BusWorks
®
900EN-S005 Ethernet Switch User’s Manual Ethernet I/O
___________________________________________________________________
_______________________________________________________________________________________
Acromag, Inc. Tel:248-624-1541 Fax:248-624-9234 Email:
http://www.acromag.com
53
That is, the packet’s “user-priority” is compared against this value. If it is
greater than or equal to it, the packet is classified as high priority. If it is less
than it, it is classified as low priority.
There is also a global Priority Classification register that is used to set the
high or low priority status of each 3-bit 802.1p tag field (a set bit specifies
high priority for the corresponding tag value). In this method, the inbound
traffic port examines the priority field in the tag and determines if it is of high
or low priority. This method works best when used with ports that have
mixed data and media traffic flow.
Priority Using The IPv4 Diffserv/DSCP Method
Diffserv is another per-frame method of determining outbound traffic priority.
The Diffserv method uses the TOS field in the IP header as a Differentiated
Services Code Point (DSCP) to determine high and low priority on a per
code point basis. Each fully decoded code point can have either high or low
priority and a wider spectrum of priority flow can be defined with this larger
code space. The most significant 6 bits of the IP header TOS field are fully
decoded into 64 possibilities and each resultant singular code is compared
against the corresponding bit in the DSCP register. If the register bit is 1, the
priority is high, and low if 0. Each port has an enable port DSCP bit that
when set, causes the DSCP field of the IP header to determine the high or
low priority. There is also a global DSCP Priority Points register (64 bits) that
is used to fully decode the priority of the 6 bit DSCP field of the IP header.
When setting up a priority scheme, other controls are also available to
regulate traffic, such as: the Priority Scheme bits which control the
interleaving of high and low priority frames. This is a global (all ports)
transmit buffer high/low interleave control (see register 5 bits 3-2). Options
allow high/low ratios of 2:1, 5:1, 10:1, up to delivering all high priority packets
first.
Another global priority control is the Priority Buffer Reserve bit that when set,
will allocate 48 buffers per output port for high priority traffic only. A separate
buffer share bit can also be used to cause all ports to share the available
buffer pool and allow any port to use more than its allocated amount (1/5 of
available buffer space) when the other ports are not busy.
On an individual port basis, we have the Enable Diffserv/DSCP, Enable
802.1p, and Enable Port-Based Priority bits described above, plus an Enable
Port Queue Split that splits the transmit queue of the desired port for high
and low priority traffic. Note that default high priority classification is a VLAN
tag value greater than 4, and low priority being 3 or less (this applies if the
802.1 global priority classification is clear or not defined).
Priority Controls
