Abr Standard With Vs/Vd Congestion Control - Cisco BPX 8620 Installation And Configuration Manual

Chapter 1 The BPX Switch: Functional Overview
Traffic and Congestion Management
The extent to which CR reduces network settling time will vary with network topology, traffic
Note
conditions and the number of CR enabled nodes in the network.
Before CR can be enabled on any node in the network, all of the nodes in a network must be upgraded
to switch software Release 9.3.30. The cnfcmparm command sets the route concurrency level to an
integer value greater than 1 but not greater than 8. Once CR has been enabled, it operates automatically.
However, it is not necessary for CR to be enabled on every node in a network for CR to function on those
nodes that are CR enabled. CR can be turned off by specifying a concurrency level of 1. For a detailed
discussion of the cnfcmparm command and other commands pertinent to the CR feature, refer to the
Cisco WAN Switching Command Reference, Release 9.3.30.
A maximum of eight concurrent routes can be configured on a node. However, a node is able only to
master two routes concurrently, any remaining concurrent routing capacity is used for routes or as a
slave. Allowing more than eight concurrent routes would have diminishing returns, because processor
utilization would become excessive. A node continues to master new route requests (provided route
candidates exist), or serve as a via node or slave node for new routes, until it reaches the route
concurrency level that is configured on the node.
CR has the potential to dramatically reduce CPU idle time. To preserve enough CPU time for other
switch features or for users to interact effectively with a node, a mechanism is implemented to limit
(throttle) route concurrency. When CPU utilization exceeds a defined threshold (throttle level), new
route activity is temporarily suspended to preserve node responsiveness. Throttling continues until CPU
utilization drops below a second threshold (resume level), which is less than or equal to the throttle level.
Allowing the resume level to be less than the throttle level provides a hysteresis mechanism to avoid
oscillation around the throttling point. The default CPU throttling values for master, via and slave routes
are set at 80 percent of CPU capacity for throttling and 60 percent of CPU capacity to resume new route
activity. Separate throttle and resume points can be set for master, via, and slave routes to allow tailoring
of route behavior. However, if you need to change the settings, contact TAC for configuring the levels.
If a node masters two or more routes that share the same via node or slave node, the routes have
overlapping paths. Due to messaging protocol limitations, a node is able only to master concurrent routes
that do not have overlapping paths. The Path Blocking algorithm checks each master route candidate that
a node might initiate to see if it overlaps with another route in progress that is mastered by the node. If
there is any overlap, the candidate is rejected and candidate selection continues. The degree to which
Path Blocking limits concurrent master routes on a node is a function of network topology and
connection provisioning. Path blocking does not affect nodes that are serving only as a via node or a
slave node.
PB is a computation-intensive process, which allows switch connections classified as more important
(based on CoS value) to "bump" connections of lesser importance. CR may be restricted if the PB feature
is enabled on a network. Both PB and CR are processor intensive. To avoid excessive processor
utilization, no new route requests are initiated or accepted on a node that participates in a PB route, until
that PB route is complete.

ABR Standard with VS/VD Congestion Control

This section describes Standard ABR with VS/VD. The BPX/IGX switch networks provide a choice of
two dynamic rate based congestion control methods, ABR with VS/VD and Optimized Bandwidth
Management (ForeSight).
When an ATM connection is configured between BXM cards for Standard ABR with VS/VD per ATM
Forum TM 4.0, Resource Management (RM) cells are used to carry congestion control feedback
information back to the connection source from the connection destination.
Cisco BPX 8600 Series Installation and Configuration
1-24
Release 9.3.30, Part Number 78-12907-01 Rev. E0, May 2005