Edge-Core AS6700-32X Cli Reference Manual page 560

Chapter 21
| Data Center Bridging Commands
Congestion Notification Commands
its congested state and that the rate of the flow entering the network should be
reduced.
Upon receiving the CN messages, rate limiting is initiated as close as possible to the
source of the congestion. This alleviates the congestion at the network core and
stops it from spreading through the network.
Congestion Notification is different from PFC in that CN messages are propagated
all the way toward the source of the congestion, while the PFC Pause is a hop-by-
hop process, and normally takes longer to slow down the congestion caused by a
specific flow.
The rate limiter parameters prompted by CN notifications are dynamically adjusted
based on feedback coming from the CPs. CN functions at Layer 2 and therefore
applies to all traffic types, using exponential decrease in traffic rates when
congestion occurs, and a linear increase when bandwidth is available.
Congestion Notification Domains
Congestion notification depends on the formation of a cooperating set of network
devices including VLAN-aware bridges and end stations to reduce frame loss. By
partitioning the bridges' and end stations' resources, frames sourced or sunk by
non-cooperating bridges or end stations can be carried across the network with
minimal contention with Congestion Controlled Flows (CCFs) for those resources.
Operational Concept
In order for CN to successfully control congestion in a Virtual Bridged Network, the
bridges and end stations in that network have to be configured with CN values that
are appropriate to the characteristics of the CCFs generated by the applications
that expect congestion controlled services. For example:
1.
If frames that were not originated from an RP can enter a CP experiencing
congestion, then the CNMs generated by that CP upon receipt of those frames
cannot correct the problem.
2.
Congestion notification cannot operate correctly if a CP's configuration is
inappropriate for the CCFs passing through it, or if priority values are
regenerated in a manner that moves frames in and out of CNPVs.
3.
Frames transmitted from an end station with a CN-TAG cannot be understood
by an end station that is not congestion aware.
Congestion aware bridges are therefore used to construct a Congestion
Notification Domain (CND), within which a particular Congestion Notification
Priority Value (CNPV) is supported. A CND is a connected subset of the bridges and
end stations in a Virtual Bridged LAN that are configured to serve a particular CNPV.
CNDs can be created by configuring the bridges and end stations in a network, or
they can be created automatically, using an additional TLV element, the Congestion
Notification TLV.
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