- Enterasys Security Router User's Guide

PPP Features
XSR User’s Guide 8-9
IP Address Assignment
In PPP, IPCP configuration option type 3 corresponds to IP address negotiation. This
configuration option provides a way to negotiate the IP address to be used on the local end of the
link.
It allows the sender of the Configure-Request to state which IP address is desired, or to request
that the peer provide the information. The peer can do this by NAKing the option, and returning a
valid IP address. If the host wants the peer to provide the IP address, it will mark the IP address
field as configuration option 0.
Upon receiving an IP-address Configure-Request with IP address field 0, IPCP may allocate a
valid IP address to the peer by sending a Configure-Nak to the received Configure-Request or it
may reject the Configure-Request.
PPP Bandwidth Allocation/Control Protocols (BAP/BAPC)
The XSR supports the PPP Bandwidth Allocation/Control protocols (BAP/BACP) as a means of
managing individual links of a multilink bundle as well as specifying which peer is responsible
for managing bandwidth during a multilink connection.
This ability to dynamically change bandwidth during a multilink connection is referred to as
Bandwidth-on-Demand (BoD). For more information on BoD, refer to “Configuring Integrated
Services Digital Network” on page 11-1 and “Configuring Dialer Services” on page 10-1.
BAP/BACP, as defined by RFC-2125, is a flexible, robust method of managing bandwidth
between two peers. BAP does this by defining Call-Control packets and a protocol that allows
peers to co-ordinate actual bandwidth allocation and de-allocation. Phone number values may be
passed in the Call-Control packets to minimize user configuration.
BAP/BACP provides the following benefits:
• Allows multilink implementations to interoperate by providing call control through the use of
link types, speeds, and telephone numbers.
• Controls thrashing caused by frequent raising/tearing down links.
• Ensures that both ends of a link are told when links are added/dropped from a multilink
bundle.
The BACP protocol must reach the Opened state using the standard PPP mechanism as defined in
RFC-1661. Once BACP reaches the Opened state on a bundle, BAP may transmit packets through
this PPP/MLPPP pipeline.
BAP datagrams are encapsulated by the PPP/MLPPP module and transmitted across the link.
Transmission and reception of BAP and BACP packets is through the same interface procedures
used by any other NCP protocol pair.
Functionality provided by BAP/BACP is summarized as follows:
•To add links:
– Negotiate phone numbers over the bundles through BAP.
– Agree with peer before trying to set up a call.
– Check for available lines before agreeing to add a link.
– Manage race conditions when both peers wish to add a link.
• To delete links:
– Agree with peer to tear down a link before disconnecting the call.