Cisco 10000 Series Configuration Manual page 114

Layer 2 Overhead Accounting
Figure 7-1
In Ethernet DSL environments, the outbound interface on the BRAS to DSLAM side is Gigabit Ethernet,
but the encapsulation from the DSLAM to CPE is ATM. When shaping the aggregate subscriber traffic
to the subscriber line rate, the BRAS does not know about the ATM encapsulation on the DSLAM-CPE
side. Therefore, the BRAS does not consider the ATM overhead in its bits-per-second calculation, which
can cause subscriber traffic to overrun the subscriber line rate. For example, suppose your configuration
uses a priority class to provision voice traffic on a link. If the BRAS does not consider Layer 2
overheads, the BRAS effectively polices the priority traffic at a higher rate than intended. As a result,
the priority traffic can take bandwidth from other classes, which can be problematic for networks
configured with very little bandwidth to spare.
The BRAS with Layer 2 overhead accounting enabled can take into account the various encapsulation
types on the BRAS-DSLAM and DSLAM-CPE sides of the network as the following list describes:
On the BRAS-DSLAM side, the encapsulation is typically IEEE 802.1Q VLAN or QinQ (qinq),
•
which the DSLAM removes. Therefore, the BRAS does not consider 802.1Q or qinq encapsulation
when calculating overhead.
On the DSLAM-CPE side, the encapsulation types are based on SNAP and MUX formats of AAL5
•
and AAL3, followed by RBE, PPPoA, PPPoE, and IP (for example, snap-rbe or mux-pppoa). The
DSLAM treats IP and PPPoE packets as payload; therefore, the BRAS does not account for these
encapsulation types in the overhead calculation.
The BRAS supports ATM overhead accounting only for the shaping and bandwidth ANCP QoS features
associated with the parent level of a hierarchical QoS policy.
For more information about ATM overhead accounting, see the
for ATM"
Configuration Guide
http://www.cisco.com/en/US/products/hw/routers/ps133/products_configuration_guide_book09186a00
805b9497.html
Cisco 10000 Series Router Access Network Control Protocol Configuration Guide
7-2
Broadband Access ANCP Network
section in the "Shaping Traffic" chapter of the
at:
Chapter 7
"Traffic Shaping Overhead Accounting
Cisco 10000 Series Router Quality of Service
Traffic Accounting
OL-14660-03