9-7
Cisco 10000 Series Router Quality of Service Configuration Guide
OL-7433-09
Chapter 9 Shaping Traffic
Traffic Shaping
The SAR uses weighted round-robin scheduling to schedule the PVP VCs onto the wire. The SAR also
does the following:
•
Assigns high rate VCs a high weighting
•
Ensures that cells are scheduled in accordance with the configured rate of the PVP
•
Offers per-VC buffering to accommodate small bursts at the VC packet level
•
Ensures that transmitted cells conform to ATM transmission standards, including cell delay
variation tolerance (CDVT)
The priority of the PVP is the same as the priority of CBR VCs on an interface. If the physical interface
experiences congestion, the PVPs and any CBR VC have priority over VBR-nrt VCs and unspecified bit
rate (UBR) VCs.
Hierarchical Shaping and Oversubscription
You can oversubscribe the PVP bandwidth by configuring the VCs in such a way that the sum of the VCs’
bandwidths exceeds the configured rate of the PVP. The
atm
over-subscription-factor
command is used
to configure oversubscription. This command sets up flow-control between the SAR and the VTMS, and
provides Caller Admission Control (CAC) based on the configured oversubscription value. Valid values
for oversubscription are from 0 (no oversubscription allowed) to 10.
You cannot oversubscribe the physical interface by oversubscribing the interface’s PVPs. For a given
interface, the sum of the PVPs’ bandwidth must be less than or equal to the physical interface speed,
minus the appropriate overhead. PVPs are subject to the same CAC mechanisms as the VCs.
The current oversubscription mechanism is optimized for VC-level fairness. When PVP congestion
occurs, the router cannot guarantee bandwidth allocation to an IP traffic class on a given VC.
The current oversubscription mechanism is optimized for VCs—it is not designed to handle PVP
congestion. If VCs are oversubscribed and PVP congestion occurs, VC shaping accuracy and bandwidth
distribution can degrade.
Active ATM VCs
Table 9-1
lists the maximum number of active VCs supported on ATM line cards when configured for
hierarchical shaping.
Table 9-1
Active VCs on ATM Line Cards
Line Card
Maximum VCs per Port
Maximum VCs per Module
VBR, CBR, Shaped UBR VCs
E3/DS3
4,096
32,768
28,672
OC-3
8,192
32,768
28,672
OC-12
16,384
16,384
16,384
