Data Center Design: Nexus 7K with VDC-core/VDC-agg model

[Adelaziz Ben Aziza]

Dear all,

I'm doing with a collapsed VDC-core/VDC-agg model on the same chassis with 2  Redundant Cisco Nexus 7010 and a pair of Cisco 6509 used as a Service  Chassis without VSS. Each VDC Core have redundant link to 2 PE based on  Cisco 7606.

After reading many design document of Cisco, I'm asking  what is the need of a Core Layer in a Data Center especially if it is  small or medium size with only 1 aggregation layer and dedicated for a Virtualized Multi-Tenanted environement? What is driving to have a core layer?

Thanx

[Chad Peterson · Accepted solution]

If your data center is small enough to not require a core, then its fine to run with a collapsed core (distribution + core as the same device).  For a redundant design you need to uplink all your distribution switches to each of your cores.  If you have no cores, then you need full mess at your distribution layer (for full redundancy).

Lets say you have only 4 distribution pairs...so 8 switches  For full redundancy each one needs uplink to each other.  This means you need 28 total ports used to connect all the switches together (n(n-1)/2).  Thats also assuming 1 link to each device.  However if you had redundant cores, the number of links used for uplinks reduces to 21 total links (this includes links between each distribution switch in a site, and link between the two cores).  So here you see your only saving 7 links.  Here your not gaining much by adding a core.

However if you have 12 distribution pairs...so 24 switches.  Full redundancy means you have 276 links dedicated for this.  If you add a core, this drops to 61 links.  Here you see the payoff.