Information Technology Reference
In-Depth Information
Alternate PPP Methodologies
PPP offers great flexibility in the manner in which it can be used. The following methodologies describe
how to connect multiple links between two or more devices, and how to scale PPP for large numbers of
inbound connections.
Multilink PPP
The Multilink Point-to-Point Protocol (MLP) feature provides load-balancing functionality over
multiple WAN links, while providing multivendor interoperability, packet fragmentation and proper
sequencing, and load calculation on both inbound and outbound traffic. Cisco's implementation of
multilink PPP supports the fragmentation and packet sequencing specifications in RFC 1717.
Multilink PPP allows packets to be fragmented and for 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 bandwidth on demand and
reduces transmission latency across WAN links.
Multilink PPP is designed to work over single or multiple interfaces of the following types that are
configured to support both dial-on-demand rotary groups and PPP encapsulation:
Asynchronous serial interfaces
•
Basic Rate Interfaces (BRIs)
•
Primary Rate Interfaces (PRIs)
•
Configuration
To configure multilink PPP on asynchronous interfaces, you configure the asynchronous interfaces to
support DDR and PPP encapsulation, and then you configure a dialer interface to support PPP
encapsulation, bandwidth on demand, and multilink PPP. At some point, however, adding more
asynchronous interfaces does not improve performance, With the default MTU size, multilink PPP
should support three asynchronous interfaces using V.34 modems. However, packets might be dropped
occasionally if the MTU is small or large bursts of short frames occur.
To enable multilink PPP on a single ISDN BRI or PRI interface, you are not required to define a dialer
rotary group separately because ISDN interfaces are dialer rotary groups, by default. If you do not use
PPP authentication procedures, your telephone service must pass caller ID information.
A load threshold number is required. For an example of configuring multilink PPP on a single ISDN BRI
interface, see the next section, “Example of Multilink PPP on One ISDN Interface.”
When multilink PPP is configured, and if you want a multilink bundle to be connected indefinitely, use
the
dialer idle-timeout
command to set a very high idle timer. (The
dialer-load threshold 1
command
does not keep a multilink bundle of
n
links connected indefinitely, and the
dialer-load threshold 2
command does not keep a multilink bundle of two links connected indefinitely.)
To enable multilink PPP on multiple ISDN BRI or PRI interfaces, you set up a dialer rotary interface and
configure it for multilink PPP. Then you configure the BRIs separately and add them each to the same
rotary group. See the “Example of Multilink PPP on Multiple ISDN Interfaces,” next.
