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Cisco ASR 1000 Series Aggregation Services Routers Software Configuration Guide
OL-16506-17
Chapter 8 Multilink PPP Support for the Cisco ASR 1000 Series Routers
Information About Multilink PPP Support for Cisco ASR 1000 Series Routers
• When packets transit the MLP transmit path, they are subject to two separate stages of queuing. The
first stage is at the MLP bundle interface, where QoS may be applied, and the second one is at the
MLP member-link interface. At the MLP bundle interface, the packets are processed according to
the applied QoS policy. Packets classified as priority are given preferential treatment over
nonpriority traffic.
For the priority classification to be honored at the MLP member-link interface, the bundle must have
ppp multilink interleave enabled. Interleaving allows a packet to be queued to a separate priority
queue at the member-link. If interleaving is not enabled on the bundle, the priority packet is placed
in the member link session default queue and the knowledge that it is a priority packet will be lost.
This is especially important if there are other PPP or MLP sessions sharing the same physical
interface or subinterface. Without interleaving, priority traffic on the other sessions are given
preferential treatment over the MLP priority packets that were reclassified as nonpriority packets at
the MLP member-link queuing stage. For additional information on interleaving, see the
Downstream LFI, page 8-13.
Multilink PPP Packet Overhead Accounting for Shaping and Policing
On the Cisco ASR 1000 Series Aggregation Services Routers, Multilink PPP adjusts the packet length
presented for shaping and policing to include the additional Layer 2 overhead added by Multilink PPP.
For MLP over Serial, overhead accounting includes the MLP and PPP Layer 2 overhead. For Broadband
MLPs such as MLPoE, MLPoEoVLAN, MLPoEoQinQ, MLPoEoA, MLPoA, and MLPoLNS, overhead
accounting includes the MLP, PPP, Ethernet, ATM, and L2TP (LNS) Layer 2 overhead. If the output
interface is ATM, such as the MLPoA or MLPoEoA, the Cisco ASR 1000 Series Aggregation Services
Routers also take into account the ATM Cell overhead for the shaper. The ATM Cell overhead is not
accounted for policing.
Shaping and policing overhead accounting does not include the additional overheads added by a SPA
such as, Ethernet CRC, preamble, IPG, serial interface CRC, start of packet (SOP) delimiter, end of
packet (EOP) delimiter, and serial-bit stuffing (the only exception being the ATM Cell overhead for the
shaper referred to earlier). The overhead added by a SPA can be included in the shaper using the QoS
shape accounting user-defined option.
If you do not define a QoS shaper for the multilink bundle interface, a default shaper is applied to the
bundle based on the aggregate bandwidth of all the links that make up the multilink bundle. The
information contained in this section applies to both the default shaper and a QoS user-defined shaper,
which the user may explicitly configure and apply to a multilink bundle.
The priority packets that are interleaved are sent PPP encapsulated and the MLP Layer 2 overhead is not
included because MLP encapsulation is not included in these packets. During overhead accounting for
link fragmentation, overhead accounting calculations are performed prior to the actual link
fragmentation and link selection for Multilink PPP load balancing.
If all the member links in the corresponding multilink bundle use the same fragment size, the number of
fragments are calculated and the overhead is adjusted to include the additional per-fragmentation Layer
2 header overhead for the shaper and policer. If one or more links in the bundle use different fragment
sizes, the number of fragments cannot be calculated with 100 percent accuracy because link selection
for load balancing and fragment size is not known until QoS processing is completed at the bundle level
(after shaping and policing). For links with unequal fragment size, a best effort attempt is made using
the largest link fragment size on the bundle. By using the largest fragment size, MLP avoids
undersubscribing the member-link interfaces. If the links become oversubscribed, MLP will
backpressure the bundle to avoid sustained oversubscription of the member links.
In Cisco IOS XE Release 3.4S on the Cisco ASR 1000 Series Aggregation Services Routers, support for
shaping and policing overhead accounting was added for Broadband Multilink PPP. In addition, support
was added for the Shape User-Defined Overhead Accounting feature using the following QoS command: