Multilink PPP Support for the Cisco 4000 Series Integrated Services Router

Multilink PPP Support for the Cisco 4000 Series Integrated Services Router

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Last Updated: October 30, 2019

The Multilink Point-to-Point Protocol (MLP) provides support to aggregate the bandwidth of low-speed WAN links into a single entity, referred to as a bundle interface. A bundle interface is a logical entity that provides a single point in which other features (Quality of Service (QoS)) can be attached. MLP provides incremental bandwidth on demand, by adding additional links to the bundle. MLP also allows interleaving of latency-sensitive priority traffic with non-priority fragmented traffic using link fragmentation and interleaving (LFI).

This document describes the features, limitations, and scaling of MLP on the Cisco 4000 series Integrated Services Router (Cisco 4000 Series ISR) running Cisco IOS XE software. The router supports MLP links for serial (T1 or E1 or NxDS0) topologies.

For further information about the configuration and operation of MLP in Cisco IOS XE software, see the “Configuring Multilink PPP” section in Configuring Media-Independent PPP and Multilink PPP.

Contents

This document includes the following sections:

• Feature Information
• Restrictions for Multilink Point-to-Point Protocol over Serial Interfaces
• Information About the Multilink Point-to-Point Protocol Feature
• Downstream Link Fragmentation and Interleaving
• Unsupported Features
• Additional References
• Feature Information for Multilink Point-to-Point

Feature Information

Your software release may not support all the features documented in this module. For the latest information about features and caveats, see the release notes for your platform and software release.

To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see Table 1 .

Use the Cisco Feature Navigator to find information about platform support and Cisco IOS and
Cisco IOS XE operating system software image support. To access the Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the corresponding command reference documentation.

Table 1 lists the features in this module and provides links to configuration information for features that were introduced or modified in Cisco IOS XE Release 3.10S or a later release.

Restrictions for Multilink Point-to-Point Protocol over Serial Interfaces

The following restrictions apply to using MLP over Serial Interfaces:

• A maximum of ten member links per MLP bundle is supported. The member links can be any combination of T1/E1 or fractional T1s/E1s (for example, NxDS0). Member-link interface speed above T1/E1 is not supported with the MLP over Serial feature. For better MLP performance, all the member links in a bundle must be of the same bandwidth.
• All the member links in a bundle must have the same encapsulation type.
• You cannot manually configure the bandwidth of an MLP bundle by using the bandwidth command on the multilink interface. The bandwidth of an MLP bundle is managed based on the aggregate bandwidth of all the active member links on the bundle. As the links are dynamically added or removed from an MLP bundle, the bandwidth is updated to reflect the aggregate of the active links. The bandwidth can be rate limited by applying an hierarchical QoS (HQoS) policy on the multilink interface and applying a shaper to the parent class-default class.
• MLP over Frame Relay is not supported; only MLP over Serial Interfaces is supported.
• MLP over ISDN is not supported.
• MLP over Frame Relay is not supported on Cisco Integrated Services Virtual Routers.
• The legacy IOS compression feature compress [ mppc | stac | predictor ] is not supported.
• LFI is supported on MLP bundles with any number of links in the bundle. When using a bundle with more than one member link, the order of the priority packets (PPP encapsulated) is not guaranteed. Priority packet distribution is handled in a manner similar to the IP per packet load sharing. MLP guarantees non-priority packet ordering that manages reordering at the peer device based on the MLP packet sequence number.
• There is an order issue for the LFI multiple member link, in the case of priority traffic. Although this can be addressed in some platforms using Multiclass Multilink Protocol (MCMP-RFC 2686), the Cisco 4000 Series ISR does not support MCMP.
• Only the MLP long-sequence number format is supported for the packet header format option.

Information About the Multilink Point-to-Point Protocol Feature

The Multilink Point-to-Point Protocol (MLP) feature provides the load balancing functionality over multiple WAN links, while providing multivendor interoperability, packet fragmentation, proper sequencing, and load calculation on both inbound and outbound traffic. The Cisco implementation of MLP supports the fragmentation and packet sequencing specifications described in RFC 1990.

Packet Overhead Accounting for Shaping and Policing

MLP adjusts the packet length presented for shaping and policing to include a Layer 2 overhead for MLP. This shaping and policing overhead accounting includes the overhead for MLP and PPP Layer 2.

In Cisco IOS XE Release 3.10S, support for shaping and policing overhead accounting was added for Serial MLP on the Cisco 4000 Series ISR.

The shaping and policing overhead accounting does not include the additional overheads added by a Network Information Module (NIM) such as serial interface CRC, start of packet (SOP) delimiter, end of packet (EOP) delimiter, and serial-bit stuffing. The overhead added by a NIM can be included at the shaper using the QoS shape accounting user-defined option in the following command:

shape [average | peak] mean-rate [ burst-size ] [ excess-burst-size ] account {{{qinq | dot1q} {aal5 | aal3} {subscriber-encapsulation}} | { user-defined offset [atm]}}

By specifying the user-defined option, you can include the additional overhead added by an NIM.

For further information on overhead accounting when apply shaping to packets, see Ethernet Overhead Accounting.

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Note You can configure and apply a QoS shaper to a multilink bundle--a “QoS user-defined shaper”. Otherwise, if you do not configure and apply a QoS user-defined 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 either a default shaper or a QoS user-defined shaper.

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Priority packets that are interleaved are sent PPP encapsulated. 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. When 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. When one or more links in the bundle use different fragment sizes, the number of fragments cannot be calculated with 100% accuracy because link selection for load balancing and fragment size is not known until the 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 under subscribing the member-link interfaces. If the links become oversubscribed, MLP will backpressure the bundle to avoid sustained oversubscription of the member links.

Enable overhead accounting using the following QoS command:

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Note The bandwidth command must not be used to define the bundle bandwidth on the virtual template interface or the multilink interface. By default, the bundle bandwidth is the aggregate of the bandwidth of the individual member links that make up the bundle.

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Downstream Link Fragmentation and Interleaving

Although Link Fragmentation and Interleaving (LFI) are thought of as a single feature, they are actually two independent features within MLP.

• MLP Link Fragmentation
• MLP Interleaving

MLP Link Fragmentation

MLP Link Fragmentation allows for larger packets to be Layer 2 fragmented by MLP, and for the fragments to be distributed across the various member links in the MLP bundle. These fragments are MLP encapsulated and sequenced. These fragments are then collected, reordered, and reassembled at the peer termination point for the MLP bundle interface.

Note For more information about interleaving with QoS, see “Quality of Service” section.

MLP Interleaving

MLP Interleaving enables you to reduce the transmission delay on delay-sensitive voice, video, and interactive application data by interleaving it with the MLP fragments. When interleaving is configured, the packets on the bundle interface that QoS classifies as priority packets are interleaved. These priority packets are PPP encapsulated and interleaved with the MLP-encapsulated fragments or packets. When the peer router receives the PPP packets, they can be immediately forwarded, whereas, the received MLP encapsulated packets have to be reordered and reassembled before being forwarded.

While LFI can be configured on any multilink bundle, it is beneficial only on bundles with link speeds of 1 Mbps or less. For higher bandwidth bundles, LFI should not be needed. In the case of these bundles, low packet transmission delays allow QoS prioritization of traffic to be sufficient.

One disadvantage of interleaving is that when there are two or more links in an MLP bundle, the order of the PPP-encapsulated packets cannot be guaranteed. In most applications sending data such as, voice, video, and telnet, this is not an issue because the gap between the packets on a given flow is large enough that the packets must not pass each other on the multiple links in the bundle. Since the order cannot be guaranteed for priority PPP-encapsulated packets that are interleaved, IP Header Compression (IPHC) is skipped on any packet that is classified as being priority-interleaved. IPHC will continue to occur for non-priority packets that are sent as MLP encapsulated because MLP guarantees reordering before the packets are forwarded to IPHC.

Although the Multi-Class Multilink Protocol (MCMP) (RFC-2686) addresses issues related to ordering of priority-interleaved packets, currently MCMP is not supported on the Cisco 4000 Series ISR.

In the context of interface multilink or interface virtual template, use the following commands to enable LFI:

• ppp multilink fragment delay (delay in milliseconds)—enables MLP link fragmentation on the member link serial interface
• ppp multilink fragment size (maximum fragment size, in bytes)—enables MLP link fragmentation on the member link serial interface
• ppp multilink interleave —enables MLP interleaving

For MLP interleaving, you must also define a QoS policy with one or more priority classes, and attach the QoS to this interface using the service-policy output policy-map name command. This command classifies the priority traffic, which is interleaved by the MLP.
If the ppp multilink interleave command is not configured, only MLP link fragmentation is enabled.

Note If the MLP bundle has only one active member link and interleaving is not enabled, MLP link fragmentation is disabled. In addition, all the packets are sent as PPP encapsulated instead of MLP encapsulated. When a second link in the bundle becomes active or interleaving is enabled, MLP link fragmentation is enabled.

See the QoS and LFI configuration examples in “Configuring Multilink PPP Connections”:

Wide-Area Networking Configuration Guide: Multilink PPP, Cisco IOS XE Release 3S

When configuring MLP link fragmentation on the various Cisco platforms, the functionality of MLP link fragmentation and interleaving support on the various platforms may differ. The following information explains the configuration options and their interpretation on the Cisco 4000 Series ISR.

Based on the values of the MLP link fragmentation configuration commands, the MLP feature calculates two values that are used during MLP link fragmentation: link weight and maximum fragment size. These parameters are calculated for each member link in the bundle.

First, a link weight must be determined for each member link. The link weight indicates the number of bytes, and the MLP uses this value to balance the data amongst the links in the bundle. This parameter is especially important when the links in a bundle are of unequal bandwidth. The link weight is based on a combination of the bandwidth of the member link and the PPP multilink fragment delay value. If you do not configure the fragment delay value, a default delay value of 30 milliseconds is used.

Link Weight = (Member Link Interface Bandwidth in bps/8) * Fragment Delay

Caution Configuring the fragment delay to a smaller value results in smaller fragment size because the fragment delay value determines the default fragment size on the member link. This in turn implies loss of bandwidth due to the added Layer 2 header overhead.

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The default maximum fragment size must be calculated per member link. The default maximum fragment size used will be the lesser value obtained from either of the following calculation:

• Link Weight – Multilink PPP + PPP Header Overhead (8)
• Interface MTU – Multilink PPP Header Overhead (4)

After the default maximum fragment size is calculated, if you have configured the
ppp multilink fragment size (maximum) command at the multilink, virtual template, or serial interface level, the default maximum fragment size is compared against the configured maximum value and is capped accordingly. If the fragment size is configured at the serial interface level and also at the multilink interface level, the serial interface configuration takes precedence.

Unsupported Features

The Cisco 4000 Series ISR does not support the following MLP features:

• In-Service Software Upgrade (ISSU) and Stateful Switchover (SSO) for MLP bundles.
• Per-user firewall.
• Lawful intercept.
• MLP with MPLS-TE FRR.
• Layer 2 input QoS Classification.
• The Multiclass Multilink Protocol (MCMP) RFC 2686 extension to LFI.
• Only the MLP long-sequence number format is supported for the packet header format option.

Additional References

The following sections provide references related to the multilink point-to-point protocol connections.

Feature Information for Multilink Point-to-Point

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator www.cisco.com/go/cfn to find information about platform support and Cisco software image support. An account on Cisco.com is not required.