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Information About Multilink PPP Support for Cisco ASR 1000 Series Routers

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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:

Cisco ASR 1000 Series Aggregation Services Routers Software Configuration Guide

11-10

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

11-13.

Chapter 11

Multilink PPP Support for the Cisco ASR 1000 Series Routers

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