IDT 89HPES48T12G2, User Manual, Page: 46 / 276

IDT Switch Core
PES48T12G2 User Manual 3 - 6 April 5, 2013
Notes The ingress and egress link bandwidth is determined by the negotiated speed and width of the links.
Table 3.5 shows the conditions under which cut-through and adaptive-cut-through occur. When the condi-
tions are met, cut-through is performed across the IFB, crossbar
1
, and EFB. Note that a packet undergoing
a cut-through transfer across the Switch Core may be temporarily delayed by the presence of prior packets
in the IFB and/or EFB. In this case, the packet starts cutting-through as soon as it becomes unblocked by
prior packets.
When cut-through routing of a packet is not possible, the packet is fully buffered in the appropriate IFB
prior to being transferred to the EFB and towards the egress link (i.e., store-and-forward operation). Once
the packet is stored in the IFB, there is no necessity to fully store it in the EFB as it is transferred towards
the egress link.
1.
During cut-through transfers, the crossbar maintains the connection between the appropriate IFB and EFB
through-out the duration of the transfer.
Ingress
Link
Speed
Ingress
Link
Width
Egress
Link
Speed
Egress
Link
Width
Conditions for Cut-
Through
2.5 Gbps x8 2.5 x8, x4, x2, x1 Always
5.0 x4, x2, x1 Always
x8 At least 50% of packet is in IFB
x4 2.5 x4, x2, x1 Always
x8 At least 50% of packet is in IFB
5.0 x2, x1 Always
x4 At least 50% of packet is in IFB
x8 At least 75% of packet is in IFB
x2 2.5 x2, x1 Always
x4 At least 50% of packet is in IFB
x8 At least 75% of packet is in IFB
5.0 x1 Always
x2 At least 50% of packet is in IFB
x4 At least 75% of packet is in IFB
x8 At least 100% of packet is in IFB
x1 2.5 x1 Always
x2 At least 50% of packet is in IFB
x4 At least 75% of packet is in IFB
x8 Never (100% of packet is in IFB)
5.0 x1 At least 50% of packet is in IFB
x2 At least 75% of packet is in IFB
x4 Never (100% of packet is in IFB)
x8
Table 3.5 Conditions for Cut-Through Transfers (Part 1 of 2)