Access Layer Switches, 2 uplinks to the core, one in blocking state

Unanswered Question
Jul 2nd, 2007

Our network is running a collapsed core design with access layer switches which are 3560-48TS are connected to the 2 6509 core switches via fiber uplinks. There is no distribution layer, how can i take advantage of both of my uplinks to the core, since one is always in blocking state.

I have this problem too.
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Edison Ortiz Mon, 07/02/2007 - 12:18

Change the uplink ports from L2 to L3 ports.

Keep in mind, with this design, you will need to create VLANs on all access switches (No VTP management) and run a routing protocol for the uplinks.

faze1sari Mon, 07/02/2007 - 12:36

Thanks for the timely responce.Ok then what i have to do is add ip addresses to those uplnk interfaces and run EIGRP on them as well? We currently run them all transparent so there is no vtp management anyway.

jkillion Mon, 07/02/2007 - 12:37

As Edison pointed out, one option is to run L3 to the core. This should work fine so long as there isn't a business reason for VLANs to span multiple closets / stacks. If you use this design, I'd also use "mls ip cef load-sharing full". By default, cef load balances based on L3 SA/DA. The cmd above will change this to include L4 port in the equation which will cause the links to balance much more even.

A second option is to split the STP roots between the two 6509's. If you do this, traffic for half the vlans will take one uplink and the other half will use the second uplink. This won't balance traffic as evenly as L3 links, but it should do the trick. Of course, if you only have a single vlan that is heavily utilized then this won't do much for you.


Jay Killion, CCIE #17873

faze1sari Mon, 07/02/2007 - 12:47

Thanks for the reply I will definitly try this out in the lab and see how it works, unfortunatly we have a vlan that spans the whole network where we put everything that needs to be migrated to our new ip scheme. After the migration project we can go back and prune our uplinks to follow your suggestion.


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