Enterasys Networks CSX7000 Manual do Utilizador descarregar pdf (Página 398)

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USER’S GUIDE

398 CyberSWITCH

PPP BACKGROUND INFORMATION

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 FAILURE DETECTION

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.

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