C
HAPTER
36
| Class of Service Commands
Priority Commands (Layer 3 and 4)
– 1165 –
DEFAULT SETTING
COMMAND MODE
Interface Configuration (Port, Static Aggregation)
COMMAND USAGE
◆ The default mapping of CoS to PHB values shown in Table 146 is based
on the recommended settings in IEEE 802.1p for mapping CoS values
to output queues.
◆ Enter a value pair for the internal per-hop behavior and drop
precedence, followed by the keyword “from” and then up to eight CoS/
CFI paired values separated by spaces.
◆ If a packet arrives with a 802.1Q header but it is not an IP packet, then
the CoS/CFI-to-PHB/Drop Precedence mapping table is used to
generate priority and drop precedence values for internal processing.
Note that priority tags in the original packet are not modified by this
command.
◆ The internal DSCP consists of three bits for per-hop behavior (PHB)
which determines the queue to which a packet is sent; and two bits for
drop precedence (namely color) which is used by Random Early Discard
(RED) to control traffic congestion.
◆ Random Tail Drop is supported for the queue descriptors and buffers
only. When Random Tail Drop is enabled, the last six bits of the queue
descriptor limit and buffer limits are randomized. This allows a
simplified Random Early Discard (RED) and breaks queue
synchronization when the queue is congested due to different sources
sending packets with the same byte count.
◆ The specified mapping applies to all interfaces.
Table 146: Default Mapping of CoS/CFI to Internal PHB/Drop Precedence
CFI
CoS
0 1
0(0,0)(0,0)
1(1,0)(1,0)
2(2,0)(2,0)
3(3,0)(3,0)
4(4,0)(4,0)
5(5,0)(5,0)
6(6,0)(6,0)
7(7,0)(7,0)
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