Oversubscription Ratios

 

 

For three-tiered designs, increasing the uplink speeds between the access and distribution layer switches may also require increasing uplink speeds between the distribution and core layer switches in order to maintain the desired oversubscription ratio.

Determining the oversubscription ratio of the uplink between the distribution and core layer switches is fairly straight forward.  You need to take into consideration the number of ports connecting the distribution layer switches to the access layer switches or switch stacks, as well as the speeds at which the ports are operating.

For example, let’s say your distribution layer switch is a StackWise Virtual pair that supports a building with 4 floors.  Each floor has two IDFs (wiring closets).  Each IDF has an access layer switch stack consisting of four 48-port switches along with a 2 x 25 Gbps uplink module in two of the switches within the stack.  The total number of 25 Gbps ports required at the distribution layer switches is 4 uplinks x 2 IDFs per floor x 4 floors = 32 ports.  

This configuration would provide up to 32 x 25 Gbps = 800 Gbps bandwidth between the distribution layer and access layer switches.  Simply keeping existing 2 x 40 Gbps uplinks would only provide up to 80 Gbps between the distribution layer and core layer switches.  This would provide an oversubscription ratio of 800:80 or 10:1 between the distribution and core layers. Depending upon your business requirements, this may be insufficient.

Increasing Uplink Speeds

You could choose to add additional 40 Gbps links between the distribution and core layer switches, possibly operating in a Layer 3 EtherChannel configuration.  However, this would require additional 40 Gbps switch ports at every distribution layer and core layer switch.  More importantly, it would require additional fiber optic pairs between the distribution layer switches and the core layer switches. 

In a large campus deployment, the core layer switches may be located in a centralized data center in a different building.  If insufficient optical pairs exist, then additional optical cabling would need to be pulled between the centralized data center and each of the buildings.  This could be a very expensive proposition, as existing conduit space between the buildings may not be capable of supporting additional cabling, and you run the risk of damaging the existing cabling in the conduit – resulting in an extended outage.  Installing new conduit may involve getting the necessary right-of-way to trench and install underground conduit – on top of the cost to install the new fiber optic cable.

An alternative may be to upgrade the uplink speeds between the distribution layer and core layer switches to 100 Gbps.

This would provide an oversubscription ratio of 800:200 or 4:1 between the distribution and core layers. 

As with the access layer, when deciding to upgrade the uplink speeds between the distribution layer switches and the core layer switches, you should keep in mind the following:

●     The optical transceiver modules which connect the distribution layer switches to the core layer switch platforms have to interoperate with each other and have to be compatible with the fiber optic cabling between buildings.

Due to the increased distances between buildings, single mode fiber (SMF) may already be installed between the distribution and core layer switches.  This may help facilitate the migration from 40 Gbps to 100 Gbps between the distribution and core layers.