Layer 2 Load Balancing over multiple hops?

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May 13th, 2008
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Is there a standardized protocol that would allow load balancing to occur over multiple hops from one point to another? LACP is intended for use with point-to-point links. I'm wondering about a routing-like load sharing for a Layer 2 redundant network. (RSTP/MST)

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Jon Marshall Tue, 05/13/2008 - 13:04
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Well yes you can do this in a layer 2 network with STP. Assuming you have redundant connections between all your switches you cna manipulate STP root/secondary and then individual STP port costs/vlan costs to ensure that one set of vlans use one set of links within your switch network and another set use a different set of links.


This can be quite time consuming if you have a large diameter of switches though.


Jon

pthomason Tue, 05/13/2008 - 13:09
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That's the conclusion I keep coming back to, splitting traffic into VLANs and engineering the paths manually via MST path costs. The new Nexus line looks like they may implement a Layer 2 solution, but I suspect the entry cost will be prohibitive. Anyone able to confirm that my search for an automagic solution is likely to end in disappointment.

Joseph W. Doherty Tue, 05/13/2008 - 15:04
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If each L2 hop load balances for its outgoing links, which could also vary per link hop, is there even a need for a multiple hop load balancing protocol?

sundar.palaniappan Tue, 05/13/2008 - 15:28
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Assuming you have multiple links setup between switches you can configure ether-channel to load balance traffic at layer 2. Though it's not a direct load balancing strategy I see a good number of networks out there make use of this method to utilize redundant links to load balance. The other option, as suggested by Jon earlier, is to use PVST and split VLAN traffic to ride different links.


HTH


Sundar

pthomason Wed, 05/14/2008 - 06:12
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I understand EtherChannel, but it demands direct connections between participating switches. Assuming a N, S, E, W layout of switches, could you tunnel an EtherChannel connection from N to S through E and W using dot1q-tunnel? This would be complicated with only one transport (trunk) link between each switch, but might be manageable.

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