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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 9.


Figure 9: 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 10 illustrates interface stacking with MLPPP.


Figure 10: Structure of MLPPP

MLPPP LCP Extensions

Multilink PPP adds the following LCP negotiation options:

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.

MLPPP Link Selection

By default, E-series routers use a round-robin algorithm to select the link on which to transmit data on an MLPPP interface. The round-robin link selection method applies to both best-effort packets, such as data, and non-best-effort (high-priority) packets, such as voice and video. Best-effort packets are encapsulated with an MLPPP header that contains a sequence number, whereas non-best-effort packets are encapsulated with a PPP header that does not contain a sequence number.

The member links in an MLPPP bundle can experience different queuing delays due to the volume of traffic transmitted on the MLPPP interface. These delays can cause packets to arrive out of order at the remote router. The effect of such delays differs for best-effort packets and non-best effort packets, as follows:

To ensure that the E-series router maintains the proper packet order when transmitting non-best-effort traffic, you can use the ppp hash-link-selection command to enable use of a hash-based algorithm to select the link on which the router transmits high-priority packets on an MLPPP interface.

When you use hash-based link selection instead of the default round-robin link selection for non-best-effort traffic, the router uses the IP source address (SA) and IP destination address (DA) of the packet as a hash to select the MLPPP member link on which to transmit the packet. Specifically, the router uses the hash algorithm to bind the transmission of all traffic between this IP SA and IP DA to the same member link in the MLPPP bundle.

If the member link selected to transmit high-priority packets becomes inoperable or is removed from the MLPPP bundle, the router must select a different link on which to transmit the packets. As a result, packets transmitted on this new link might sometimes arrive at the remote destination before the traffic sent on the previously selected member link.

You can configure hash-based MLPPP link selection in any of the following ways:

For a detailed description and examples of using the ppp hash-link-selection command, see ppp hash-link-selection.


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