MVTX2801
Data Sheet
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 MVTX2801's approach to ensuring bounded delay and minimum bandwidth for high priority flows.
7.9.1 Unicast Flow Control
For unicast frames, flow control is triggered by source port resource availability. Recall that the MVTX2801'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 MVTX2801'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.
7.9.2 Multicast Flow Control
When port based Vlan is not used, a global buffer counter (64 packets) triggers flow control for multicast frames.
When the system exceeds a programmable threshold of multicast packets, Xoff is triggered. Xon is triggered when
the system returns below this threshold. MCC register programs the threshold. When port based Vlan is used, each
Vlan has a global buffer counter.
In addition, each source port has an 8-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 8-bit vector is reset
to zero.
The MVTX2801 also provides the option of disabling VLAN multicast flow control.
Note: If port flow control is on, QoS performance will be affected. To determine the most efficient way to program,
please refer to the QoS Application Note.
7.10 Mapping to IETF Diffserv Classes
The mapping between priority classes discussed in this chapter and elsewhere is shown below.
MVTX2801
P7
P6
P5
P4
P3
P2
P1
P0
IETF
NM
EF
AF0
AF1
AF2
AF3
BE0
BE1
Table 4 - Mapping between MVTX2801 and IETF Diffserv Classes for Gigabit Ports
As the table illustrates, P7 is used solely for network management (NM) frames. P6 is used for expedited
forwarding service (EF). Classes P2 through P5 correspond to an assured forwarding (AF) group of size 4. Finally,
P0 and P1 are two best effort (BE) classes.
Features of the MVTX2801 that correspond to the requirements of their associated IETF classes are summarized in
the table below.
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