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參數(shù)資料
| 型號： | MVTX2604 |
| 廠商： | Zarlink Semiconductor Inc. |
| 英文描述： | Managed 24-Port 10/100 Mb + 2 Port 1 Gb Ethernet Switch |
| 中文描述： | 管理24端口的10/100 Mb 2端口千兆以太網(wǎng)交換機1 |
| 文件頁數(shù)： | 40/173頁 |
| 文件大小： | 2097K |
| 代理商： | MVTX2604 |
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MVTX2604
Data Sheet
42
Zarlink Semiconductor Inc.
7.10 MVTX2604 Flow Control Basics
Because frame loss is unacceptable for some applications, the MVTX2604 provides a flow control option. When
flow control is enabled, scarcity of buffer space in the switch may trigger a flow control signal; this signal tells a
source port that is sending a packet to this switch, to temporarily hold off.
While flow control offers the clear benefit of no packet loss, it also introduces a problem for quality of service. When
a source port receives an Ethernet flow control signal, all microflows originating at that port, well-behaved or not,
are halted. A single packet destined for a congested output can block other packets destined for uncongested
outputs. The resulting head-of-line blocking phenomenon means that quality of service cannot be assured with high
confidence when flow control is enabled.
In the MVTX2604, each source port can independently have flow control enabled or disabled. For flow control
enabled ports, by default all frames are treated as lowest priority during transmission scheduling. This is done so
that those frames are not exposed to the WRED Dropping scheme. Frames from flow control enabled ports feed to
only one queue at the destination, the queue of lowest priority. This means that if flow control is enabled for a given
source port then we can guarantee that no packets originating from that port will be lost but at the possible expense
of minimum bandwidth or maximum delay assurances. In addition, these “downgraded” frames may only use the
shared pool or the per-source reserved pool in the FDB; frames from flow control enabled sources may not use
reserved FDB slots for the highest six classes (P2-P7).
The MVTX2604 does provide a system-wide option of permitting normal QoS scheduling (and buffer use) for
frames originating from flow control enabled ports. When this programmable option is active, it is possible that
some packets may be dropped even though flow control is on. The reason is that intelligent packet dropping is a
major component of the MVTX2604’s approach to ensuring bounded delay and minimum bandwidth for high priority
flows.
7.10.1 Unicast Flow Control
For unicast frames, flow control is triggered by source port resource availability. Recall that the MVTX2604’s buffer
management scheme allocates a reserved number of FDB slots for each source port. If a programmed number of a
source port’s reserved FDB slots have been used then flow control Xoff is triggered.
Xon is triggered when a port is currently being flow controlled and all of that port’s reserved FDB slots have been
released.
Note that the MVTX2604’s per-source-port FDB reservations assure that a source port that sends a single frame to
a congested destination will not be flow controlled.
7.10.2 Multicast Flow Control
In unmanaged mode, flow control for multicast frames is triggered by a global buffer counter. When the system
exceeds a programmable threshold of multicast packets Xoff is triggered. Xon is triggered when the system returns
below this threshold.
In managed mode, per-VLAN flow control is used for multicast frames. In this case, flow control is triggered by
congestion at the destination. How so The MVTX2604 checks each destination to which a multicast packet is
headed. For each destination port, the occupancy of the lowest-priority transmission multicast queue (measured in
number of frames) is compared against a programmable congestion threshold. If congestion is detected at even
one of the packet’s destinations then Xoff is triggered.
In addition, each source port has a 26-bit port map recording which port or ports of the multicast frame’s fanout
were congested at the time Xoff was triggered. All ports are continuously monitored for congestion and a port is
identified as uncongested when its queue occupancy falls below a fixed threshold. When all those ports that were
originally marked as congested in the port map have become uncongested, then Xon is triggered and the 26-bit
vector is reset to zero.
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| MVTX2604AG | Managed 24-Port 10/100 Mb + 2 Port 1 Gb Ethernet Switch |
| MVTX2801 | Unmanaged 4-Port 1000 Mbps Ethernet Switch |
| MVTX2801AG | Unmanaged 4-Port 1000 Mbps Ethernet Switch |
| MVTX2802 | Managed 4-Port 1000 Mbps Ethernet Switch |
| MVTX2802AG | Managed 4-Port 1000 Mbps Ethernet Switch |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| MVTX2604A | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:MVTX260x Physical Port Control |
| MVTX2604AG | 制造商:ZARLINK 制造商全稱:Zarlink Semiconductor Inc 功能描述:Managed 24-Port 10/100 Mb + 2 Port 1 Gb Ethernet Switch |
| MVTX2801 | 制造商:ZARLINK 制造商全稱:Zarlink Semiconductor Inc 功能描述:Unmanaged 4-Port 1000 Mbps Ethernet Switch |
| MVTX2801A | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Unmanaged 4 port Gigabit Ethernet switch |
| MVTX2801AG | 制造商:ZARLINK 制造商全稱:Zarlink Semiconductor Inc 功能描述:Unmanaged 4-Port 1000 Mbps Ethernet Switch |
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