ICND2 – Chapter 14 – Point-to-Point WANs

I’ve always considered myself a WAN Engineer as opposed to a Network Engineer since my previous experience was weak in LANs an heavy in ATM, point-to-point T1s/T3s, and Frame-Relay. Therefore, I looked forward to the WAN chapters in the book. The first one up mainly covered PPP. Here are my notes:

One of the most important features of the PPP standard that is not included in the HDLC standard is that of a protocol field to identify the type of packet being carried. (Although Cisco proprietary HDLC overcomes this hurdle as well)

PPP or Point-to-Point protocol uses some additional protocols as well.

Link Control Protocol (LCP) provides 4 main features:

1. Looped Link Detection
2. Enhanced Error Detection
3. Multilink Support
4. Authentication

LCP uses Magic Numbers to determine if a link is loped. If it sees its own Magic Number in an LCP message then it knows it is looped.

LCP uses Link Quality Monitoring (LQM) to analyze error rates. LQM only helps when redundant links are present.

Multilink PPP load balances traffic equally across multiple links that appear as a single-link. Multilink PPP fragments packets and sends one packet over each link.

PAP and CHAP can be used to authenticate end-points.

PAP is insecure because it passes the password in plain text.

CHAP uses MD5 hashes which is more secure.

CHAP uses a random number that it adds to the password and then hashes that value.

Basic PPP Configuration

Router(config-if)#encapsulation ppp

Configuring CHAP

Router(config)#hostname R1
Router(config)#username R2 password mypass (username is hostname of far-end router)
Router(config)interface Serial0/0/0
Router(config-if)#encapsulation ppp
Router(config-if)#ppp authentication chap

Keepalives on a serial link should be enabled on both ends or disabled on both ends.

Use the debug ppp authentication to find the root cause of PPP authentication issues.


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