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 telco 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 electrical signal sent by the CPE device and sends the same electrical current right back to the same device.

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

Routing protocol convergence can be sped up by LCP’s 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 fact that the link is down.  If a router does not notice that the link has been looped, the routing protocol must wait for timeouts – things such as not hearing from the router on the other end of the link for some period of time.

LCP notices looped links quickly using a feature called magic numbers.  When using PPP, the router sends PPP LCP messages instead of Cisco-proprietary keepalives across the link; these messages include a magic number, which is different on each router.  If a line is looped, the router receives an LCP message with its own magic number instead of getting a message with the other router’s magic number.  When a router receives its own magic number, that router knows that the frame it sent has been looped back, so the router can take down the interface, which speeds up convergence.

Enhanced Error Detection

Similar to many other data-link protocols, PPP uses an FCS field in the PPP trailer to determine if an individual frame has an error.  If a frame is received in error, it is discarded.  However, PPP can monitor the frequency with which frames are received in error so that it can take down an interface if too many errors occur.

PPP LCP analyzes the error rates on a link using a PPP feature called Link Quality Monitoring (LQM).  LCP at each end of the link sends messages describing the number of correctly received packets and bytes.  The router that sent the packets compares this number of in-error frames to the number of frames and bytes it sent, and it calculates percentage loss.  The router can take down the link after a configured error rate has been exceeded.

The only time LQM helps is when you have redundant routes in the network.  By taking down a link that has many errors, you can cause packets to use an alternative path that might not have as many errors.