Cisco Ccie Fundamental Network Design An

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.Rerouting in multilink transmission groupsWhen a router acknowledges an Information frame, it must ensure delivery of that frame to thereceiving NCP.If, after the frame is locally acknowledged, the corresponding link in the receivingtransmission group is lost, the receiving router reroutes the frame onto another SDLC link in thesame transmission group.CoSThe sending NCP performs CoS.Each PIU is assigned a sequence number.The best service therouters can perform is to try to preserve the CoS as assigned by the sending NCP via sequencenumbers.Therefore, all SNA data PIUs are treated equally with the goal to preserve PIU order.However, virtual route-pacing responses flow at SNA network priority level and do not havesequence numbers (that is, they have a sequence number of 0).The router prioritizes all SNAnetwork priority PIUs higher than SNA data to achieve more efficient virtual route pacing.Note The router cannot use the PIU to determine whether traffic is interactive or batch.Even if the routercould make this determination, prioritizing one type of traffic over another would cause the receivingNCP to waste CPU time resequencing the PIUs.This would also degrade throughput because thereceiving NCP would hold PIUs longer when resequencing.Flow control tuning for better CoS operationThe tcp-queue-max keyword of the stun route address tcp global configuration command allowsyou to tune the size of the outbound TCP queue so that when the WAN becomes congested,frames generated by an NCP can be stored in the router as well as in the NCP.When the size of theoutbound TCP queue is small, back-pressure via SDLC RNRs is applied to sending NCPs sooner,causing the NCP to hold more frames.The more frames that are held by the NCP, the more framesto which the NCP's CoS algorithm is applied.The size of the outbound TCP queue should beconfigured to 70 or above.Transmission Group and CoS Design Guidelines and NotesThe following guidelines should be considered when implementing transmission groups and CS:1.Bandwidth of the WAN should be greater than, or equal to, the aggregate bandwidth of all theserial lines.If other protocols are also using the WAN, bandwidth of the WAN should be greaterthan the aggregate bandwidth of all the serial lines.2.If the network delay associated with one line of an NCP transmission group is different from thenetwork delay associated with another line in the same NCP transmission group, the receivingNCP spends additional time resequencing PIUs.This happens when one or more of the NCPtransmission group lines is routed and one or more lines is directly connected between NCPs.3.The Software Release 9.1 prioritizing algorithm ensures that only the highest priority traffic isguaranteed to get through.Software Release 9.21 prioritization is enhanced by the addition ofcustom queuing.Custom queuing can be used to guarantee specific bandwidth allocated toprotocols with respect to bandwidth allocated to other protocols.http://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2005.htm (15 of 30) [9/16/2000 5:05:40 PM]Designing SDLC, SDLLC, and QLLC InternetworksIf you are using Software Release 9.1 and an SNA WAN as a multiprotocol backbone, give SNA trafficthe highest priority and assign the next highest priority to other mission- critical protocols.In addition,make sure that your WAN bandwidth is significantly greater than your aggregate SNA serial linebandwidth so that your SNA traffic does not monopolize the WAN.Table 5-4 lists equivalent commands for configuring priority queuing and custom queuing.Table 5-4: Comparison of Priority Queuing and Custom Queuing Configuration CommandsPriority Queuing Custom Queuingpriority-list 4 protocol ip high tcp 1994 queue-list 2 protocol ip 1 tcp 1994priority-list 4 protocol ip medium tcp 1992 queue-list 2 protocol ip 2 tcp 1992priority-list 4 protocol ip normal tcp 1991 queue-list 2 protocol ip 3 tcp 1991priority-list 4 protocol ip low tcp 1990 queue-list 2 protocol ip 4 tcp 19904.When NCPs are directly connected, their poll-and-pause timers should be configured formaximum throughput using the NCP PAUSE statement.Configuration of this parameter dependson whether the NCP is acting as a primary or secondary SDLC station.Table 5-5 outlines thedefaults and recommendations as specified in the IBM publication Tuning and Problem Analysisfor NCP SDLC Devices.Table 5-5: NCP PAUSE Parameter GuidelinesPause Statement Parameter IBM GuidelineNCP primary PAUSE Specifies the time the NCP will wait between sending polls if ithas no data to send.(Default is 0.2 seconds; 0 is recommended.)NCP secondary PAUSE Specifies the time that the secondary NCP will wait beforereturning a frame with the final bit set.(Default is 0.2 seconds;recommended to be high -0.2 to 1.0 seconds.)Adding routers with local acknowledgment creates two SDLC sessions instead of one.The result is thatthe two SDLC sessions do not preserve the original characteristics of the original NCP-to-NCP SDLChttp://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2005.htm (16 of 30) [9/16/2000 5:05:40 PM]Designing SDLC, SDLLC, and QLLC Internetworkssession.To adapt a secondary NCP to the router environment, change its system generation PAUSEstatement to a value between 0.0 and 0.1 seconds, inclusive.SNA Host Configuration Considerations for STUNWhen designing STUN-based internetworks featuring routers and IBM SNA entities, you must carefullyconsider the configuration of SNA nodes and routing nodes [ Pobierz całość w formacie PDF ]

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