There are many organizations whose campuses do not have the luxury of new buildings, and some are 50+ years old. What this means is that running fiber in old multistory buildings is expensive, difficult and very time consuming. It's safe to say that the in-building fiber infrastructure is fairly fixed. As the user population grows, the network grows. The preferred SOP is to dual home each switch to the STP root and secondary root. This means that for N number of switches you need 4N fiber strands. That's one TX/RX pair to the STP root and another TX/RX pair to the STP secondary root. As you may have guessed, being low on fiber infrastructure is a regular issue. Because of this, many switches only go to one upstream switch, or switches get daisy chained, which not a best practice. The implications of this is that any network maintenance or failure on the aggregation box impacts the smaller leaf switches. One possible solution is to use a larger chassis solution, like a 4500 or a 2970/3750 switch stack to reduce the number of switches and uplinks, but this is not always a viable solution. Below is a sample cookie cutter topology.

There is a fix for this cabling infrastructure limitation, assuming that you're using Single Mode Fiber, SFPs that only use one strand for both TX and RX! These are commonly referred to as BX optics. They work by having TX and RX operate on a different frequency, using an internal optical splitter to separate the frequencies. One side will transmit on 1310nm with the other side receiving on 1310nm, and the reverse path riding on 1490nm. Yes, this does require 2 different SFPs which are referred to as D side and U side. The D optic transmits on 1490nm while the U optic transmits on 1310nm.

For more information about these optics, the Cisco product page is a good reference:

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