MLPPP Overview
Multilink Point-to-Point Protocol (MLPPP) aggregates multiple PPP physical links into a single virtual connection, or 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.
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.
MLPPP for subscriber access is supported starting in Junos OS Release 14.1.
This section contains the following topics:
Traditional MLPPP Application
MLPPP is used to bundle multiple low speed links to create a higher bandwidth pipe such that the combined bandwidth is available to traffics from all links, and to support link fragmentation and interleaving (LFI) support on the bundle to reduce the transmission delay of high priority packets. LFI interleaves voice packets with fragmented data packets to ensure timely delivery of voice packets. Figure 1 shows how incoming packets are distributed and aggregated into an MLPPP 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.
MLPPP LCP Negotiation Option
Multilink PPP adds the multilink maximum received reconstructed unit (MRRU) option for LCP negotiation. The MRRU option has two functions:
It informs the other end of the link the maximum reassembled size of the PPP packet payload that the router can receive.
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 default value set to 1500 bytes (user-configurable option) 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.
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.