Cross-Link Sizing Considerations in a Collapsed Core/DL Design

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Apr 19th, 2009
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In a collapsed Core/DL U-shaped design with 2x10G port-channeled links to each access switch what would be recommended for the L3 link between the Core from a Sizing perspective. I would assume that ideally all things being equal this link should be used to a minimum (routing updates) and will mostly be used to pass traffic in case of a failure. However if all is not equal than this might be an issue. Could someone please advise on what would determine the capacity of this link keeping in mind that this is the link between the Core as opposed to a distribution. It would be great to know in general what the recommended best practice would be for sizing these links regardless if it was between the Core or the Distribution. Thx for your help.

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lamav Sun, 04/19/2009 - 19:51
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Can you give a little more detail? is this a campus module or a server farm?

Why did you choose the loop-free inverted U? It mitigates redundancy.

If this is the design you're going to go with, keep in mind that you will have a good deal of crosslink traffic since each access switch will have only one uplink for all vlans, so, assuming you stagger your HSRP active and STP root from vlan to vlan, about 50 percent of your session traffic will traverse your crosslink. So you may want to make it substantial. Make it 20-Gbps, like the uplinks.



vdadlaney Mon, 04/20/2009 - 07:03
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Hi Victor,

This is a Server Farm Module. The design is a Loop Free U not a Loop Free Inverted U. The reason to go with the Loop Free U was to span VLANs across the switches since this is a Server Farm Module. This is a Collapsed Core/DL Design. The link between the Core/DL Switches is a L3 P2P link. The downlink to the Access Switches are Trunked Layer 2 links (2 x 10G bundled).

Could you please elaborate on why the traffic would traverse that crosslink. I agree that if there is a failure on the downlink towards the Access Swtiches than Data traffic would definetly traverse but cannot quite grasp why under normal working conditions there would be traffic on that Crosslink. If Traffic arrives on either core it will just arp for the ip and forward it out its link towards the access switches. Most definetly there will be traffic traversing the Crosslink between the Access Switches but I am more interested in the traffic between the L3 crosslink on the Core's. Thanks for your help.

vdadlaney Thu, 04/23/2009 - 20:01
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Hi, could someone please share their experiences and what criteria did they use to decide on the size of the crosslinks. Would really appreciate any help. Thx


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