Bandwidth consumed by PXCs based on faceplate ports correlates with the faceplate’s port capacity. Because each PXC allocates a faceplate port for exclusive use, the PXC capacity cannot exceed the card capacity that is already allocated for the faceplate ports. In other words, a PXC based on a faceplate port does not add any additional bandwidth to the forwarding complex. This is in contrast to the MAC-based PXCs, where bandwidth consumed on each PXC is added to the faceplate port capacity. If bandwidth is not carefully managed on cards with MAC- based PXC, extensive periods of oversubscription can occur. Operating under extended periods of congestion is not recommended and should be avoided. Therefore, practical use of a MAC-based PXC makes sense in an environment where the utilization of faceplate ports is relatively low so the remaining bandwidth can be used for traffic flowing through the MAC-based PXC.
The bandwidth management in the PXC environment is performed through existing QoS mechanisms, and in addition, for MAC-based PXCs by careful selection of the MAC chip and in some instances in fabric taps. The user can control the selection of these entities.