Users Guide

Although the system contains 9 MB of space for shared buers, a minimum guaranteed buer is provided to all the internal and
external ports in the system for both unicast and multicast trac. This minimum guaranteed buer reduces the total available shared
buer to 7,787 KB. This shared buer can be used for lossy and lossless trac.
The default behavior causes up to a maximum of 6.6 MB to be used for PFC-related trac. The remaining approximate space of 1
MB can be used by lossy trac. You can allocate all the remaining 1 MB to lossless PFC queues. If you allocate in such a way, the
performance of lossy trac is reduced and degraded. Although you can allocate a maximum buer size, it is used only if a PFC
priority is congured and applied on the interface.
The number of lossless queues supported on the system is dependent on the availability of total buers for PFC. The default
conguration in the system guarantees a minimum of 52 KB per queue if all the 128 queues are congested. However, modifying the
buer allocation per queue impacts this default behavior.
By default the total available buer for PFC is 6.6 MB and when you congure dynamic ingress buering, a minimum of least 52 KB
per queue is used when all ports are congested. By default, the system enables a maximum of two lossless queues on the S4820T
platform.
This default behavior is impacted if you modify the total buer available for PFC or assign static buer congurations to the
individual PFC queues.
Behavior of Tagged Packets
The below is example for enabling PFC for priority 2 for tagged packets. Priority (Packet Dot1p) 2 will be mapped to PG6 on
PRIO2PG setting. All other Priorities for which PFC is not enabled are mapped to default PG – PG7.
Classication rules on ingress (Ingress FP CAM region) matches incoming packet-dot1p and assigns an internal priority (to select
queue as per Table 1 and Table 2).
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 classication – 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 buering the packets with Dot1p 2. This causes PG6 buer 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.
Conguration 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 congure 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
!
250
Data Center Bridging (DCB)