
IDT Switch Core
PES48T12G2 User Manual
3 - 5
April 5, 2013
Notes
Figure 3.2 Architectural Model of Arbitration
Port Arbitration
Each egress port does port arbitration among multiple ingress ports for packets. Each egress port
contains a port arbiter. Ingress port(s) that wish to transfer one or more packets to the egress port partici-
pate in arbitration.
Prior to participating in port arbitration, each ingress port does packet ordering. Based on this, each
ingress port selects zero, one, or multiple packets as candidates for transfer towards the EFBs.Port arbitra-
tion is done according to the configuration of the egress port’s VC Capability Structure. The PES48T12G2
ports operating in upstream switch port or downstream switch port mode support port arbitration using
Hardware Fixed Round-Robin (default) and 32-phase Weighted Round-Robin (WRR) algorithms
1
. The arbi-
tration algorithm and WRR arbitration parameters are programmed independently for each of the ports via
the VC Capability Structure located in the port’s configuration space.
Cut-Through Routing
The PES48T12G2 utilizes a combined input and output buffered cut-through switching architecture to
forward PCIe TLPs between switch ports. Cut-through means that while a TLP is being received on an
ingress link, it can be simultaneously routed across the switch and transferred on the egress link. The entire
TLP need not be received and buffered prior to starting the routing process (i.e., store-and-forward). This
reduces the latency experienced by packets as they are transferred across the switch.
Typically, cut-through occurs when a TLP is received on an ingress link whose bandwidth is greater than
or equal to the bandwidth of the egress link. For example, a TLP received on a x4 Gen2 port and destined
to a x1 Gen2 port is cut-through the switch. This rule ensures that the ingress link has enough bandwidth to
prevent ‘underflow’ of the egress link. In addition to this, the PES48T12G2 does “adaptive cut-through”,
meaning that packets are cut-through even if the egress link bandwidth is greater than the ingress link
bandwidth. In this case, the cut-through transfer starts when the ingress port has received enough quantity
of the packet such that the packet can be sent to the egress link without underflowing this link.
1.
Weighted round robin arbitration with 32-phases is implemented by converting the PCIe port arbitration table
into weighted round robin counts. Therefore, over short intervals grants may not match the phase table configura-
tion.
Port 0 IFB
VC 0
Port 1 IFB
VC 0
Port 13 IFB
VC 0
EFB
VC 0
VC 0
Port
Arbitration
VC
Arbitration
(not
applicable)
Port 0 Egress Arbitration
TC/
VC
Map
EFB
VC 0
VC 0
Port
Arbitration
VC
Arbitration
(not
applicable)
Port 13 Egress Arbitration
TC/
VC
Map
Содержание 89HPES48T12G2
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Страница 22: ...IDT Register List PES48T12G2 User Manual xiv April 5 2013 Notes...
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