Router on a stick config

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Jan 25th, 2010
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I have a 2821 router connected to a 2950 switch.  On the 2821 I configured sub-interfaces.  On the 2950 I configured 4 VLAN's.  The port that connects to the router I configured as a trunk port.  The problem I am having is on the 2950 it will only let me have 1 active VLAN.  If a VLAN is active and then I enable one of the other VLAN's the other shuts down.  Hmmmm  I have deploy the same router on a stick with 2960's and they do not function this way.  Is this how 2950's function?  Would I need a layer 3 switch like a 3550? 


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sachinraja Mon, 01/25/2010 - 13:36
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Hi Harrison

Layer 2 switches like 2950's can have only one active Layer 3 SVI configured for a VLAN. Since it is layer 2, the Layer 3 IP is used only to manage the box. If you are configuring trunk between router and switch, why do you need to define seperate layer 3 interfaces on the switch ? have layer 2 vlans on the switch, and associated Layer 3 IPs on the routers. Have the PC's / servers on the switch , configured with a default gateway address defined on the router !! eg, define vlan 2, 3, 4, 5 etc on the switch (on layer 2), and have sub-interfaces on the router with layer 3 addresses for these vlans. default gateway for components in these vlans would be the router's sub-int IP address.

BTW - You said " I have deploy the same router on a stick with 2960's".. do you mean, u could defind more than one VLAN SVI in 2960 ??


HMidkiff Mon, 01/25/2010 - 13:47
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Thank you for replying to my post...

With 2960 I have been able to define multiple VLANs and they all come up.  I have to have the router with the sub-interfaces in order to route traffic between them.  Am confused what could be happening here?

sachinraja Mon, 01/25/2010 - 13:51
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2960 is also layer 2 in nature.. thats why i asked..

are u saying u had multiple layer 2 vlans in 2960, and everything was up ? or are u referring to multiple layer 3 SVIs on 2960 ?

Incase you have any of the layer 2 vlans which are in down state, it can just be due to the cause that there are no ports active on that VLAN.. again.. why would u need to define layer 3 on 2960/2950 when doing router on a stick ? If you need distributed architecture, you should have a layer 3 switch like 3560/3750 etc and have layer 3 VLAN SVI terminated on the switch. The layer 3 switch will then have a back-to-back /30 link with the router.



sachinraja Mon, 01/25/2010 - 14:22
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My bad..

Thanks for the info.. I was thinking 2960's just as other L2 switches !!! learnt something new today

Harrison - You have your answer now regarding multiple SVI's on 2960s, but i still dont get as to why you would need multiple SVI's on Layer 2 switches (for the fact that they cant route traffic between themselves) ?

Thanks again Jon


glen.grant Mon, 01/25/2010 - 16:02
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  Again the lines continue to blur between layer 2 and 3  .  I'm pretty sure a 2960 is even capable of doing things like being a dhcp server because I think we actually did this once .   They also do port acl's which to me is also a layer 3 function as it would have to inspect the packet to do these .  I wonder if it being able to have multiple layer 3 interfaces whether it could be a dhcp server for multiple subnets  even though it can't route , sounds like an interesting lab test.


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