3.4 PPP Compression
3.4.4 Multilink operation and configuration
MLP is controlled by the addition of a two-, four-, or eight-byte sequencing header in the PPP frame that indicates sequencing for the fragments.

MLP Packet Header

RFC 1990 provides for only four-byte headers. The first fragment of a multilink packet in PPP has two headers, one for the fragment, followed by the header for the packet itself.

During PPP negotiation, some hardware platforms could use different header lengths for MLP, causing potential problems. Please check with the specific platform documentation if you have problems with MLP.

Transmission channels in the bundle need not be the same types. Asynchronous and synchronous links, for example, can be used to simultaneously transmit fragments of one datagram.

During the PPP LCP option negotiation, a system indicates to its peer that it is willing to use multilink by sending the MRRU option as part of the initial LCP option negotiation. Multilink systems must be able to do the following:

  • Combine multiple physical links into one logical link (bundle)
  • Receive and reassemble upper-layer protocol data units (PDUs)
  • Receive PDUs of a negotiated size

The ppp multilink command activates multilink on an interface:

Router(config-if)#ppp multilink

The following shows an example of the show ppp multilink command. At the bottom of the output, you can see the exact number of channels participating in the bundle:

Router#show ppp multilink
Bundle rudder, 3 members, first link is BRI0: B-channel 1
0 lost fragments, 8 reordered, 0 unassigned, sequence 0x1E/0x1E rcvd/sent