Service Manual

The internal Priority assigned for the packet by Ingress FP is used by the memory management unit
(MMU) to assign the packet to right queue by indexing the internal-priority to queue map table (TABLE 1)
in hardware.
PRIO2COS setting for honoring the PFC protocol packets from the Peer switches is as per above Packet-
Dot1p->queue table (Table 2).
The packets come in with packet-dot1p 2 alone are assign to PG6 on ingress.
The packets come in with packet-dot1p 2 alone use Q1 (as per dot1p to Queue classification – Table 2)
on the egress port.
• When Peer sends a PFC message for Priority 2, based on above PRIO2COS table (TABLE 2), Queue 1 is
halted.
• Queue 1 starts buffering the packets with Dot1p 2. This causes PG6 buffer counter to increase on the
ingress, since P-dot1p 2 is mapped to PG6.
• As the PG6 watermark threshold is reached, PFC generats for dot1p 2.
Configuration Example for DSCP and PFC Priorities
Consider a scenario in which the following DSCP and PFC priorities are necessary:
DSCP
0 – 5, 10 - 15 20 – 25, 30 – 35
Expected PFC
Priority
1 2
To configure the aforementioned DSCP and PFC priority values, perform the following tasks:
1. Create class-maps to group the DSCP subsets
class-map match-any dscp-pfc-1
match ip dscp 0-5,10-15
!
class-map match-any dscp-pfc-2
match ip dscp 20-25,30-35
2. Associate above class-maps to Queues Queue assignment to be based on the below table .
NOTE: Although, each port on the S4810, S4820T, and S5000 devices support 8 QoS queues,
you can configure only 4 QoS queues (0-3)to manage data traffic. The remaining 4 queues
(4-7) are reserved for control traffic.
Table 16.
Internal-
priority
0 1 2 3 4 5 6 7
Queue 0 0 0 1 2 3 3 3
3. Dot1p->Queue Mapping Configuration is retained at the default value.
Default dot1p-queue mapping is,
Dell#show qos dot1p-queue-mapping
Data Center Bridging (DCB)
291