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.All SNAphysical unit (PU) types are supported.This section focuses on the SDLC data-link protocol and itsvarious configurations and then explains how to implement STUN.Figure 5-1 illustrates elements of STUN configuration in an environment that includes front-endprocessors (FEPs) and cluster controllers.Figure 5-1: Sample STUN network configuration.http://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2005.htm (2 of 30) [9/16/2000 5:05:39 PM] Designing SDLC, SDLLC, and QLLC InternetworksSDLC Data LinkSDLC is the synchronous, bit-oriented protocol used by the SNA data-link control layer.As formallydefined by IBM, SDLC is a line discipline for managing synchronous, code-transparent, seriallytransmitted bit information over a data link.Transmission exchanges can be full duplex or half duplexand can occur over switched or nonswitched links.The configuration of the link connection can bepoint-to-point, multidrop, or loop.Common physical link-layer implementations are V.24 (EIA/TIA-232, formerly RS-232), V.35, andX.21.This section describes SDLC as it applies to STUN.The SDLC data link allows a reliable exchange of information over a communication facility betweenSNA devices.The protocol synchronizes receivers and transmitters and detects transmission errors.Itaccomplishes these functions by acknowledging frame receipt and by performing a cyclic redundancycheck (CRC) on the data.Supported Data-Link ConfigurationsThis section provides information related to router-specific hardware implementation.Table 5-1 providesa matrix of SDLC support for V.24.Table 5-1: SDLC Support for V.24 (EIA/TIA-232)Product Type NRZ/NRZI DTE/DCE Full Duplex Half MaximumDuplex MTUCisco 7000 Both Both Yes Yes 4 KBCisco 7010 Both Both Yes Yes 4 KBAGS+ Both Both Yes Yes 4 KBMGS Both Both Yes Yes 4 KBhttp://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2005.htm (3 of 30) [9/16/2000 5:05:39 PM] Designing SDLC, SDLLC, and QLLC InternetworksCisco 2500 Both Both Yes Yes 8 KBCisco 4000 Both Both Yes 4T card only 8 KBCisco 4500 Both Both Yes 4T card only 8 KBCisco 3104 Both Both Yes Dual serial card 8 KBonlyCisco 3204 Both Both Yes Dual serial card 8 KBonlyThe following notes apply to the entries in Table 5-1:For the Cisco 7000, Cisco 4000, Cisco 4500, and Cisco 3000 products, support of data terminalequipment (DTE) or data communications equipment (DCE) functionality depends on which cableis used.For the AGS+ and MGS, if you are using a nonreturn to zero inverted (NRZI) applique, thesystems support DCE natively.A special cable is required to support DTE mode operation.Priorto the availability of the NRZI applique, customers specifically ordered a DCE or DTE applique.Half-duplex support is available for the AGS+ and MGS with Software Release 9.1(7) or later.The NRZI applique, three-port SCI card, and Software Release 9.1(7) or later are all required forhalf-duplex support.Prior to software releases 8.3(6), 9.0(3), or 9.1(2), only 2-KB frame sizes were supported.Whenincreasing maximum transmission unit (MTU) size, consider interface card buffer memory sizeconstraints.SDLC Frame FormatThe SDLC frame format is illustrated in Figure 5-2.Figure 5-2: SDLC frame format.http://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2005.htm (4 of 30) [9/16/2000 5:05:39 PM] Designing SDLC, SDLLC, and QLLC InternetworksThe Flag field starts and ends the frame and initiates and terminates error checking.When the link isidle, the router sends streaming flags to maintain link synchronization, but this is not necessary to keepthe link up.The Addr field contains the SDLC address of the secondary station regardless of whether the frame iscoming from the primary or secondary station.The Addr field can contain a specific address, a groupaddress, or a broadcast address.Routers support specific addresses and support broadcast addressing on alimited basis.The Control field is a 1-byte field (for modulo 8 frames) or a 2-byte field (for modulo 128 frames).Theextra byte is required for modulo 128 frames to accommodate larger send and receive frame count fields.The value of the Control field identifies three different frame formats, as shown inTable 5-2.Table 5-2:Hex EquivalentBinary [P/F (P/FFormat Configuration off) on) Command Name AcronymUnnumbered 03 13 Unnumbered Info UI000 P1/F2 001107 17 Request RIM000 F 011107 17 Initialization SIM000 P 01110F 1F Mode DM000 F 111143 53 Set Initialization RD010 F 001143 53 Mode DISC010 P 011163 73 Disconnect Mode UA011 F 001183 93 Request SNRM100 P 0011CF DF Disconnect SNRME110 P 111187 97 Disconnect FRMR100 F 0111AF BF Unnumbered Ack XID101 P/F 1111E3 F3 Set Normal TEST111 P/F 0011Response.Set NormalResponse.Modehttp://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2005.htm (5 of 30) [9/16/2000 5:05:39 PM] Designing SDLC, SDLLC, and QLLC InternetworksEx.Frame REJECTExchange IDTestSupervisory x1 Receive Ready RRRRR3 P/F 0001 x14x5 Receive Not RNRRRR P/F 0101 x5x9 Ready REJRRR P/F 1101 x9RejectInformation xx xx Numbered Info TransferRRR P/F SSS05Present1P = Poll bit2F = Final bit3RRR = Nr (receive count)4x = Any single digit hexadecimal value5SSS = Ns (send count)Components of the Control FieldThe Info field is a variable-length field containing a path information unit (PIU) or exchangeidentification (XID) information.Table 5-3 lists supported PIUs
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