5.6 Optional Configuration
5.6.3 Multilink PPP

BOD has been further enhanced by basing its implementation on industry-standard MLP. The MLP feature provides load balancing over multiple WAN links, while providing multivendor interoperability, packet fragmentation, proper sequencing, and load calculation on both inbound and outbound traffic. As shown in Figure , the Cisco implementation of MLP supports the fragmentation and packet sequencing specified in RFC 1990, which replaces RFC 1717.

PPP Multilink Terminology
Even though PPP is a Layer 2 protocol and therefore should use the term frame, RFC 1990 defines individual fragments as packets for a multilink protocol.

MLP-based BOD is available in Cisco IOS Release 11.0 and later, and it is available in Cisco 700 series routers with Release 3.2 and later.

MLP allows packets to be fragmented and the fragments to be sent at the same time over multiple point-to-point links to the same remote address. The multiple links come up in response to a dialer load threshold that you define. The load can be calculated on inbound traffic, outbound traffic, or either, as needed for the traffic between the specific sites. MLP provides BOD and reduces transmission latency across WAN links.

Use the dialer load-threshold load command, which has the keywords inbound, outbound, or either, for MLP-based BOD. For remote users, configure the load threshold outbound. Then, configure ppp multilink, which allows you to turn MLP on or off. This command is placed either on the physical interface or in a dialer interface, depending on the interface type, quantity of interfaces configured, and whether dialer rotary groups or dialer profiles are used. The following is an example of multilink activation:

Router(config)#int bri 0
Router(config-if)#dialer load-threshold load [inbound | outbound | either
Router(config-if)#ppp multilink

The ppp multilink command can be configured on the following:

  • Asynchronous serial interfaces in dialer rotary groups-MLP is configured in the dialer rotary interface (int dialer number).
  • Individual BRIs-MLP is configured on the BRI interface (int bri number).
  • Synchronous serial- MLP is configured on the serial interface (int serial number).
  • Multiple BRIs in dialer rotary groups- MLP is configured in the dialer rotary interface (int dialer number), not on the BRI interfaces.
  • Multiple BRIs in dialer profiles-For incoming calls, MLP is configured on the BRI interfaces. For outgoing calls, MLP is configured in the dialer interface. For both incoming and outgoing calls, MLP is configured on both the BRI and dialer interfaces.
  • PRI B channels in dialer rotary groups- MLP is configured on the serial interfaces associated with the PRI interface.

By default, both B channels of a BRI are in a two-link rotary group. PRIs can have up to 23 links per rotary group for T1 or 30 links for E1.

Note: Click on the topology to view the configuration of BRI 0 for MLP and the rotary group interface configured for MLP. .

Configuring Load Threshold Value
Only one end of a link can be configured for a load threshold. If you need to configure both the caller and answering routers (they might both have to perform DDR at multipoint), set the threshold at different values. If Router-A is set for an outbound threshold of 50 and the answering router—Router-B—is set for an inbound threshold of 50, they might both attempt to bring up the second link simultaneously. They thus get a busy signal from the other end. This is improbable with ISDN because the call setup is very low. It can, though, be a problem with slower call setup technologies, such as asynchronous links.

In small offices, use the either option for load threshold to ensure that the maximum of the inbound and outbound traffic is calculated as the load threshold. As shown in Figure , if multiple rotary groups on a router are configured for multilink, one large bundle is created between the two routers, based on the authentication name (provided that the rotary group dials the same destination).

PPP authentication plays a part in MLP. The bundle decision is based on the authentication name of the remote router independently on each side of the link. Each router should use a unique hostname for authentication, with a shared password. If authentication is not configured correctly, MLP will not work correctly.

When single multilink BRIs or a rotary group with multiple BRIs are calling a PRI, do not put a dialer load-threshold on the PRI or multiple BRI rotary group. Use the dialer load-threshold command on the single BRI interface. This will prevent attempts to bring up a third B channel to a single BRI destination.