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5139 Views 10 Replies Latest reply: Oct 5, 2011 8:22 AM by Difan Zhao RSS

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why layer 2 switches need mac address for it's port?

Apr 18, 2011 9:41 PM

asif 5 posts since
Apr 18, 2011

In layer 2 switches, there will be one mac adress for the switch and one mac address for it's every port. why layer 2 switches need mac for it's port?

  • Amjad Abdelhalim 9 posts since
    Dec 4, 2009
    Currently Being Moderated
    1. Apr 18, 2011 9:43 PM (in response to asif)
    Re: why layer 2 switches need mac address for it's port?

    there are many reasons, one of these reasons is to use it for spanning-tres calculations.

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  • Amjad Abdelhalim 9 posts since
    Dec 4, 2009
    Currently Being Moderated
    3. Apr 18, 2011 9:54 PM (in response to asif)
    Re: why layer 2 switches need mac address for it's port?

    assume you have two paths from switch A to switch B, and switch B is the root, stp uses the mac address for the ports to assign one of the two ports as a root port.

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  • samarjitdutta 59 posts since
    Jan 6, 2010
    Currently Being Moderated
    4. Apr 18, 2011 9:54 PM (in response to asif)
    Re: why layer 2 switches need mac address for it's port?

    MAC Address on a VLAN (SVI) or L3 Interface on Catalyst Switches

    /image/gif/paws/41263/catmac_41263a.gif

     

    By default, Catalyst switches come with the same MAC address configured      on all interfaces. The diagram in this section shows a Catalyst 6500 with      Supervisor Engine 2 and MSFC2. However, the MAC address on all three VLAN      interfaces is the same, even though the IP addresses are different.

     

    Catalyst switches have varied support for the ability to change the MAC      address for a VLAN (SVI) or L3 interface. You do not need to change the      burned-in MAC address if the network devices support multiple IPs to a single      MAC Address Resolution Protocol (ARP) table, which is common. Also, you do not      need to change the MAC address if the switches support a per-VLAN MAC address      table. Cisco switches support a per-VLAN MAC address table or      content-addressable memory (CAM) table. This support allows the switches to      maintain a MAC address table per VLAN. Therefore, the switches can have the      same MAC address on multiple VLAN interfaces without issue.

     

    Note: A Hot Standby Router Protocol (HSRP) group uses the same virtual MAC          address if the HSRP group ID is reused on multiple interfaces. So you must          understand and use different HSRP groups when possible. In order to understand          the HSRP group limitation on the Catalyst 6500/6000, refer to this document:

     

     

    The Catalyst 3550 switches, Catalyst 4500/4000 switches with Supervisor      Engine III/IV, and Catalyst 6500 switches with Supervisor Engine 720 support up      to 256 unique HSRP group IDs in the 0 to 255 range.

     

    MAC Addresses on Layer 2 Interfaces

     

    MAC addresses of Layer 2 Interfaces (Switchports) are unique and are      assigned to that particular line module. In Cisco 6500/6000, 4500/4000, 3750,      3560, 3550, and 2970 series switches, you are not able to change the MAC      address on a switchport. In Cisco 2940, and 2950/2955 series switches you can      change the MAC address of switch ports using the command      mac-address, under the interface configuration      mode.

     

    MAC Addresses for Spanning Tree Computation

     

    MAC addresses used for Spanning Tree calculations are stored in an      EEPROM present in the Supervisor module. Regardless of the types of line      modules installed, the Layer 2 MAC addresses for VLANs do not change unless you      replace the Supervisor module. If you do replace the Supervisor module, the      Layer 2 MAC addresses of all VLANs change to those specified in the address      allocator on the new Supervisor module. In the fixed configuration Catalyst      switches, MAC addresses for VLANs cannot be changed.

     

    Catalyst Switch with Support for CLI Configuration of a Unique MAC Address per Interface

     

    This section discusses switches that support a change in MAC addresses      per interface.

     

    Catalyst 6500/6000 Supervisor Engine 720 and Supervisor Engine I with MSFC1, MSFC2, or MSFC3 That Runs CatOS System Software

     

    This output is from the MSFC1 in which the default MAC address is the      same for all interfaces:

     

    cs-6506-24a#show interfaces | include line | address
    Vlan1 is down, line protocol is down
      Hardware is Cat6k RP Virtual Ethernet, address is 00d0.bcf1.ee5c (bia 00d0.bcf1.ee5c)
      Internet address is 14.18.2.182/16
    Vlan2 is down, line protocol is down
      Hardware is Cat6k RP Virtual Ethernet, address is 00d0.bcf1.ee5c (bia 00d0.bcf1.ee5c)
    cs-6506-24a#

     

    Use the mac-address      mac_address interface configuration      command in order to change the MAC address. Here is an example:

     

    cs-6506-24a#configure terminal
    Enter configuration commands, one per line.  End with CNTL/Z.
    cs-6506-24a(config)#interface vlan 1
    cs-6506-24a(config-if)#mac-address 0007.0001.0001
    cs-6506-24a(config-if)#exit
    cs-6506-24a(config)#interface vlan 2
    cs-6506-24a(config-if)#mac-address 0007.0001.0002
    cs-6506-24a(config-if)#end
    cs-6506-24a#

     

    Verify the change in the MAC address in this way:

     

    cs-6506-24a#show interfaces | include line | address
    Vlan1 is down, line protocol is down
      Hardware is Cat6k RP Virtual Ethernet, address is 0007.0001.0001 (bia 00d0.bcf1.ee5c)
      Internet address is 14.18.2.182/16
    Vlan2 is down, line protocol is down
      Hardware is Cat6k RP Virtual Ethernet, address is 0007.0001.0002 (bia 00d0.bcf1.ee5c)
    cs-6506-24a#
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  • Amjad Abdelhalim 9 posts since
    Dec 4, 2009

    I think i made a mistake, stp uses port number as a tie breaker,  not the mac address

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  • samarjitdutta 59 posts since
    Jan 6, 2010
    Currently Being Moderated
    7. Apr 19, 2011 2:11 AM (in response to asif)
    Re: why layer 2 switches need mac address for it's port?

    normal layer 2 managagle switchport do not use and port mac address ,

    it uses a base mac address for the purpose of STP, or Management purpose,

    suppose u give interface vlan an ip address, then only this virtual interface will have an ip address, which is same as it's base mac-address

    if you configure multiple vlan interface on a switch but all these port will have same mac-address

    If you want you can change the mac-address of these virtual lan interface as i have indicated in my last post

     

    stp root port election

    lowest path cost

    if cost is same....

    is the multiple ways to the root switch via same switch or different switch

    if they are through different switch then choose the switch with lower bridge id (priority + base mac)

    if both the path are via same switch then bridge id for  both path are same ...

    choose the port with lowest port id.

     

    here base mac address is playing an importent route to decide the root bridge and also to find out the best root port

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  • jbg 2 posts since
    Oct 5, 2011
    Currently Being Moderated
    8. Oct 5, 2011 3:32 AM (in response to asif)
    Re: why layer 2 switches need mac address for it's port?

    per port macs help identify misconfigurations for 802.3ad/lacp and when both sides of a connection aren't using the same stp topology (802.1d vs pvst/802.1s etc).

     

    without the two+ ports using distinct MACs you have no mechanism to distinguish these conditions.

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  • Rizwan Ayub 120 posts since
    May 6, 2009
    Currently Being Moderated
    9. Oct 5, 2011 8:14 AM (in response to asif)
    Re: why layer 2 switches need mac address for it's port?

    Is this Asif ali mian??? If yes this is Rizwan

    The answer is not varified but is according to my own understanding so it can be wrong. The switch has MAC Identifier's stored in the back plane and it is used in BID. Where as the switch port MAC Identifier is used as the source MAC Identifier for the Frame that contains the BPDU This is one and there are many reasons

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  • Difan Zhao 226 posts since
    May 30, 2009
    Currently Being Moderated
    10. Oct 5, 2011 8:22 AM (in response to asif)
    Re: why layer 2 switches need mac address for it's port?

    I know that when you do 802.1x, the EAP packets are from the port MAC address. I think CDP message and keepalive messages use unique MAC addresses on the port too. I also think Cisco just have TOO many MAC addresses of their own so they can afford wasting some.

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