Switches.




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  • amjadhalim

    Thanks.

    Then why the Switch refused to send the frame from fas 0/1 to fas 0/24?

    Of course, the different VLANs.

    Judging from Ethernet frame of what?

    What part?

  • <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> The switch isolate the two ports, virtually they are in different LANs or different switches. There is no vlan tag in this ethernet packet.

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    From: Castor <[email protected]>
    Date: 28 Jul 2010 20:46:57 -0700
    Subject: Re: [CCIE R&S] Is Vlan Tag added within one Switch?

    amjadhalim

    Thanks.

    Then why the Switch refused to send the frame from fas 0/1 to fas 0/24?

    Of course, the different VLANs.

    Judging from Ethernet frame of what?

    What part?




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  • amjadhalim

    Thanks.

    You have mentioned;

    "There is no vlan tag in this ethernet packet."

    Then why the Switch decide not to send the frame from A to B?

    There must be some difference between frames which is belongs to VLAN_A and B. 

  • <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> I told you they are virtually separated, the switch will not let the two ports send each other because they are in different vlans. The switch will only send the traffic to a L3 device only

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    From: Castor <[email protected]>
    Date: 28 Jul 2010 21:14:00 -0700
    Subject: Re: [CCIE R&S] Is Vlan Tag added within one Switch?

    amjadhalim

    Thanks.

    You have mentioned;

    "There is no vlan tag in this ethernet packet."

    Then why the Switch decide not to send the frame from A to B?

    There must be some difference between frames which is belongs to VLAN_A and B. 




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  • You have mentioned:

    "because they are in different vlans. "

    Because they are in different vlans, the Switch decided not to send.

    Then,

    why the Switch figured out both of them belong to different vlans?

    What did the Switch check?

    What part of the Ethernet frame did the Switch check?

  • <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> The switch didn't check the packet, the switch can know that from its configuration. Did you forget that you configured these two ports in different vlans!

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    From: Castor <[email protected]>
    Date: 28 Jul 2010 21:41:02 -0700
    Subject: Re: [CCIE R&S] Is Vlan Tag added within one Switch?

    You have mentioned:

    "because they are in different vlans. "

    Because they are in different vlans, the Switch decided not to send.

    Then,

    why the Switch figured out both of them belong to different vlans?

    What did the Switch check?

    What part of the Ethernet frame did the Switch check?




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  • Thanks.

    Let me sleep on it.

     

  • Castor,

    This goes back to CCNA studies - the switch has a
    separate virtual subnet for each VLAN. Unless the interface is a Trunk,
    there is no tagging or ISL encapsulation - the switch knows that the
    interface belongs to the configured access VLAN (switchport access vlan
    x). Inside the switch only vlan x ports can communicate with other vlan x
    ports - they cannot communicate with vlan y ports.

    In the case of
    inter-VLAN routing, either the switch, if it is Layer-3, or a separate
    router, receives traffic from one VLAN/subnet and routes it to a
    different VLAN/subnet.

    The only place where there are different
    types of frames is in a trunk link - there some frames may not have a
    VLAN tag (native VLAN in the case of dot1q), and other frames will have a
    tag (either ISL or dot1q) identifying which VLAN it belongs to.

    You have mentioned:

    "because they are in different vlans. "

    Because they are in different vlans, the Switch decided not to send.

    Then,

    why the Switch figured out both of them belong to different vlans?

    What did the Switch check?

    What part of the Ethernet frame did the Switch check?

     

  • DarrellEscola

    Thanks.

    You have mentioned;

    "the switch knows that the interface belongs to the configured access VLAN (switchport access vlan x).

    Inside the switch only vlan x ports can communicate with other vlan x ports

    - they cannot communicate with vlan y ports."

     

    I understood.

    The question is;

    put yourself in the switch's shoes.

    What would you do when a host of vlan x try to communicate with the host in vlan y?

    Of course, you would not permit him to do so.

    Judged by what?

    Destination address?

    I understand x and y cannot communicate.

    I do not understand what part of the frame does the switch check.

  • <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> Dear
    We are talking about L2
    In L2, when a host is trying to communicate with another one, the switch get the mac address of the receiver and try to find where he is located. If its location is unknown, then the switch broadcast the message which means it sends the message to all the ports that are in the same vlan of the sender. Is that clear now?

    Sent on the Sprint® Now Network from my BlackBerry®


    From: Castor <[email protected]>
    Date: 28 Jul 2010 23:58:19 -0700
    Subject: Re: [CCIE R&S] Is Vlan Tag added within one Switch?

    DarrellEscola

    Thanks.

    You have mentioned;

    "the switch knows that the interface belongs to the configured access VLAN (switchport access vlan x).

    Inside the switch only vlan x ports can communicate with other vlan x ports

    - they cannot communicate with vlan y ports."

     

    I understood.

    The question is;

    put yourself in the switch's shoes.

    What would you do when a host of vlan x try to communicate with the host in vlan y?

    Of course, you would not permit him to do so.

    Judged by what?

    Destination address?

    I understand x and y cannot communicate.

    I do not understand what part of the frame does the switch check.




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  • The switch can always look back into it's local VLAN Database to find VLAN information. From which it knows that which ports belong to which vlan as entire vlan information is stored in VLAN.dat file only.

    Now talking back about vlans, the whole idea of making VLAN is to reduce size of broadcast domain. In other words you can say that Virtual LAN means deviding up the physical switch into multiple virtual switches which are going to be act separately from each other. Now the only way these virtually separate switches can communicate is either through setting up internal trunks (which is ofcourse not possible) or by introducing a Layer 3 device or functionality.

     

    Now as far theory of adding vlan tags to the L2 frames goes, it can be done by trunk ports only.

     

    HTH...

    Deepak Arora

    http://deepakarora1984.blogspot.com/

  • The one additional situation I didn't mention is the voice vlan - but this is still not a situation where a host could choose to send layer-2 frames to a destination on a different VLAN. I have some Cisco phones connected to a 3550 with PoE - the phones expect traffic tagged with the voice vlan, and pass untagged traffic on to the attached computer. Even when the computer is attached directly to the phone, the mini-switch in the phone will not permit the computer to communicate directly with the phone - the computer's frames are sent through untagged to the switch.

    The only way for the computer to communicate with the directly attached phone would be through a router or level-3 switch.

    Now as far theory of adding vlan tags to the L2 frames goes, it can be done by trunk ports only.

    HTH...

    Deepak Arora

    http://deepakarora1984.blogspot.com/

     

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