Cisco ICND1 Flashcard: WAN Protocols and Serial Encapsulation, PPP, HDLC

The Usual Scenario that describes most WANs
Use serial point to point connection to connect the LAN to service provider WAN
Have serial point to point connections within the LAN
Use Circuit Switching technology (ICND1 Topic)

ICND2: Packet Switching in Frame Relay and ATM

The Telco provides clocking info for CSUé DSU. The DCE provides clocking, set the clock rate command here, while the receiving device say the customer`s router is a DTE.

What is a T1
T1: 24 DSO's each 64 k
1 DSO is the bandwidth is required for an uncompressed, digitized phone call
a point to point leased line bandwidth specified by a DS number (DS0, DS1 etc)

T1: 1.544Mbps, 24 DSO`s 64 kbps each, 8 kbps overhead
E1: 2.048 MBps, 32 DSO 64 kbps channels

Circuit Switching
A dedicated path is established, maintained, terminated through a carrier network for each session.
Therefore circuit switching creates a dedicated physical connection running PPP, HDLC on Layer 2. Most likely this will be a leased line at fixed capacity, dedicated for the WAN connection.  The Point to Point serial line to form a preestablished WAN communications path

HOW TO Configure a Serial Interface
The serial interface will connect WAN to routers at a remote site

conf t
interface serial 0/0/0
bandwidth 64
clock rate 64000
encapsulation hdlc
no shutdown

Notes: by default Cisco devices are DTE devices but may be configured as DCE
bandwidth: metric used by IGRP routing protocol
clockrate: set clockrate on DCE interfaces in bps, possible 1200, 2400, 4800, 9600, 19200, 38400, 56000, 64000, 72000, 125000 to name a few and 4000000

To configure the clock rate for the hardware connections on serial interfaces, use the clock rate interface configuration command. Use the no form to remove the clock rate if you change the interface from a DCE to a DTE device. Using the no form of this command on a DCE interface sets the clock rate to the hardware-dependent default value.

clock rate bpsno clock rate

The default value could be no clock rate configured, or on a serial interface card I plugged in, it was 2000000 bps.

Clock rate vs Bandwidth
My summary taken from cisco discussion pages on this subject.

Take the example of simple serial PPP linke: on the DCE side of the circuit (that would be the internal part connecting to the CPE of the Service Provider) put "clock rate 64000".  Depending on the IOS version, on the DTE side, you may be able to see this with "show controllers (intf) | include clock" ; reveals the actual tx/rx clock.  The clock rate is required to match the clocks on the receiver and transmitter on remote and local router.the two routers need to sync up their clocks in order to decode the packets coming on their interfaces

Then on the DTE side, look at "show interface (intf) | include BW" and the regular serial link is showing 1544K even though it's only physically possible to send 64K. By default, the routers do not have any mechanism to detect the actual bandwith of a serial line and its is set to a default value of 1.544 MBPs. If there is one 64K serial line and another T1 line on the same router, if the bandwidth value on the 64K serial line is not changed, the router will treat both as T1 lines. Its a method to tell the router that it is a slower link so that actual metrics can be computed accurately.

This bandwidth command tells IOS how to perceive the speed of any particular interface in order to manipulate routing metrics (EIGRP, OSPF); note that the bandwidth command doesn't physically change the speed of an interface like the clock rate command does.

Other commands
show controller serial 1/0, displays information about the physical interface, including clock rate
show interface

HDLC - High Level Data Link Control protocol
HDLC is one of two major data-link protocols, the encapsulation method for data on synchronous serial data links. Error checking built in, enables flow control and error checking using ack, control characters, checksum. However HDLC is not compatible between different vendors. Remember, it uses a frame delimiter to mark the start/ end of each frame

HDLC has a type field that may not be compatible with equipment from other vendors.

Cisco HDLC
Cisco HDLC is a datalink protocol for point to point WAN connections. It is the default encapsulation for serial lines. There is no windowing no flow control, only point to point. Some extensions allow multiprotocol support before ppp was specified
* will not interoperate with other HDLC implementations
* use PPP when interoperability is required, for example if two Nortel and Cisco devices were connecting

ISDN
Different ISDN services - voice and data can run over existing telephone lines. The BRI (basic rate interface) uses two B channels (64 kbps each, may be combined) and one D channel (16 kbps). B for bearer for voice and data; D for data for call signalling or clocking.

An ISDN interface can run these protocols:
E protocols for ISDN on existing telephone network.
I protocols for concepts, terminology and services.
Q protocols refer to switching and signaling.

A Service Provider may use Signaling System 7 (SS7) between the two switches—the same protocol used inside phone company networks to set up circuits for phone calls.  ISDN PRI in North America is like a digital T1 circuit

ISDN BRI and PRI Reference Point Diagrams

PPP
Point to Point protocol is a data-link protocol, provides router to router and host to network connections over both synchronous and asynchronous circuits. So, it transports Layer 3 packets across the data-link layer. PPP can be applied to these physical interfaces:
1) asynchronous connection - think of a dial up connection
2) synchronous connection - think of a leased line, like ISDN media
3) High Speed Serial Interface HSSI

What features not available in HDLC but found in PPP:
1) link quality management feature to monitor quality of link. Too many errors detected, ppp takes down the link
2) supports Password Authentication Protocol PAP and CHAP (three way hash authentication)

Three phases of PPP
The method for encapsulating multiprotocol datagrams
* Link establishment phase- LCP extensible link control protocol, establish, configure, test the WAN link
* authentication phase of ppp is optional (choose PAP or CHAP)
* Network Layer protocol phase - NCP network control protocol, to establish and configure different network layer protocols, example IPCP, Appletalk Control Protocol, Novell IPX Control Protocol, Cisco Systems CP, Systems Network Architecture (SNA) CP, Compression CP

Main components:
E1A/ T1A 232C - connector, physical layer standard for serial comms
HDLC - high level data link control, for encapsulating datagrams over serial links
LCP negotiates traffic, maintaining or terminating traffic
NCP encapsulates traffic, multiple network layer protocols.

LCP configuration options
Authentication - identifying the sender, PAP or CHAP
Compression - Cisco uses Stacker and Predictor compression methods
Error Detection - Quality and Magic Numbers
Multilink - splits the load over two or more parallel circuits, or a bundle

PAP
- Password Authentication Protocol; passwords are sent in the cleartext, PAP is only for the initial link establishment

CHAP
- Challenge Authentication Protocol; used at the initial startup of the link and at periodic checkup times to make sure the router is still communicating with the same host. Router sends challenge request to the remote device, expects a value calculated by the one way hash function MD5. If the values don't match, the link is terminated.

Configuring PPP and authentication
* hostname RouterX, assign a hostname to RouterX
* username RouterY password B007! , identify the username RouterY and password of remote router
* conf t, then go to the serial interface in question
* encapsulation ppp, enable ppp encapsulation
* ppp authentication chap, enable chap authentication or use pap instead

Sample configuration
conf t
int s0
encapsulation ppp

Conf t
hostname routerX
username routerY privilege 15 secret 0 password B007!!
encapsulation ppp
ppp authentication chap
(or ppp authentication pap)

debug ppp authentication
Verify
show interfaces
show interface serial
show interface s0