Serial Connections

• Connections over a WAN rather than a LAN

• This chapter describes the Cisco proprietary HDLC protocol and the PPP protocol

Connectivity Options• Leased Line – Provides a point-to-point dedicated WAN

communications path between a service provider and a remote network. Fast and high bandwidth but costly

• Circuit-Switched – A dedicated circuit path exists between sender and receiver for the duration of a call (i.e., over telephone or ISDN lines with a modem). Used in environments where WAN use is sparse

• Packet-Switched – Devices share a single point-to-point link to transport packets across a carrier network. Physical connections are provided by programmed switching devices. Headers identify destination. Cheaper than leased lines but slower

WAN Terminology• Customer Premises Equipment (CPE) – Devices

located on the customer premises• Demarcation (Demarc) – Juncture where CPE

ends and service begins• Local Loop (“Last Mile”) – Cabling from demarc

to WAN central office• Central Office – Telco switching facility that

provides nearest presence of WAN service• Toll Network – The collective telco switches and

facilities (called trunks) inside the WAN provider’s cloud

Serial Cables

Cisco serial router cable is a DB-60 shielded serial transition cable with appropriate connector:– EIA/TIA-232– EIA/TIA-449– V.35– X.21– EIA-530

Equipment

• DTE – Your customer premises equipment

• DCE – The device used to convert the user data from the DTE into a form acceptable to the WAN service provider (i.e., Channel Service Unit/Data Service Unit (CSU/DSU))

WAN Layer 2 - Encapsulation

Serial devices must encapsulate data in a frame format at OSI model layer 2. Different services use different framing formats. Choice of protocol depends on WAN technology– Leased Line – HDLC, PPP, SLIP– Circuit-Switched – PPP, SLIP, HDLC– Packet-Switched – X25, Frame Relay, ATM

Encapsulation Types

• High-Level Data Link Control (HDLC)

• Point-to-Point Protocol (PPP) – Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. Designed to work with several network layer protocols such as IP and XIP.

• Serial Line Internet Protocol (SLIP)

• X.25/Link Access Procedure, Balanced (LAPB)

• Frame Relay

• Asynchronous Transfer Mode (ATM)

HDLC Encapsulation• Often used for leased line links between Cisco routers.

• By default synchronous serial lines use HDLC serial encapsulation

• If communicating with a non-Cisco device, PPP is a more viable option

• It is a bit-oriented synchronous data link layer protocol

• Limitation – HDLC does not inherently support multiple protocols on a single link there is no standard way to indicate which protocol it is carrying

HDLC Frame Formats

Cisco offers a proprietary version of HDLC with a proprietary field that acts like a protocol field

Cisco HDLC

Flag Address

Control Data FCS Flag

Flag Address

Control Proprietary

Data FCS Flag

HDLC

PPP Encapsulation

• PPP RFCs are at:

ftp://ftpeng.cisco.com/fred/rfc-index/rfc.html• PPP is NOT proprietary• Used to connect dissimilar vendor devices such as:

– Asynchronous serial

– HSSI (High-Speed Serial Interface)

– ISDN

– Synchronous serial

PPP Sublayers

IP IPX Layer 3 Protocols

IPCP IPXC Many Others

Network Control Protocol

Authentication, Other Options

Link Control Protocol

Synchronous or Asynchronous

Physical Media

PPP

Network

Layer

Data Link

Layer

Physical

Layer

PPP Components

• Functionally, PPP is a data link layer protocol with network layer services

• Therefore, PPP can be broken down into two sublayers NCP and LCP which enhance functionality

NCP/LCP

• NCP is used to encapsulate multiple protocols

• LCP is used to negotiate and set up control options on the WAN data link

LCP OptionsFeature How it Operates Protocol

Authentication Requires a password

Performs challenge handshake

PAP

CHAP

Compression Compresses data at the source;

decompresses at destination

Stacker or

Predictor

Error Detection

Monitors data dropped on link. Avoids frame looping

Magic Number

Quality

Multilink Loads balancing across

multiple links

Multilink

Protocol (MP)

PPP Establishment• Link Establishment Phase (Dialup or Circuit-Switched

Network) – Each PPP device sends LCP packets to configure and test the data link. LCP packet contain a Configuration Option field that allows devices to negotiate use of options such as max receive unit, compression of certain PPP fields, and link authentication protocol

• Optional Authentication Phase – PPP supports two authentication protocols: PAP and CHAP (CHAP is the preferred protocol)

• Network Layer Protocol Phase – PPP devices send NCP packets to choose and configure one or more network layer protocols such as IP

PAP Two-Way Handshake Authentication

• Remote router sends name (santacruz) and password (boardwalk) to central-site router

• Central-site router consults local list of remote router names and passwords and returns accept/reject

• Passwords are sent in clear text• There is no protection from playback or repeated

trial-and-error attacks• Acceptable if token-type passwords are used that

change with each authentication

CHAP Three-Way Handshake

• Local router sends a challenge message to the remote router

• Remote router responds with a value calculated using a one-way hash function (typically MD5)

• Local router checks response against its own calculation

• If values match, the local router responds with an acknowledge (if not router responds with reject)

PPP Authentication Tasks

Receiving Router Initiating Router

ppp encapsulation ppp encapsulation

hostname hostname

username username

ppp authentication

ppp authentication

ppp pap sent-username (PAP only)

Authentication Configuration PAP

Router(config)#hostname RouterA

RouterA(config)#username RouterB password mustmatch

RouterA(config)# interface serial 0

RouterA(config-if)#ip address 10.0.1.1 255.255.255.0

RouterA(config-if)#encapsulation ppp

RouterA(config-if)#ppp authentication pap

Router(config)#hostname RouterB

RouterB(config)#username RouterA password mustmatch

RouterB(config)# interface serial 0

RouterB(config-if)#ip address 10.0.1.2 255.255.255.0

RouterB(config-if)#encapsulation ppp

RouterB(config-if)#ppp authentication pap

Authentication Configuration CHAP

Router(config)#hostname RouterA

RouterA(config)# interface serial 0

RouterA(config-if)#ip address 10.0.1.1 255.255.255.0

RouterA(config-if)#encapsulation ppp

RouterA(config-if)#ppp authentication chap

Router(config)#hostname RouterB

RouterB(config)# interface serial 0

RouterB(config-if)#ip address 10.0.1.2 255.255.255.0

RouterB(config-if)#encapsulation ppp

RouterB(config-if)#ppp authentication chap

show interface command

• After configuring a serial interface at a router type:

RouterA#show interface s0

to get configuration information

ppp Debugging

RouterA#debug ppp authentication

Causes ppp debugging information to be output

Summary of Serial CommandsCommand Description

encapsulation hdlc Enables HDLC encapsulation on an interface

encapsulation ppp Enables PPP on a PPP interface

ppp authentication pap Enables PAP authentication on a PPP interface

ppp authentication chap Enables CHAP authentication on an interface

username name password password Establishes a username-based authentication system

show interface Shows the status of an interface, including encapsulation method

debug ppp authentication Debigs the PAP or CHAP authentication process