Subinterfaces or Secondary Interfaces in VLANS ?

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Nov 8th, 2007
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I need to combine two VLANS into one.


What is Cisco's recommendation regarding the use of subinterfaces or secondary interfaces in the same VLAN ?


Is one better than the other for any particular reason ?


Any implications regarding HSRP ?

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bvsnarayana03 Thu, 11/08/2007 - 07:37
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Do you intend to ask, that the 2 vlans must communicate to each other.


If these are L2 vlans on L2 switch, u definitely need subinterfaces with router-on-stick solution.


If its an L3 switch, "ip routing" will do the trick irrespective of whether it is L2 or L3 vlan.

Richard Burts Thu, 11/08/2007 - 07:50
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Richard


If your question is about the use of subinterfaces or secondary interfaces in the same VLAN then there is a simple answer. I do not believe that you can not use subinterfaces within the same VLAN. It is easy and the better choice to configure a secondary address on the interface to handle the second address range within the same VLAN.


B V S


It looks to me like your answer assumes that there will still be two distinct VLANs and is correct for that assumption. But my understanding of the question was that 2 VLANs were being combined and that there would be 1 VLAN on the switch. If my understanding is not correct then I hope that Richard will clarify the situation.


HTH


Rick

rbauer Thu, 11/08/2007 - 09:13
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Hi Rick,


It will be one VLAN. That one VLAN is already in an HSRP configuration though, so if I want to add a secondary gateway, what does that configuration look like ?


Rich

Richard Burts Thu, 11/08/2007 - 10:47
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Rich


Thanks for confirming that I correctly understood the situation. If the VLAN is already in an HSRP group then you probably need to do 2 things in the interface configuration. You will need to configure the secondary address and you will need to add the secondary address to the HSRP group. If you had something that looked somewhat like this:

interface vlan 10

ip address 10.10.10.2 255.255.255.0

standby 1 ip 10.10.10.1

then you would add something that looks somewhat like this:

ip address 10.20.20.2 255.255.255.0 secondary

standby 1 ip 10.20.20.1 secondary


HTH


Rick

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