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This Question is Not Answered 1 Correct Answer available (4 pts) 2 Helpful Answers available (2 pts)
5252 Views 6 Replies Latest reply: Oct 14, 2010 4:44 AM by MarkG RSS

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Subnet & Vlan Relationship

Oct 12, 2010 11:43 PM

Hardik 92 posts since
Feb 1, 2009

Hi All,

 

I'm aware about, that we can have one subnet par VLAN. But is it possible to have multiple Subnets par VLAN?

 

 

Regards,

Hardik...

  • Matt.Wang 2 posts since
    Aug 21, 2009
    Currently Being Moderated
    1. Oct 13, 2010 12:01 AM (in response to Hardik)
    Re: Subnet & Vlan Relationship

    hi Hardik

     

    i think there is nothing possible to have multi subnet in per vlan

     

    if we use VLSM to divided many subnets ,we must fix some gateway ,one gateway seems one vlan

     

    i don't be sure about my opinion ,

     

    many thanks

     

    matt

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  • Damian McCourt 50 posts since
    Jun 15, 2009
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    2. Oct 13, 2010 12:28 AM (in response to Hardik)
    Re: Subnet & Vlan Relationship

    I think it's quite possible to do, but not recommended - this is something that confused me a bit when I read it too. A VLAN is a layer 2 arrangement, which is why it needs an SVI (or an actual router) to route packets to other vlans. So a vlan doesn't know or care about subnets, it will quite happily accept packets from any member port.

     

    I think what will happen is that if there are several subnets on the same vlan is that the cam tables are all going to have layer 2 routes for the various hosts, regardless of what layer 3 subnet they're on, so the packets will get routed regardless. Which is probaby why it's recommended to keep a single vlan per subnet.

     

    Anyone know if this is the case?

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  • Claudius 117 posts since
    Jan 7, 2009
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    3. Oct 13, 2010 1:15 AM (in response to Damian McCourt)
    Re: Subnet & Vlan Relationship

    Hi Guys,

     

    A VLAN it's defined as a single Broadcast domain (L2) you can have different L3 addressing (subnets) for host into the same VLAN or broadcast domain, however remember how the hosts obtain L2 information by using ARP. All the packets for different subnets will be sent to the default gateway in it's own subnet, so we need L3 routing a L3 interface to route packets between those hosts. Packets for the same subnet will obtain L2 info by using ARP.

     

    We can also have several several VLANs within a single L3 subnet in this case router's proxy ARP can route the packets between VLANs.

     

    That's my understanding.

     

    HTH,

     

    Carlos.

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  • MarkG 108 posts since
    Oct 6, 2008
    Currently Being Moderated
    4. Oct 13, 2010 8:35 AM (in response to Hardik)
    Re: Subnet & Vlan Relationship

    Hi Hardik,

     

     

     

    Yes, it is possible to have multiple subnets per vlan, but be careful unless you really understand what you're doing.  An example of such a configuration is the "ip address <ip> <netmask> secondary" interface configuration.  With this config, the switch will respond on both subnets on vlan66. You could just as easily put two routers on the same vlan and have each router on a separate subnet.

     

     

     

    interface Vlan66
    ip address 192.168.1.1 255.255.255.0 secondary
    ip address 172.16.1.1 255.255.255.0
    end

     

     

     

    Just be careful with secondary IP addresses as there are some subtle issues that can arise.  One such example as how routing protocols treat the secondary IP address.

     

     

     

    https://supportforums.cisco.com/docs/DOC-4618;jsessionid=732186816571A38A5792F7BE44E184CF.node0

     

     

     

    Regards,

    Mark

     

     

     

    PS - I highly doubt this is a CCNA topic (I haven't checked).

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  • Matt.Wang 2 posts since
    Aug 21, 2009
    Currently Being Moderated
    5. Oct 13, 2010 6:26 PM (in response to MarkG)
    Re: Subnet & Vlan Relationship

    hi

     

    so u mean u can key this command on one interface

     

    i really don't know this command true meaning ,but i guess ,is it for any other application for one interface?such as VOIP,

     

    do u test it ,one interface access per vlan ,but on this interface u do the setting ,set the second IP address ,can u reach other interface with the same vlan ,

     

    the two subnets can communication freely ??

     

    this is just my doubt ?

     

    many thanks

     

    matt

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  • MarkG 108 posts since
    Oct 6, 2008
    Currently Being Moderated
    6. Oct 14, 2010 4:44 AM (in response to Matt.Wang)
    Re: Subnet & Vlan Relationship

    the two subnets can communication freely ??

     

    this is just my doubt ?

     


     

     

     

    Hi Matt,

     

    Think about another example.  You can have an IPv4 subnet riding on the same vlan as an IPv6 subnet.  Similarly,  you'll have a IPv6 link-local subnet on the same vlan as your routable IPv6 address.

     

    In all examples provided, you would still need some layer-3 routing between the various subnets for layer-3 communication to take place, but they can all coexist on the same layer-2 vlan.

     

    Best,

    Mark

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