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Cisco 10000 Series Router Quality of Service Configuration Guide
OL-7433-09
Chapter 18 Regulating and Shaping Subscriber Traffic
Queuing PPP Sessions on ATM VCs
Bandwidth Sharing and ATM Port Oversubscription
An ATM port can operate in reserved bandwidth mode or shared bandwidth mode.
When a port is not oversubscribed (the sum of the bandwidths of all VCs on the port is less than the port
bandwidth), the port operates in reserved bandwidth mode—a specific amount of bandwidth is reserved
for each VC on the port. If a VC does not use all of its allocated bandwidth, the unused bandwidth is not
shared among the VCs on the port.
When the ATM port is oversubscribed (the sum of the bandwidths of all VCs on the port is greater than
the port bandwidth), the port operates in shared bandwidth mode. In this mode, any unused bandwidth
is available for re-use by the other VCs on the port, up to the VC’s respective shape rate—traffic on a
VC cannot exceed the shape rate of that VC.
Oversubscription at the Session Level
Oversubscription at the session level occurs after session traffic shaping and when the aggregate session
traffic exceeds the subinterface shape rate. After all priority traffic is accounted, the router distributes
the remaining bandwidth on the VC to the sessions according to the value specified in the
bandwidth
remaining ratio
command configured in the parent policy of the policy applied to the sessions. If the
bandwidth remaining ratio
command is not specified in the parent policy, the router uses a default ratio
of 1.
Prerequisites for PPP Session Queuing on ATM VCs
•
PPPoA or PPPoEoA sessions must be enabled.
•
Create traffic classes using the
class-map
command and specify the match criteria used to classify
traffic.
•
For dynamic PPPoA or PPPoEoA session queuing using RADIUS, you must:
–
Enable authentication, authorization, and accounting (AAA) on the router
–
Configure the RADIUS server for dynamic QoS
–
Create the subscriber’s user profile on the RADIUS server
Restrictions and Limitations for PPP Session Queuing on ATM VCs
•
You cannot configure PPP session queuing on unshaped VCs—VCs without a specified peak cell
rate (PCR) or sustained cell rate (SCR).
•
Although you can configure oversubscription at the VC level, the router does not guarantee priority
queuing (PQ) and fair treatment among VCs during congestion.
•
VCs with session queuing polices cannot be part of a shaped virtual path (VP).
•
PPP session queuing does not allow you to simultaneously configure queuing policies on a VC and
on a session of that VC, although the router permits the configuration.
•
The maximum number of VCs with PPP session queuing policies cannot exceed 16,000 VCs system
wide.
