Redundant Links - Spanning Tree or ?

Unanswered Question

Hi All,

I have a pair of 6509s running as our core switch. I also have a few 4510s that we are looking at bringing up over 10GE. I would like to set up a redundant link to the other core as fall back but am unsure of the direction to go.

1. Leave it up to spanning tree in rapid-pvst

2. Setup L3 routing from the 4510s to the 6509s. If going this route what needs to be done with vlans.

3. ? some better fast way I can't think of.

If anyone has a good place to start looking dealing with L3 forwarding from the core to distribution switch let me know.



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asaykao73 Mon, 06/22/2009 - 16:33
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Hi Mike,

We have access layer switches that have one trunk into core1 and another trunk to core2. Both cores are 7606 with 48 port line card.

We've used spanning-tree cost on both ends of the access layer switch and core to manually determine which way to forward traffic.

This helps me visualize the traffic flow better knowing which one is the primary link and which one is the redundant link instead of leaving it up to spanning-tree.


interface GigabitEthernet1/2

description To switch2:Gi0/1 (Primary)


interface GigabitEthernet2/2

description To switch2:Gi0/2 (Secondary)

spanning-tree cost 20


interface GigabitEthernet0/1

description To core2:Gi1/2 (Primary)

interface GigabitEthernet0/2

description To core1:Gi2/2 (Secondary)

spanning-tree cost 20

Traffic going via Gi0/1 and blocking on Gi0/2.

switch2#sh spanning-tree interface g0/1

Vlan Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

VLAN0010 Root FWD 3004 128.25 P2p

VLAN0011 Root FWD 3004 128.25 P2p

VLAN0020 Root FWD 3004 128.25 P2p

switch2#sh spanning-tree interface g0/2

Vlan Role Sts Cost Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

VLAN0010 Altn BLK 3020 128.26 P2p

VLAN0011 Altn BLK 3020 128.26 P2p

VLAN0020 Altn BLK 3020 128.26 P2p




davy.timmermans Wed, 06/24/2009 - 10:16
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Hi Andy,

it's much easier to accomplish this (example below with loadbalancing):

for example core 1 is spanning-tree root for odd vlans and backup root for even vlans.

core 2 is spanning tree root for even vlans and backup root for odd vlans

if the access switch has two equal uplinks to core 1 and core 2, the port on the access switch leading to the backup root is always blocking.

core 1

spanning-tree vlan 1,3,5 priority 8192

spanning-tree vlan 2,4,6 priority 16384


spanning-tree vlan 2,4,6 priority 8192

spanning-tree vlan 1,3,5 priority 16384

you can also eliminate spanning-tree by using a L3 link between core 1 and core 2.

Your solution depends on your design and requirements.

asaykao73 Wed, 06/24/2009 - 16:35
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Hi Davy,

Totally understand your logic.

We did explore this option but based on our design and requirements, we decided to go with the spanning-tree cost option.



Tony.henry Wed, 06/24/2009 - 13:32
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I would think that trunking between the access switches and the core switches would be worth thinking about as well.

That way if you ever wanted to split the access switch up for whatever reason it would be pretty trivial.



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