Cabletron Systems CSX1000 - Manual (Page 336)

USER’S GUIDE
336 CyberSWITCH
However, the PPP link exists on an end-to-end basis with the remote peer, a domain which exceeds
that controlled by the signalling-type entities just cited. Thus, not every end-to-end failure will be
detected. Some examples of such failures include:
• an ISDN peer’s D-channel “process” is functional, but it’s B-channel “process” has failed
• the underlying physical circuit has an end-to-end fault in one or both directions which does not
affect the D-channel or control path
• the underlying physical circuit has been mistakenly looped back
In such cases, the Link Failure Detection feature can discern the fault(s). A properly functioning
remote device is obligated to return an Echo-Reply to each Echo-Request, which verifies the full
end-to-end path of the point-to-point link. Furthermore, the Echo-Request frames carry a PPP
element known as the “Magic Number” which can be used to ascertain if an inbound Echo-Request
truly came from the peer or was looped back.
PPP Link Failure Detection can be enabled or disabled within the PPP Options configuration menu.
When enabled, two other configurable parameters then control the mechanism. Upon entrance of
a PPP link into Network Phase (the point at which device data transfer is allowed), Echo-Requests
will be sent at a configured frequency. As long as Echo-Replies are received, the link is deemed to
be functional.
A second parameter specifies the maximum number of Echo-Request attempts which will be
transmitted without a reply. If this limit is reached, a message is logged and the link is reported as
faulty. Thus, the configured frequency multiplied by the configured maximum attempts yields the
approximate time it will take to detect a failed link.
Note: Within the CyberSWITCH, there are certain accesses which also present the ability to
enable/disable the periodic transmission of link maintenance type packets. For example,
the packet-based Frame Relay access supports the configurable enable/disable of
“Keepalives” in order to avoid the extra per-packet costs which the periodic Echo-Request
frames might incur. The PPP Link Detection Failure feature will honor such access-based
configuration, in addition to the explicit enable/disable configuration status of the PPP
feature itself.
PPP R EFERENCE D OCUMENTS
Point-to-Point Protocol (PPP) is also described in more detail in the set of RFCs listed below:
•
RFC 1661 The Point-to-Point Protocol
• RFC 1638 PPP Bridging Control Protocol (BCP)
• RFC 1549 PPP in HDLC Framing
• RFC 1547 Requirements for an Internet Standard Point-to-Point Protocol
• RFC 1334 PPP Authentication Protocols
• RFC 1332 The PPP Internet Protocol Control Protocol (IPCP)