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Understanding Link and Multilink Services

 

Multilink-based protocols enable you to split, recombine, and sequence datagrams across multiple logical data links. The goal of a multilink operation is to coordinate multiple independent links between a fixed pair of systems, providing a virtual link with greater bandwidth than any of the members. In addition to providing incremental bandwidth, bundling multiple links can add a level of fault tolerance to your dedicated access service, because you can implement bundling across multiple PICs, protecting against the failure of any single PIC.

Junos OS supports several multilink-based protocols including Multilink Point-to-Point Protocol (MLPPP) and Multilink Frame Relay (MLFR). MLPPP enables you to bundle multiple PPP links into a single logical link. MLFR enables you to bundle multiple Frame Relay data-link connection identifiers (DLCIs) into a single logical link. MLPPP and MLFR provide service option granularity between low-speed T1 and E1 services and higher-speed T3 and E3 services. You use MLPPP and MLFR to increase bandwidth in smaller, more cost-effective increments.

The multiclass extension to the MLPPP extension enables multiple classes of service using MLPPP. For more information, see RFC 2686, The Multi-Class Extension to Multi-Link PPP. The Junos OS PPP implementation does not support the negotiation of address field compression and protocol field compression PPP NCP options. The software always sends a full 4-byte PPP header.

Standards

The standards for MLPPP, MLFR FRF.15, and MLFR FRF.16 are defined in the following specifications:

  • RFC 1990, The PPP Multilink Protocol (MP)

  • FRF.15, End-to-End Multilink Frame Relay Implementation Agreement

  • FRF.16.1, Multilink Frame Relay UNI/NNI Implementation Agreement

    Note

    Endpoint Discriminator Class compatibility checking is enabled on MLPPP interfaces. Prior to Junos OS Release 8.0, when a Juniper Networks router received an unsupported Endpoint Discriminator Class message from an MLPPP session peer, it returned an ACK response.

Each Multilink Services or Link Services PIC can support a number of bundles. A bundle can contain up to eight individual links.

For Multilink Services PICs, the links can be T1, E1, or DS0 physical interfaces, and each link is associated with a logical unit number that you configure. For Link Services PICs, the links can be E1, T1, channelized DS3-to-DS1, channelized DS3-to-DS0, channelized E1, channelized STM1 interfaces, or channelized IQ interfaces. For MLFR FRF.16 bundles, each link is associated with a channel number that you configure.

You must configure a link before it can join a bundle. Each bundle should consist solely of one type of link; the mixing of physical interfaces of differing speeds within a bundle is not supported.

Three versions of Multilink Services and three versions of Link Services PICs are available, as shown in Table 1. The PIC hardware is identical, except for different faceplates that enable you to identify which version you are installing. The software limits the unit numbers and maximum number of physical interfaces you assign to the PIC.

Table 1: Multilink and Link Services PIC Capacities

PIC Capacity

Unit Numbers

Maximum Number of T1/DS0 Interfaces

Maximum Number of E1 Interfaces

4-bundle PIC

0 through 3

32 links

32 links

32-bundle PIC

0 through 31

256 links

219 links

128-bundle PIC

0 through 127

292 links

219 links

A single PIC can support an aggregate bandwidth of 450 megabits per second (Mbps).

You can configure a larger number of links, but the Multilink Services and Link Services PICs can reliably process only 450 Mbps of traffic. A higher rate of traffic might degrade performance.

Note

In Junos OS releases 9.0 and above you are not allowed to configure a unit number greater than the maximum unit number available on your link services PIC. Attempting to do so will cause an error message.

Support on Service PICs

Junos OS supports multilink-based protocols on services PICs such as the Multilink Services PIC and the Link Services PIC, as well as the link services intelligent queuing (IQ) and voice services configured on the Adaptive Services (AS) and MultiServices PICs. For more information about link services IQ, see Layer 2 Service Package Capabilities and Interfaces. For more information about voice services, see Configuring Services Interfaces for Voice Services.

Starting with Junos OS Release 12.1, the following channelized MICs on MX240, MX480, and MX960 routers support Multilink Point-to-Point Protocol (MLPPP)-based services:

  • 4-port Channelized SONET/SDH OC3/STM1 (Multi-Rate) MIC with SFP (MIC-3D-4CHOC3-2CHOC12)

  • 8-port Channelized SONET/SDH OC3/STM1 (Multi-Rate) MIC with SFP (MIC-3D-8CHOC3-4CHOC12)

  • 8-port Channelized DS3/E3 MIC (MIC-3D-8CHDS3-E3-B)

For more information about MLPPP-based services MICs, see Multilink Interfaces on Channelized MICs Overview.

The Link Services and Multilink Services PICs support the following encapsulation types:

  • MLPPP

  • MLFR

Starting with Junos OS Release 12.1, support for the following encapsulation types and protocols has been extended to the MX240, MX480, and MX960 routers with Multiservices DPCs:

  • MLPPP

  • Multiclass MLPPP

  • MLFR end-to-end (FRF.15)

  • MLFR UNI NNI (FRF.16) (also referred to as MFR)

  • Compressed Real-Time Transport Protocol (CRTP)

Note

Only MLPPP is supported on ACX Series routers. MLFR is not supported on ACX Series routers.

At the logical unit level, the Multilink Services and Link Services PICs support the MLPPP and MLFR Frame Relay Forum (FRF) 15 encapsulation types. At the physical interface level, the Link Services PIC also supports the MLFR FRF.16 encapsulation type.

Note

On M Series Multiservice Edge Routers, only one DS3 link is allowed in an MLFR bundle. MLPPP bundles can include two DS3 links.

On ACX Series routers, even if the PIC can support up to 4xDS3 total throughput, each aggregate can only run a volume of traffic equal to one DS3 in bandwidth. Aggregating DS3 links is not supported.

Support on Interface Types

MLPPP and MLFR FRF.15 are supported on interface types ml-fpc/pic/port, ls-fpc/pic/port, and lsq-fpc/pic/port. For MLFR FRF.15, multiple permanent virtual circuits (PVCs) are combined into one aggregated virtual circuit (AVC). This provides fragmentation over multiple PVCs on one end and reassembly of the AVC on the other end.

MLFR FRF.16 is supported on a channelized interface, ls-fpc/pic/port:channel, which denotes a single MLFR FRF.16 bundle. For MLFR FRF.16, multiple links are combined to form one logical link. Packet fragmentation and reassembly occur on a per-VC basis. Each bundle can support multiple VCs. Link Services PICs can support up to 256 DLCIs per MLFR FRF.16 bundle. The physical connections must be E1, T1, channelized DS3-to-DS1, channelized DS3-to-DS0, channelized E1, channelized STM1, or channelized IQ interfaces. When you bundle channelized interfaces using the link services interface, the channelized interfaces require M Series Enhanced Flexible PIC Concentrators (FPCs).

The ml- interface type is used to configure interfaces on the Multilink Services PIC and does not support class-of-service (CoS) features. The ls- interface type is used for limited CoS configurations on the Link Services PIC, and the lsq- interface type is used for full CoS configurations on the Adaptive Services and MultiServices PICs. The bundle interfaces are configured on the Multiservices DPC as link services IQ (lsq) interfaces and virtual LSQ redundancy (rlsq) interfaces.

For link services IQ (lsq) interfaces, Junos OS CoS components are fully supported and are handled normally on M Series and T Series routers, as described in the Class of Service User Guide (Routers and EX9200 Switches). For more information on link services IQ configuration, see Layer 2 Service Package Capabilities and Interfaces.

When running MLPPP or MLFR on a non-QPP interface, you cannot mix logical units that are members of an aggregate with logical units configured using other families, such as inet. For example, the following configuration is not valid:

Multiservices Modular Interface Cards (MICs) enable you to perform multiple services on the same MIC by configuring a set of services and applications such as voice services and Layer 2 Tunneling Protocol (L2TP) services. On Juniper Networks MX Series 5G Universal Routing Platforms, the Multiservices DPC provides essentially the same capabilities as the Multiservices PIC. The interfaces on both platforms are configured in the same way. The Multilink interfaces are hosted on a channelized MIC. The bundle interfaces are configured on Multiservices DPC as virtual LSQ redundancy (rlsq) interfaces.

Starting with Junos OS Release 12.1, the following channelized MICs on MX240, MX480, and MX960 routers support Multilink Point-to-Point Protocol (MLPPP)-based services:

  • 4-port Channelized SONET/SDH OC3/STM1 (Multi-Rate) MIC with SFP (MIC-3D-4CHOC3-2CHOC12)

  • 8-port Channelized SONET/SDH OC3/STM1 (Multi-Rate) MIC with SFP (MIC-3D-8CHOC3-4CHOC12)

  • 8-port Channelized DS3/E3 MIC (MIC-3D-8CHDS3-E3-B)

The following encapsulations, interfaces, protocol, and packet types are supported on the aforementioned MICs:

  • Multilink Point-to-Point Protocol (MLPPP)—Supports Priority-based Flow Control (PFC) for data packets and Link Control Protocol (LCP) for control packets. Compressed Real-Time Transport Protocol (CRTP) and Multiclass MLPPP are supported for both data and control packets.

  • Multilink Frame Relay (MLFR) end-to-end (FRF.15)—Supports Ethernet Local Management Interface (LMI), Consortium LMI (C-LMI), and Link Integrity Protocol (LIP) for data and control packets.

  • Multilink Frame Relay (MFR) UNI NNI (FRF.16)—Supports Ethernet Local Management Interface (LMI), Consortium LMI (C-LMI), and Link Integrity Protocol (LIP) for data and control packets.

  • Link fragmentation and interleaving (LFI) non multilink MLPPP and MLFR packets.

Layer 2 services and voice services functionality are implemented on the Multiservices Dense Port Concentrators, which supports the following two kinds of traffic that are routed by the Packet Forwarding Engine:

  • Customer-end to provider-end (also, known as customer traffic)—Here, the Multilink fragments from the customer end arrive at the Multiservices interfaces configured on the channelized MIC. These fragments are then transmitted to the Multiservices DPC for Layer 2 processing such as CoS and are reassembled by the Multiservices software running on the Multiservices DPC. These reassembled packets are sent to the Packet Forwarding Engine where they go through the regular router lookup process and are finally sent over the Internet to the provider end. The voice packets also go through the same process.

  • Provider-end to customer-end (also, known as Internet traffic)—Here, the data packets that are sent from the Internet provider end are received at any generic ingress interface in the Packet Forwarding Engine. These packets are then sent to the Multiservices DPC for Layer 2 processing. The Multiservices software running on Multiservices DPC fragment these data packets and send it to the Packet Forwarding Engine. These Multilink fragments are sent over the channelized MIC interfaces to the customer end. The voice packets also go through the same process.

Note

All the features that are supported on Multilink and Link Services PICs are also supported on the Multilink Services or Link Services MICs. For more information about Multilink and Link Services PICs, see Multilink and Link Services PICs Overview.

Support for the following encapsulations, interfaces, protocol, and packet types are now extended to the aforementioned MICs:

  • Multilink Point-to-Point Protocol (MLPPP)—Supports priority-based flow control (PFC) for data packets and Link Control Protocol (LCP) for control packets. Compressed Real-Time Transport Protocol (CRTP) and multiclass MLPPP are supported for both data and control packets.

  • Multilink Frame Relay (MLFR) end-to-end (FRF.15)—Supports Ethernet Local Management Interface (LMI) and Consortium LMI (C-LMI) for data and control packets.

  • Multilink Frame Relay (MLFR) UNI NNI (FRF.16)—Supports Ethernet Local Management Interface (LMI), Consortium LMI (C-LMI), and Link Integrity Protocol (LIP) for data and control packets.

  • Link fragmentation and interleaving (LFI) on multilink MLPPP and MLFR packets—Reduces delay and jitter on links by breaking up large data packets and interleaving delay-sensitive voice packets with the resulting smaller packets.