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Jun 14th, 2009
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1.I have two switches connected together with two cables,Switch A is the root bridge for my vlan 2 traffic,so both ports are in designated fowarding. I want my traffic to preffer one of the port over the other, so I reduced the priority for vlan 2 on the port on switch A.

My question is ,do I need to reduce the priority at switch B too? Does this applies to port-cost too.

2. In a switched network with a root bridge for many spanning-tree vlan instance,will traffic from one leaf switch to another leaf switch on same vlan ,go the root bridge first

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thotsaphon Sun, 06/14/2009 - 10:35
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1. You need not reduce priority on the switchB. You can modify by changing cost on the switchB as well. Priority will work for the downstream switch.

2. No, It depends on where hosts on that vlan are connecting to.



scottmac Sun, 06/14/2009 - 13:23
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Being the root doesn't affect the traffic, it only affects which device controls the spanning-tree.

Traffic will always switch through the shortest available path (shortest can be defined physically or logically, depending on the specifics).

If you want to engineer traffic flows without MPLS, you need to organize your VLANs, HSRP/VRRP interfaces, and spanning-tree instances.

Spanning tree only prevents multiple parallel L2 links (i.e. those that are not aggregate links) for the given broadcast domain.

Good Luck

achruuf Tue, 12/01/2009 - 07:11
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1- yes you need to configure it on both ends [priority or cost], since each switch treats its interface from its poing of view [designated from one side is root from the other side]

2- yes, leaf to leaf passes through the root, else why do we have a tree topology ?



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