Overview

MLFR aggregates multiple physical links into a single logical bundle. More specifically, MLFR bundles multiple link-layer channels into a single network layer channel.

The routers joined by the multilink each assign the same unique name to the bundle. A bundle can consist of multiple physical links of the same type—such as multiple asynchronous lines—or can consist of physical links of different types—such as leased synchronous lines and dial-up asynchronous lines.

The router treats MLFR like nonmultilink Frame Relay. Packets received with an MLFR header are subject to sequencing. Packets received without the MLFR header cannot be sequenced and can be delivered only on a first-come, first-served basis.

T1/E1 Connections

Some users need more bandwidth than a T1 or an E1 channel can provide, but cannot afford the expense or do not need the bandwidth of T3 or E3. Equal-cost multipath (ECMP) is one way to achieve a bandwidth greater than DS1 service without going to the expense and infrastructure required for DS3 service. MLFR is commonly used as an alternative to ECMP to deliver NxT1 service. Cost-analysis of NxT1 versus DS3 service typically imposes a practical limit of 8xT1 service; that is, aggregation of no more than eight T1 or E1 connections into an MLFR bundle.

This implementation of MLFR logically aggregates up to eight T1 or E1 connections into a single virtual connection, known as a bundle, to a given customer site. The connections can terminate at a CPE (Figure 5) or a Multilink Frame Relay bridge (Figure 6).

Figure 5: MLFR Aggregation of T1 Lines into a Single Bundle

MLFR Aggregation of T1 Lines into a Single
Bundle

Figure 6: Terminating the Bundle at an MLFR Bridge

Terminating the Bundle at an MLFR Bridge

MLFR Link Integrity Protocol

Member links in an MLFR bundle support the MLFR Link Integrity Protocol (LIP). LIP offers several types of messages, which allow member links to join and leave a bundle. See Table 9.

Table 9: LIP Messages and Functions

LIP Message

Function

Add-Link

Member link sends this message to request to join a bundle.

Add-Link-Ack

Member link sends this message when it receives an Add-Link message.

Add-Link-Rej

Member link sends this message to reject a request to join a bundle.

Hello

Member link sends this message to check the status of another member.

Hello-Ack

Member link sends this message when it receives a Hello message.

Remove-Link

Member link sends this message to request to leave a bundle.

Remove-Link-Ack

Member link sends this message when it receives a Remove-Link message.

The DTE creates a link management interface (LMI) with the network by encapsulating the Frame Relay frame within an MLFR frame. You assign one or more data link control identifiers (DLCIs) to a bundle.

Interface Stacking

Because MLFR aggregates multiple link-layer channels onto a single network layer IP interface, protocol layering within the router is different than it is for nonmultilink Frame Relay. See Figure 7.

Figure 7: Structure of MLFR

Structure of MLFR

The MLFR Link Integrity Protocol runs on each link in a bundle. However, from the major Frame Relay interface (the bundle) upward, the interface stacking is the same as for nonmultilink Frame Relay. For example, LMI runs only on the bundle. The bundle sends and receives all MLFR packets.