MLPPP Overview

Multilink PPP (MLPPP; also referred to as PPP Multilink, MLP, and MP) aggregates multiple physical links into a single logical bundle. More specifically, MLPPP bundles multiple link-layer channels into a single network-layer channel. Peers negotiate MLPPP during the initial phase of Link Control Protocol (LCP) option negotiation. Each router indicates that it is multilink capable by sending the multilink option as part of its initial LCP configuration request.

An MLPPP 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 acts on MLPPP like another PPP Network Control Protocol (NCP). Packets received with an MLPPP header are subject to fragmentation, reassembly, and sequencing. Packets received without the MLPPP header cannot be sequenced and can be delivered only on a first-come, first-served basis.

Application

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 the desired bandwidth. MLPPP is commonly used as an alternative to ECMP to deliver NxT1 service. NxT1 service provides bandwidth greater than DS1 service without going up to the expense and infrastructure required for DS3 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 MLPPP bundle.

The NxT1 implementation of MLPPP logically aggregates up to eight T1 or E1 connections into a single virtual connection, or bundle, to a given customer site, as shown in Figure 1.

Figure 1: MLPPP Aggregation of T1 Lines into a Single Bundle

Because MLPPP aggregates multiple link-layer channels onto a single network-layer IP interface, protocol layering within the router is different than for non-multilink PPP.

Figure 2 illustrates interface stacking with MLPPP.

Figure 2: Structure of MLPPP

MLPPP LCP Extensions

Multilink PPP adds the following LCP negotiation options:

• Multilink maximum received reconstructed unit (MRRU) option—The MRRU option has two functions. First, it informs the other end of the link the maximum size of the PPP packet payload that the router can receive. Second, it informs the other end that the router supports MLPPP. When you enable multilink on your router, the router includes the MRRU option in LCP negotiation with the value set to the maximum received unit (MRU) value for PPP. If the remote system rejects this option, the local system determines that the remote system does not support multilink PPP and it terminates the link without negotiation.

  Note: The router does not bring up a link if the MRU value received from a peer device differs from the MRRU value received from the peer.

• Short sequence number (SSN) header format option (not currently supported)—The SSN option indicates that the transmitting router wants to use a short sequence number (12 bits) in the MLPPP header rather than a long sequence number (24 bits). The router currently supports only long sequence numbers.
• Endpoint discriminator option—The endpoint discriminator option identifies the router transmitting the packet. If the receiving router determines that packets on another link have the same endpoint discriminator option, this link must be joined to that bundle. If the receiving router determines that no packets on other links have the same option, the receiving router must create a new bundle from this link.

  The endpoint discriminator is generated internally; you cannot configure it. The endpoint discriminator option is the same for all links on one end of the bundle; at the other end, all links also share a common endpoint discriminator. The two endpoint discriminators are different if the MLPPP bundle is set up between two E Series routers.