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Matt_70198's avatar
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Feb 09, 2012

Trunk to two separate switches?

We have two separate HA production switches, Sw1 and Sw2, with an etherchannel between them. We'd like to have redundant links to the LB so we have 1 link from Sw1 to LB 1.1, and 1 link from Sw2 to LB 1.2





g1/6 g1/6


\ /


\ /


1.1 1.2





On the LB I created trunk 'Trunk_External', with interfaces 1.1 and 1.2, no LACP.



I created VLAN 'External' with Untagged interfaces 'Trunk_External'. And then a self IP with vlan 'External'.



I've tried STP mode pass-through, and STP mode STP, and all interfaces on both switches and LB trunk show forwarding either way, not sure if this how it's supposed to be, I just don't want a bridging loop.



Is that the desired way to do this even though all ports are forwarding? STP info below.







Gi1/0/6 Desg FWD


Po1 Desg FWD







Gi1/0/6 Desg FWD


Po1 Root FWD







+-> STP INSTANCE 0 priority 61440 root bridge B8:BE:BF:E4:90:80 (Sw1)


+-> STP VLAN 0/External +-> STP TRUNK 0/Trunk_External | path cost 20000 priority 128 role root | state forward (forward) link auto not edge



VLAN External tag 30 00:23:E9:09:22:C3 MTU 1500


+-> VLAN MEMBER External/Trunk_External untagged


+-> TRUNK Trunk_External(Id: 2) 00:23:E9:09:22:CF LACP DISABLED members: 2 (2 working)





2 Replies

  • does the switches support cross-stack etherchannel? if not, i do not think you should create trunk on bigip side.



    i guess the switches do not act like one virtual switch. so, can you just assign interface 1.1 and 1.2 to vlan external as untagged? for stp, pass-through mode should be fine.
  • Correct no ability to etherchannel the switchports going to the LB.



    A coworker steered me down the wrong path by saying I need to do this with a trunk and not selecting LACP. I did what you suggested nitass and removed the trunk and added 1.1 and 1.2 to External vlan.




    With STP in pass-through I see Sw2 Gi1/0/6 in BLK. With STP enabled on the LB I see 1.1 in FWD and 1.2 in BLK.




    During testing I found pass-through doesn't work so well, I shut down 1/6 on Sw1 and instead of Sw2 1/6 going to FWD it went to block loop inconsistent state for some reason, had to shut no shut interface for it to come up.




    With LB mode set to STP I shut down Sw1 1/6, 1.1 went to BLK, 1.2 went to FWD, all was good. So in this type of setup STP is preferred vs pass-through.




    Thanks for the advice nitass!