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PPP 405
The difference between Cisco and IBM HDLC is that the IBM version can carry only
one protocol, TCP/IP. Cisco's modified version can carry multiprotocol traffic, even at the
same time.
No error recovery is available with Cisco HDLC. If there's an error, the packet is dropped. As
mentioned in Chapter 10, when an error occurs with LAPB encapsulation, the packet is
retransmitted. The extra error recovery overhead in LAPB decreases performance, so you
should use LAPB only on lines that are prone to errors.
Multilink PPP (MLPPP)
MLPPP was first introduced in Cisco IOS Release 11.0(3) and is referenced by RFC 1990
(replacing RFC 1717). It specifies multivendor interoperability. MLPPP is a data-link protocol.
It sits between PPP and the Network Control Protocol. The sending peer receives protocol data
units (PDUs) from the network control layer above it.
MLPPP converts the PDUs into packets for transmission by adding addressing information
and MLPPP headers.
MLPPP also lets you aggregate the bandwidth across multiple interfaces, usually ISDN
interfaces. For example, a 64 K BRI can combine with a 64 K serial link, totaling 128 K of
bandwidth into one logical pipe. For example, if a customer needed more bandwidth but did not
want to affect the existing network, this method would be a solution. Design engineers can
increase bandwidth without affecting routing tables on the network. This means that network
designers can use MLPPP to simplify fault management and build redundancy into the network
without affecting users. Therefore, it reduces the effort and cost of maintaining a network.
MLPPP provides load balancing over dialer interfaces, including
·
ISDN interfaces
·
Synchronous interfaces
·
Asynchronous interfaces
Before the introduction of MLPPP, two or more ISDN B channels could not be used in a
standardized way while ensuring sequencing. MLPPP solves several of the problems associated
with load balancing across multiple serial links.
Bundles
During LCP negotiation, a router can indicate to another router that it can combine multiple
physical links into what is known as a bundle. MLPPP uses packet sequencing and load
calculation to negotiate the maximum received reconstructed unit (MRRU) option during the
PPP LCP negotiation. The MRRU option is used to negotiate the size of information fields in
reassembled packets. This negotiation indicates that MLPPP is controlled by the addition of a
87200333.book Page 405 Wednesday, August 22, 2001 2:53 PM