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312 Chapter 9: Point-to-Point Leased Line Implementation

configured for PPP encapsulation automatically tries to bring up the appropriate control protocols for each Layer 3 protocol (for example, IPCP). Cisco routers also use a PPP CP for supporting CDP traffic, called CDPCP.

LCP provides a variety of optional features for PPP. You should at least be aware of the concepts behind these features. They are summarized in Table 9-6.

Table 9-6 PPP LCP Features


LCP Feature






Link Quality

PPP can take down a link based on the percentage of errors



on the link. LQM exchanges statistics about lost packets



versus sent packets in each direction. When compared to



packets and bytes sent, this yields a percentage of errored



traffic. The percentage of loss that causes a link to be taken



down is enabled and defined by a configuration setting.




Looped link

Magic number

Using different magic numbers, routers send messages to



each other. If you receive your own magic number, the link



is looped. A configuration setting determines whether the



link should be taken down when looped.





Multilink PPP

Fragments of packets are load-balanced across multiple








PAP and

Exchanges names and passwords so that each device can



verify the identity of the device on the other end of the link.




The next few pages discuss how two of these features work. Chapter 10 covers Multilink PPP (MLP) support.

Looped Link Detection

Error detection and looped link detection are two key features of PPP. Looped link detection allows for faster convergence when a link fails because it is looped. What does “looped” mean? Well, to test a circuit, the phone company might loop the circuit. The technician can sit at his desk and, using commands, cause the phone company’s switch to loop the circuit. This means that the phone company takes the bits you send in and sends them right back to you.

The routers cannot send bits to each other while the link is looped, of course. However, the router might not notice that the link is looped, because the router is receiving something! PPP helps the router recognize a looped link quickly so that it can bring the interface down and possibly use an alternative route.

In some cases, routing protocol convergence can be sped up by LCP recognition of the loop. If the router can immediately notice that the link is looped, it can put the interface in a “down and down” status, and the routing protocols can change their routing updates based on the