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Dell EMC Networking OS Configuration Guide for the S5048F–ON System 9.14.2.8

Buffer Organization

This section describes the buffer organization on the platform.

A single chip architecture can allocate or share all its resource on all the ports. However, the system runs on a different 2x2 chip design.

In this design, all ports are assigned to four port-sets. These sets are built as follows:
  • A or B for ingress, referred to as layers
  • R or S for egress, referred to as slices

There are 4 XPEs with each XPE containing 5.5MB of buffer space making the total available buffer space to be 22MB. Out of these 4 XPEs, one of the XPE is used for a particular traffic flow based on its ingress and egress port pipes. For example, if a traffic flow enters through a port that is a part of the ingress pipe 0 and exits through a port that is part of the egress pipe 0, then XPE A from MMU Slice R is used. If a traffic flow enters through the pipe 1 and exits through the pipe 3, then XPE B from MMU Slice S is used.

The following example shows default DCB buffer values:
DellEMC#show dcb

DCB Status: Enabled, PFC Queue Count: 2

Total Buffer:         Total available buffer excluding the buffer pre-allocated
                      for guaranteed services like global headroom, queue's min
                      guaranteed buffer and CPU queues.

PFC Total Buffer:     Maximum buffer available for lossless queues.

PFC Shared Buffer:    Buffer used by ingress priority groups for shared usage.

PFC Headroom Buffer:  Buffer used by ingress priority group for shared headroom usage.
PFC Available Buffer: Current buffer available for new lossless queues to be
                      Provisioned.

stack-unit Total Buffer PFC Total Buffer PFC Shared Buffer PFC Headroom Buffer PFC Available Buffer
      PP            (KB)             (KB)              (KB)                (KB)                 (KB)
-----------------------------------------------------------------------------------------------------
1     0.0    4727         2656             1040              1040                576
1     0.1    4727         2656             1040              1040                576
1     0.2    4727         2656             1040              1040                576
1     0.3    4727         2656             1040              1040                576

Z9100R145U29#show dcb

DCB Status: Enabled, PFC Queue Count: 2

stack-unit 	Total Buffer 	PFC Total Buffer 	PFC Shared Buffer 	PFC Available Buffer
          PP 	(KB) 		(KB) 			(KB) 			(KB)
------------------------------------------------------------------------------------------------------------------------------------------
1        0.0 	4096 		3664 			208 			3456 
1        0.1 	4096 		3664 			208 			3456 
1        0.2 	4096 		3664 			208 			3456 
1        0.3 	4096 		3664 			208 			3456

DellEMC#
DellEMC#show dcb

DCB Status: Enabled, PFC Queue Count: 2

Total Buffer:         Total available buffer excluding the buffer pre-allocated
                      for guaranteed services like global headroom, queue's min
                      guaranteed buffer and CPU queues.

PFC Total Buffer:     Maximum buffer available for lossless queues.

PFC Shared Buffer:    Buffer used by ingress priority groups for shared usage.

PFC Headroom Buffer:  Buffer used by ingress priority group for shared headroom usage.
PFC Available Buffer: Current buffer available for new lossless queues to be
                      Provisioned.

stack-unit Total Buffer PFC Total Buffer PFC Shared Buffer PFC Headroom Buffer PFC Available Buffer
      PP            (KB)             (KB)              (KB)                (KB)                 (KB)
-----------------------------------------------------------------------------------------------------
1     0.0    3399         2656             1040              1040                576
1     0.1    3399         2656             1040              1040                576
1     0.2    3399         2656             1040              1040                576
1     0.3    3399         2656             1040              1040                576
DellEMC#

The following table shows the PFC buffer required for one loss less queue on various supported interface speeds:

DCB Buffer value after PFC configuration:

When PFC is configured on a particular port, two XPEs corresponding to the ingress pipe to which the port belongs is updated with the buffer values. If the port is on the ingress pipe 0 or 3, both the XPE A are updated. Where as, if the port is on ingress pipe 1 or 2, then both the XPE B are updated.

The following example shows DCB buffer values after one PFC lossless queue configured on a port that belongs to ingress pipe 0 [10G speed]:
DellEMC#show dcb

DCB Status: Enabled, PFC Queue Count: 2

Total Buffer:         Total available buffer excluding the buffer pre-allocated
                      for guaranteed services like global headroom, queue's min
                      guaranteed buffer and CPU queues.

PFC Total Buffer:     Maximum buffer available for lossless queues.

PFC Shared Buffer:    Buffer used by ingress priority groups for shared usage.

PFC Headroom Buffer:  Buffer used by ingress priority group for shared headroom usage.
PFC Available Buffer: Current buffer available for new lossless queues to be
                      Provisioned.

stack-unit Total Buffer PFC Total Buffer PFC Shared Buffer PFC Headroom Buffer PFC Available Buffer
      PP            (KB)             (KB)              (KB)                (KB)                 (KB)
-----------------------------------------------------------------------------------------------------
1     0.0    4727         2656             1040              1040                576
1     0.1    4727         2656             1040              1040                576
1     0.2    4727         2656             1040              1040                576
1     0.3    4727         2656             1040              1040                576
Z9100R145U29#show dcb

DCB Status: Enabled, PFC Queue Count: 2

stack-unit 	Total Buffer 	PFC Total Buffer 	PFC Shared Buffer 	PFC Available Buffer
          PP 	(KB) 		(KB) 			(KB) 			(KB)
------------------------------------------------------------------------------------------------------------------------------------------
1        0.0 	4096 		3664 			208 			3402 
1        0.1 	4096 		3664 			208 			3402 
1        0.2 	4096 		3664 			208 			3456 
1        0.3 	4096 		3664 			208 			3456


DellEMC#
DellEMC#show dcb

DCB Status: Enabled, PFC Queue Count: 2

Total Buffer:         Total available buffer excluding the buffer pre-allocated
                      for guaranteed services like global headroom, queue's min
                      guaranteed buffer and CPU queues.

PFC Total Buffer:     Maximum buffer available for lossless queues.

PFC Shared Buffer:    Buffer used by ingress priority groups for shared usage.

PFC Headroom Buffer:  Buffer used by ingress priority group for shared headroom usage.
PFC Available Buffer: Current buffer available for new lossless queues to be
                      Provisioned.

stack-unit Total Buffer PFC Total Buffer PFC Shared Buffer PFC Headroom Buffer PFC Available Buffer
      PP            (KB)             (KB)              (KB)                (KB)                 (KB)
-----------------------------------------------------------------------------------------------------
1     0.0    3399         2656             1040              1040                567
1     0.1    3399         2656             1040              1040                576
1     0.2    3399         2656             1040              1040                567
1     0.3    3399         2656             1040              1040                576
DellEMC#

Following table shows the XPE numbering scheme in Dell EMC Networking OS:

Table 1. XPE Numbering on Dell EMC Networking OSXPE Numbering on Dell EMC Networking OS
XPE Numbering
XPE A of MMU Slice R 0.0
XPE A of MMU Slice S 0.1
XPE B of MMU Slice R 0.2
XPE B of MMU Slice S 0.3

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