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 FlagAddressControlDataFCSFlag AddressControlProprietaryDataFCSFlag 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 Options FeatureHow it OperatesProtocol AuthenticationRequires a password Performs challenge handshake PAP CHAP CompressionCompresses data at the source; decompresses at destination Stacker or Predictor Error DetectionMonitors data dropped on link. Avoids frame looping Magic Number Quality MultilinkLoads 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 RouterInitiating Router ppp encapsulation hostname username 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 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 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 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 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 Commands CommandDescription encapsulation hdlcEnables HDLC encapsulation on an interface encapsulation pppEnables PPP on a PPP interface ppp authentication papEnables PAP authentication on a PPP interface ppp authentication chapEnables CHAP authentication on an interface username name password passwordEstablishes a username-based authentication system show interfaceShows the status of an interface, including encapsulation method debug ppp authenticationDebigs the PAP or CHAP authentication process