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New Member

Transient VLANS

Hi,

Vlan2---Layer3 switch.A----Layer3 switch.B----Layer3 switch.C---Vlan2

My VLAN 2 span across the Layer 3 hops, so please suggest how my VLAN2 will communicate across the Layer3 switches.

Note: the switches are connected over the Router port (L3 port) & IP address assignment for VLANs is through DHCP.

Thanks in advance.

6 REPLIES

Re: Transient VLANS

Hi,

The packets in the same vlan should be switched instead of being routed. Because they are in the different switches, you will need to build one trunking between A and B,and one trunking between B and C (or one trunking between A and C directly, if possible). Make sure that vlan 2 is allowed in the trunking and not pruned.

Hope this help

SSLIN

Re: Transient VLANS

Hi Friend,

since the L3 switches are connected on the router port, you have basically 2 seperate instances of VLAN 2 independent of each other.

VLAN 2 subnets will be different on both sides and traffic will be routed not switched.

IF you need to switch the packets then you need create tranks as suggested in earlier post

HTH

Narayan

New Member

Re: Transient VLANS

Hi,

Thanks for the info , but as a work around can't I bridge the Traffic across the switches??

I can't configure Trunks between the switches because of my design limitation.

Re: Transient VLANS

OK, create a tunnel between A and C and make the tunnel in the same subnet as Vlan 2. Is this acceptable?

HTH

SSLIN

Re: Transient VLANS

Sorry, this workaround has a problem. Two interfaces can't not be in the same subnet unless ... ip unnumber?? May be. Well, I'm sleepy, can't think now. I will try this in my lab tomorrow. If you accept this workaround, let me know.

SSLIN

Re: Transient VLANS

I have tried, but I failed. The fact is simple: A packet with the source and destination in the same subnet can only be switched. It cannot be routed. When a L3 switch receive this packet, it will try to switch the packet to other switchports in the same vlan. It will not switch the packet to any other routed ports. No chance!

To solve your problem in this senario, either change one side into a different vlan(so that they can be routed), or change the routing enviroment into a switching enviroment(so they can be switched).If you insist such a Layer2 over Layer3 senario, I know some advanced techonology can do it, like EoMPLS, VPLS. There happen to be a relateed topic in progress:

http://forums.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Service%20Providers&topic=MPLS&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1ddd265f

Maybe you can find your answer there.

Have a nice weekend

SSLIN

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