Владимир Семиколенных » 15 дек 2005, 20:04
Покажите это "иженеру" 3Сом.
SuperStack 3 Switch 4200 - Priority traffic 'fails' between ports on different ASIC's
Problem: Priority traffic 'fails' between ports on different ASIC's
Fact: SuperStack 3 Switch 4226T
Fact: SuperStack 3 Switch 4250T
Fact: 3C17300
Fact: 3C17302
Fact: ESD4537
Fact: 802.1p
Fact: Priority
Fact: Low priority traffic prioritisation between ports on different ASIC device
Problem: Priority traffic fails between ports on different ASIC's
Problem: Prioritization of low priority traffic between ports may not work as expected.
Cause: Prioritization of low priority traffic between ports may not work as expected.There are basically two scenarios, the one of which will work as expected and the other which will not.Consider three ports - ingressPort(A) and ingressPort(B), are both transmitting low priority data to egressPort(C). The expectation is that the ratio of traffic received on egressPort(C) from ingressPort(A) : ingressPort(B) is 50 : 50.Scenario 1:If all three ports, A, B &C are located on the same ASIC device (the ports numbers depend on whether a 24port or 48port switch is being used), the ratio of received traffic ingressPort(A) : ingressPort(B) is 50 : 50.Similarily, if each port A, B and C are each on different ASIC devices the ratio of received traffic will be 50 : 50.This is OK.Scenario 2:If one of the ingress ports, say ingressPort(A), and egressPort(C) are on the same ASIC device, whilst the other ingress port, ingressPort(B), is on another ASIC device, the ratio of traffic received from ingressPort(A) : ingressPort(B) will be approximately 40 : 60 (or even 30 : 70). This is not OK.
Cause: In the problem scenario 2, the ratio of low priority traffic destined for the egressPort(C) is biased towards traffic being sent from the port which is located on a different ASIC, i.e ingressPort(B). This is due to the different buffer thresholds set for the external 10/100 ports verses that for the internal G-LINK (the G-LINK provides the communication path between all the ASIC devices in the Switch). Consequently, during congestion, head of line (HOL) thresholds will kick-in before the G-LINK buffer thresholds - this results in more ingress traffic being dropped on ports that reside on the same ASIC as the egress port.
Fix: There is no current fix using the Galileo 483xx ASIC switch chipset.A possible get-around is to ensure that (in an isolated case) the ports are either all on the same ASIC or on different ASICs. If multiple groups of ports are involved, the balance of low priority traffic received from the transmitter ports is not deterministic and will rely entirely on the buffer levels an HOL at any point in time.
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