Wide Area Networks

February 4, 2009 at 10:26 am · Filed under Uncategorized

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PPP LCP Features
Function	LCP Feature	Description
Looped link detection	Magic number	Detects if the link is looped, and disables the interface, allowing rerouting over a working route.
Error detection	Link Quality Monitoring (LQM)	Disables an interface that exceeds an error percentage threshold, allowing rerouting over better routes.
Multilink support	Multilink PPP	Load-balances traffic over multiple parallel links.
Authentication	PAP and CHAP	Exchanges names and passwords so that each device can verify the identity of the device on the other end of the link.

Likely Reasons for Data-Link Problems on Serial Links
Line Status	Protocol Status	Likely Reason
Up	Down (stable) on both endsor Down (stable) on one end, flapping between up and down on the other	Mismatched encapsulation commands
Up	Down on one end, up on the other	Keepalive is disabled on the end in an up state
Up	Down (stable) on both ends	PAP/CHAP authentication failure

Summary of Symptoms for Mismatched Subnets on Serial Links
Symptoms When IP Addresses on a Serial Link Are in Different Subnets	HDLC	PPP
Does a ping of the other router’s serial IP address work?	No	Yes
Can routing protocols exchange routes over the link?	No	No

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Step 1.	Configure the physical interface to use Frame Relay encapsulation (encapsulation frame-relay interface subcommand).
Step 2.	Configure an IP address on the interface or subinterface (ip address subcommand).
Step 3.	(Optional) Manually set the LMI type on each physical serial interface (frame-relay lmi-type interface subcommand).
Step 4.	(Optional) Change from the default encapsulation of cisco to ietf by doing the following: a. For all VCs on the interface, add the ietf keyword to the encapsulation frame-relay interface subcommand. b. For a single VC, add the ietf keyword to the frame-relay interface-dlci interface subcommand (point-to-point subinterfaces only) or to the frame-relay map command.
Step 5.	(Optional) If you aren’t using the (default) Inverse ARP to map the DLCI to the next-hop router’s IP address, define static mapping using the frame-relay map ip dlci ip-address broadcast subinterface subcommand.

Step 6.	On subinterfaces, associate one (point-to-point) or more (multipoint) DLCIs with the subinterface in one of two ways: a. Using the frame-relay interface-dlci dlci subinterface subcommand b. As a side effect of static mapping using the frame-relay map ip dlci ip-address broadcast subinterface subcommand

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