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Assuming I've got four C3750X, each side stacked with Stackwise Plus and using RSTP:

C1 -----10Gbit----- C2
|                    |
|                    |
SW                  SW
|                    |
|                    |
C3 -----10Gbit----- C4

Would the second 10Gbit-uplink in RSTP alternate or backup state?

Edit: The two 10Gbit lines are provided by two different carriers with two different round-time-trips (due to different fibre lengths)

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3 Answers

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It depends on how you configure STP.

PVSTP and PVSTP+ supports per-VLAN STP, wich means that you can chose one uplink for a certain set of VLAN's and another uplink for other VLAN's.

Either way - there is no need to run separate uplinks when you have a switch stack on each end. Just configure a etherchannel between the switches, using one port on each physical switch. This will give you the redundancy (and load balancing) you are after.

For more information: http://www.cisco.com/en/US/docs/switches/lan/catalyst3750x_3560x/software/release/12.2_53_se/configuration/guide/swethchl.html#wp1342634

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Definitely would use etherchannel. – SpacemanSpiff Sep 14 '11 at 13:24
Etherchannel might add some problems due to the different round-time trips. – CyberOptic Sep 14 '11 at 13:46
So make them routed connections with a higher metric on the faster line. – SpacemanSpiff Sep 14 '11 at 14:19
Can you explain what you mean with routed connections? Layer 3 routing would be not an option. – CyberOptic Sep 14 '11 at 14:45
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up vote 2 down vote

One of the 10Gig links would be in alternate blocking state and one would be forwarding if I read that diagram correctly.

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Yes, but is the blocking state backup or alternate? It matters because of the RSTP convergence times. – CyberOptic Sep 14 '11 at 13:45
CyberOptic: can't you just log in to the switch and check? I can't see much need for a 3rd link in this setup. – HampusLi Sep 14 '11 at 14:25
The switches are yet to be bought. C1 & C3 are onsite, C2 & C4 are remote. There are two carriers providing fibre, one shall be backup. – CyberOptic Sep 14 '11 at 14:43
Sorry, should have read more closely - it would be in ALTN-BLK state. I've updated my answer accordingly. – paulos Sep 14 '11 at 15:51
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up vote 1 down vote

In that topology, the blocked port will be in Alternate state.

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