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2136 Views 9 Replies Latest reply: Jan 13, 2011 11:17 AM by Mark Heick RSS

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VLAN/IP mapping

Jan 13, 2011 5:15 AM

e.magidson 3 posts since
Nov 24, 2010

Hello,

 

Imagine that we have a switch with 1 VLAN configured and 2 ports in this particular VLAN.

Further, we have 2 Windows PC plugged in these 2 ports on the switch with different IP configured on their NIC let's say PC1 has 192.168.1.1/24 and PC2 has 192.168.2.1/24.

 

The question is would PC1 be able to ping PC2 in this case and what is PC's logic?

  • Pete Nugent 1,256 posts since
    Dec 8, 2008
    Currently Being Moderated
    1. Jan 13, 2011 5:38 AM (in response to e.magidson)
    Re: VLAN/IP mapping

    Not as you have described the pcs will not ping each other as they  are oin different subnets.

     

    PC1 IP 192.168.1.1 /24 is in network 192.168.1.0

    PC2 IP 192.168.2.1 /24 is in network 192.168.2.0

     

    You would need inter vlan routing or a layer 3 device to route between them such as a router.

     

    If PC 2 had an IP adress of 192.168.1.2 /24 then yes they would be able to ping each other as they are both in the same network and the switch is a layer 2 device.

     

    Hope that helps

  • Bryan Middleton 51 posts since
    Sep 4, 2008
    Currently Being Moderated
    3. Jan 13, 2011 6:09 AM (in response to e.magidson)
    Re: VLAN/IP mapping

    Hi

     


    petenugent is absolutely right, just to expand though as to what would happen. PC1 would see that 192.168.2.1 is on a different network and therfore it will send out an ARP request to resolve the MAC address of its default-gateway (if not already in its ARP cache) - it will not send an ARP request for something on a different subnet (192.168.2.1) so even though they are physically connected into the same layer 2 broadcast domain it will not work, if you try to remove the default-gateway and then ping you will probably get a ping transmit failure.

     

    One way that you might be able to technically get around this would be to add static ARP entries to each of the hosts for the corresponding IP/MAC addresses and to then test connectivity between the two but whether or not this would work might be hit/miss depending on the OS of the host machines, NIC driver etc. - it is not something I have tried and practically speaking would be a waste of time although interesting to see the result :-)

     

    HTH

  • Catalyst52 197 posts since
    Dec 22, 2010
    Currently Being Moderated
    4. Jan 13, 2011 8:46 AM (in response to Bryan Middleton)
    Re: VLAN/IP mapping

    Bryan, static ARPs would NOT make that work under any circumstance, because once the PC gets the IP-MAC resolution, the workstation routing table still needs to be processed. If there is no gateway, none of the rows in the workstation routing table will match, and you will 100% get a "Destination Host Unreachable" message.

  • sreenivas_naidu 78 posts since
    Sep 29, 2009
    Currently Being Moderated
    5. Jan 13, 2011 8:59 AM (in response to e.magidson)
    Re: VLAN/IP mapping

    Firstly to answer the question, PC1 will not be able to ping PC2.
    the logic:

    1)PC1 does not have the layer2 address of PC2 to complete successful
    packet delivery and recieve the response.

    2)The PC1 identifies the destination ip address in question is beyond its
    subnet based on its ip address/subnet mask combination.
    I believe you would agree that that it wouldnt be worth the effort for
    PC1 to send a arp broadcast for a device outside its network.

    3)The net result would be the PC1 has to direct the packet to its default
    gateway and let it deliver the packet to the destination.

    4)Once the DG recieves the packet from PC1,DG has to identify the destination
    network and the next hop device which will lead to that network.
    It checks for its route table,if no route is found it will drop
    the packet,else will forward the packet to the next device.

    5)The process would continue till the packet reaches the destination and
    the icmp echo reply follows the same process from PC2 to PC1.

    This logic will work if both the devices are in the same vlan or
    different vlans.

    Now assuming in your question you have already configured DG for each
    host and DG have a route each of the network, that my friend is your
    Intervlan routing in itself where the DG are the layer 3 devices(As already
    pointed by Pete and Brian).

    Lastly as brian suggested you can go around setting up static arp entries on
    PC1 and PC2 and it might work,bottom line its not scalable.

    HTH.
    Sreeni

  • Pete Nugent 1,256 posts since
    Dec 8, 2008
    Currently Being Moderated
    6. Jan 13, 2011 9:14 AM (in response to sreenivas_naidu)
    Re: VLAN/IP mapping

    The question states 1 VLAN 2 seperate networks therefore you need a layer 3 device either intervlan routing on the switch or a router. If you meant on the same subnet then yes it will arp for the second pc if it does not know its mac, as its not on the same subnet it will arp for the default gateways mac if not already known and send the packet to the default gateway. I do believe that is what the question asked.

     

    As there is only one VLAN it will never reach PC2 as you will need to create a second VLAN and create routes between the 2 as when the packet hits the default gateway it will just drop it as it does not know where to send it, hence no connectivity.

  • Mark Heick 45 posts since
    Dec 30, 2009
    Currently Being Moderated
    7. Jan 13, 2011 9:58 AM (in response to Pete Nugent)
    Re: VLAN/IP mapping

     

    As there is only one VLAN it will never reach PC2 as you will need to create a second VLAN ...

     

    You don't necessarily need to create separate vlan. You have one vlan and just configure the first subnet defualt gateway address as the primary, and use the second gateway address as the secondary address on the SVI.

     

    I do agree that you will need routing to get between the two subnets.

  • Pete Nugent 1,256 posts since
    Dec 8, 2008
    Currently Being Moderated
    8. Jan 13, 2011 10:47 AM (in response to Mark Heick)
    Re: VLAN/IP mapping

    Mark

     

    Thanks for that, you know I have never come across this, either that or I forgot about it completely, what a cool idea! Sorry maybe I should know this but for whatever reason I simply had no idea,

  • Mark Heick 45 posts since
    Dec 30, 2009
    Currently Being Moderated
    9. Jan 13, 2011 11:17 AM (in response to Pete Nugent)
    Re: VLAN/IP mapping

    petenugent,

     

    No problem. One implementation is when you might be readdressing a location and can't do it all at once so you can have the secondary addresses exist and still maintain connectivity.

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