Enterasys Networks CSX6000 - Manual (Page 398)

USER’S GUIDE
398 CyberSWITCH
PPP B ACKGROUND I NFORMATION
Point-to-Point Protocol (PPP) can provide standard interoperability for remote devices.
Interoperability will allow remote devices made by different manufacturers to operate and
exchange information on the same network.
PPP consists of three main parts:
1. A method of encapsulating datagrams so that they can be more easily transmitted over point-
to-point links.
2. A Link Control Protocol (LCP) for establishing, configuring, and testing the data-link
connection.
3. A family of Network Control Protocols (NCPs) for establishing and configuring different
network-layer protocols.
Link Control Protocol (LCP) is used to:
•
automatically agree upon the encapsulation formation options
• handle the varying limits on sizes of packets
• authenticate the identity of the remote device on the link
• determine when a link is functioning properly
• detect common misconfiguration errors
• terminate the link
After a link is established through LCP, the Network Control Protocols (NCPs) manage the specific
needs required by each device’s network-layer protocol.
PPP L INK F AILURE D ETECTION
On a point-to-point link, there are a variety of failures which can occur on the intervening
communications path and/or within the remotely connected system. Often times, such failures are
detectable via a signalling mechanism associated with the link. For example, a failure of an ISDN
B-channel usually leads to a corresponding failure of the associated D-channel, an event which is
suitable for concluding that the B-channel has failed. Similarly, the Local Management Interface
(LMI) facility of a Frame Relay circuit may provide feedback suitable for determining that an end-
to-end Virtual Circuit has failed.
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.