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8531 Views 4 Replies Latest reply: Aug 16, 2009 5:13 AM by Chetan RSS

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Switching Concepts

Aug 15, 2009 6:16 AM

thiyagarajankalaiselvan 253 posts since
Apr 26, 2009

Can anybody help me to know,

 

Why we have to move all the ports of the Default VLAN?

 

What a native VLAN is and how exactly does it work?

 

Regards,

Thiyagarajan

  • shivaji 28 posts since
    Dec 30, 2008
    Currently Being Moderated
    Re: Switching Concepts

    Hi Thiyagarajan,

     

    I guess you are Thyagarajan,

     

    But any how, by default Switch places all interfaces in VLAN 1 which is also considered as default native .VLAN.  You do not have to move all the ports out of default VLAN. As you need and as per your requiement of the network, you move the ports to specific VLAN.

     

    The advantage of native VLAN, is that the traffic of  this VLAN cab be sent without VLAN tag on the trunk links. The uplink switch interprets all the untagged traffic as Navtive VLAN traffic. This reduces an additional job of adding tag and removing a tag. By practice, If any VLAN, in which you have more number of systems connected, then you make that VLAN as native VLAN.

     

    Good Time

    bye

  • Chetan 539 posts since
    Jul 26, 2008
    Currently Being Moderated
    Re: Switching Concepts

    Que 1 : Why we have to move all the ports of the Default VLAN?

     

    Answer:

     

    Before understanding VLANs, you must first have a specific understanding of the definition of a LAN. Although you can think about LANs from many perspectives, one perspective in particular can help you understand VLANs:

     

    A LAN includes all devices in the same broadcast domain.

     

    A broadcast domain includes the set of all LAN-connected devices that when any of the devices sends a broadcast frame, all the other devices get a copy of the frame. So, you can think of a LAN and a broadcast domain as being basically the same thing. Without VLANs, a switch considers all its interfaces to be in the same broadcast domain; in others words, all connected devices are in the same LAN.

     

    With VLANs, a switch can put some interfaces into one broadcast domain and some into another, creating multiple broadcast domains. These individual broadcast domains created by the switch are called Virtual LANs.  A broadcast sent by one host in a VLAN will be received and processed by all the other hosts in the same VLAN, but not by hosts in a different VLAN.

     

     

    Some other reasons to move ports out of the Default VLAN and putting them in other VLANs are :

     

    ■ To create more flexible designs that group users by department, or by groups that work together, instead of by physical location

    ■ To segment devices into smaller LANs (broadcast domains) to reduce overhead caused to each host in the VLAN

    ■ To reduce the workload for the Spanning Tree Protocol (STP) by limiting a VLAN to a single access switch

    ■ To enforce better security by keeping hosts that work with sensitive data on a separate VLAN

    ■ To separate traffic sent by an IP phone from traffic sent by PCs connected to the phones

  • Chetan 539 posts since
    Jul 26, 2008
    Currently Being Moderated
    Re: Switching Concepts

    Question 2: What a native VLAN is and how exactly does it work?

     

    Answer : The concept of native VLAN was introduced by 802.1Q.

     

    802.1Q defines one VLAN on each trunk as the native VLAN.

     

    By definition, 802.1Q simply does not add an 802.1Q header to frames in the native VLAN.

     

    By default, the 802.1Q native VLAN is VLAN 1. 

    When the switch on the other side of the trunk receives a frame that does not have an 802.1Q header, the receiving switch knows that the frame is part of the native VLAN. Note that because of this behavior, both switches must agree which VLAN is the native VLAN.
    The 802.1Q native VLAN provides some interesting functions, mainly to support connections to devices that do not understand trunking. For example, a Cisco switch could be cabled to a switch that does not understand 802.1Q trunking. The Cisco switch could send frames in the native VLAN—meaning that the frame has no trunking header—so the other switch would understand the frame.The native VLAN concept gives switches the capability of at least passing traffic in one VLAN (the native VLAN), which can allow some basic functions, like reachability to telnet into a switch.

  • Chetan 539 posts since
    Jul 26, 2008
    Currently Being Moderated
    Re: Switching Concepts

    Also visit the following thread to see more about NATIVE VLANs.

     

    https://learningnetwork.cisco.com/message/5106

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