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In a traditional SNA environment, serial lines connect FEPs in a master-slave topology, as shown in Figure 20-1. The primary FEP is connected to the IBM host, which is typically an IBM 3090 mainframe. Synchronous modems connect the FEPs.
The software running on the FEP is called the Network Control Program (NCP). This section describes NCP configuration parameters and optional NCP features that network administrators must consider when they introduce routers into an FEP environment.
Typically, a serial port on a line interface card in the FEP connects the FEP to a synchronous modem. Depending on the type of line interface card, the serial port may be EIA/TIA-232 or V.35. The modem acts as data communications equipment (DCE) and provides clocking and synchronization. The FEP acts as data terminal equipment (DTE). The NCP statement that configures the FEP for DTE is CLOCKNG=EXT.
The FEPs dynamically determine their primary and secondary roles. Typically, the FEP with the higher subarea address becomes the primary FEP. In some versions of NCP, the role parameter is configurable. Typically, the local FEP (the closest to the mainframe) is the primary FEP, whereas the remote FEP is the secondary FEP.
The NRZI parameter specifies whether the FEP should operate in nonreturn-to-zero inverted (NRZI) mode or in nonreturn-to-zero (NRZ) mode. Both techniques encode binary data on a synchronous serial line. The specification depends on the way the modem operates. Old modems and satellite links that are not sensitive to a pattern of repeated binary ones and zeros (that is, 101010...) operate in NRZI mode. Modems that are sensitive to repeated patterns operate in NRZ mode.
The NCP statement that configures the FEP for NRZI is NRZI=YES, which is the default and is correct for most IBM modems. The NCP statement that configures the FEP for NRZ is NRZI=NO, which is correct for most non-IBM modems.
The MODULO parameter specifies the number of information frames (I-frames) that NCP can send to the remote device before receiving an acknowledgment. The statement MODULO=8 allows NCP to send seven unacknowledged I-frames, whereas the statement MODULO=128 and the statement MAXOUT=127 allows NCP to send 127 unacknowledged I-frames. (Note that when the MODULO parameter is set to 128, the NCP MAXOUT parameter specifies the number of I-frames that can be sent before receiving an acknowledgment. MAXOUT can range from 8 [the default] to 127.)
Typically, network administrators configure NCP to allow a high number of outstanding I-frames (that is, MODULO=128 and MAXOUT=127) for slow links or for satellite links. Allowing a high number of outstanding I-frames uses the link more efficiently by reducing the number of acknowledgments and by preventing session timeouts. When the MODULO parameter is 128, make sure the TCP output queue on the router is greater than 128.
The SDLC STUN implementation supports setting the MODULO parameter to 8 as well as 128. Note, however, that setting the MODULO parameter to any legal value other than 8 causes the router to use additional buffers to store unacknowledged I-frames.
When local acknowledgment is configured to reduce supervisory frame traffic and to prevent session timeouts, 8 is the only supported value of the MODULO parameter. When the MODULO value is 8, the router does not use additional buffers unnecessarily.
The ADDRESS parameter has the following format: ADDRESS=(line-number, mode).
If mode is FULL, the FEP can send and receive data at the same time. When mode is HALF, the FEP is limited to sending data and then receiving data. The default value of mode is FULL. The value of mode affects the operation of the DUPLEX parameter. For more information, see the "DUPLEX Parameter" section later. The value of line-number specifies the channel adapter position or the relative line number of all the telecommunication links defined for this NCP.
When implementing SDLC STUN or SDLC-Transmission Group STUN, the network administrator must specify SDLC addresses in the configuration of the router. The addresses specified in the router configuration are based on the order in which the ADDRESS parameter appears in the NCP configuration. Consider the following NCP configuration:
*********************************************************************
* LOCAL NCP LINKS -- PRIMARY FEP *
*********************************************************************
LINK04 GROUP LNCTL=SDLC, GROUP LEVEL X
NPACOLL=YES, <== 3745 Dallas X
DUPLEX=FULL, X
NEWSYNC=NO, X
NRZI=NO, X
SDLCST=(CPRI4,CSEC4), X
RETRIES=(10,5,10), PU LEVEL X
IRETRY=YES, X
MAXOUT=7, X
PASSLIM=254, X
SERVLIM=254, X
ISTATUS=ACTIVE, VTAM-ONLY LEVEL X
OWNER=CMC
*
*--------------------------------------------------------------------
*
X1010442 LINE ADDRESS=(005,FULL) <== 3745 Chicago (01)
S1010442 PU PUTYPE=4
*
X1030442 LINE ADDRESS=(132,FULL) <== 3745 Raleigh (02)
S1030442 PU PUTYPE=4
*
X1010446 LINE ADDRESS=(068,FULL),MODULO=128,ISTATUS=ACTIVE, <== 3745 Houston (03) X
SPEED=56000,SDLCST=(S04PRI,S04SEC)
S1010446 PU PUTYPE=4,MAXOUT=63
*
X1020412 LINE ADDRESS=(100,FULL),MODULO=128,ISTATUS=ACTIVE, <== 3745 Lafayette (04) X
SPEED=56000,SDLCST=(S04PRI,S04SEC)
S1020412 PU PUTYPE=4,MAXOUT=63
*
X1010412 LINE ADDRESS=(112,FULL),SPEED=56000,ISTATUS=ACTIVE <== 3745 Atlanta (05) X
S1010412 PU PUTYPE=4
*
X1010462 LINE ADDRESS=(080,FULL), <== 3745 San Francisco (06) X
NRZI=NO, X
NEWSYNC=NO, X
DUPLEX=FULL, X
ISTATUS=ACTIVE, X
SERVLIM=254, X
SDLCST=(S04PRI,S04SEC), X
MODULO=128, X
SPEED=56000
S1010462 PU PUTYPE=4, X
MAXOUT=63
*
**********************************************************************
Given this configuration, the router configuration uses address 01 for Chicago, address 02 for Raleigh, address 03 for Houston, address 04 for Lafayette, address 05 for Atlanta, and address 06 for San Francisco.
The DUPLEX parameter specifies whether the communication line and the modem operate in full- or half-duplex mode, and controls the Request To Send (RTS) signal. If the ADDRESS parameter specifies that the mode is FULL, the value of the DUPLEX parameter has no effect, and RTS is always high (that is, permanent RTS). If the ADDRESS parameter specifies that the mode is HALF, the following applies:
This section describes the following enhanced NCP features that are supported by Cisco routers: transmission groups, echo addressing, and remote NCP loading. Note, however, that the basic STUN and SDLC STUN implementations do not support transmission groups.
A transmission group is one or more parallel SDLC links that connect FEPs. Transmission groups increase the reliability of the logical link connection between FEPs and provide additional bandwidth. When one link fails or congests, NCP uses one of the other links in the group to send data (see Figure 20-2).
NCP uses virtual routes to provide more than one route between two FEPs. This multiple active routing mechanism increases the probability that an SDLC route is available when a session needs to be established.
When a local FEP is loading a remote FEP with a new NCP configuration, the local FEP uses a nonstandard form of SDLC to complete the remote NCP load. This violation of the SDLC standard is supported because it occurs only between FEPs.
Figure 20-3 illustrates the topology of an FEP-based network that includes routers. In this multiprotocol topology, the routers already handle traffic between Token Rings and the IBM host. When used to handle traffic between the FEPs, the routers replace the modems and lines that formerly connected the FEPs.
An EIA/TIA-232 (formerly RS-232) cable or a V.35 cable connects each router to its FEP, and a serial T1 line connects each router to the wide-area network (WAN). The FEPs continue to act as DTE devices, and, by providing clocking and synchronization, the serial interfaces on the routers act as DCE devices.
When configured for STUN, Cisco routers can take advantage of the following advanced router features: priority queuing, custom queuing, and local acknowledgment.
Priority queuing allows the network administrator to set priorities on the traffic that passes through the network. Packets are classified according to various criteria, including protocol and subprotocol type, and then queued on one of four output queues: high, medium, normal, or low.
A FEP-to-FEP STUN implementation can use priority queuing to prioritize SNA traffic over other protocols that share the same link. The following commands distribute transmission control protocol (TCP) traffic among the four queues and assign STUN traffic encapsulated in TCP to the high queue:
priority-list 1 ip high tcp 1994 priority-list 1 ip medium tcp 1990 priority-list 1 ip normal tcp 1991 priority-list 1 ip low tcp 1992 priority-list 1 stun high ! interface serial 0 encapsulation stun stun group 1 sdlc address 01 stun route address 01 tcp 1.1.1.2 local-ack priority-group 1
Custom queuing, available in Software Release 9.21 and subsequent software releases, is a queuing strategy that imparts a measure of fairness not provided by priority queuing. The network administrator can control on each interface the minimum percentage of bandwidth allocated to a particular kind of traffic.
When custom queuing is enabled on an interface, the router maintains for that interface eleven output queues (numbered 0 to 10). The router reserves queue number 0 for its own use. The router cycles sequentially through queue numbers 1 to 10, delivering packets in the current queue before moving to the next queue.
Each output queue has an associated configurable byte count that specifies how many bytes of data the router should deliver from the current queue before it moves to the next queue. When the router processes a particular queue, it sends packets until the number of bytes sent exceeds the queue byte count or until the queue is empty.
Custom queuing can be used instead of, but not in addition to, the priority-group interface configuration command in a single interface. The following configuration commands place STUN traffic on queue 1 with a byte-count limit of 4000 bytes and a maximum of 40 queues:
stun peer-name 1.1.1.1 stun protocol-group 1 sdlc-tg ! interface serial 0 encapsulation stun stun route address 01 tcp 1.1.1.2 local-ack ! interface serial 1 encapsulation hdlc custom-queue-list 1 ! queue-list 1 protocol stun 1 queue-list 1 protocol novell 2 queue-list 1 default 3 queue-list 1 queue 1 byte-count 4000 queue-list 1 queue 1 limit 40
Local acknowledgment is a router feature that prohibits supervisory frames from traversing the WAN, as shown in Figure 20-4.
Cisco recommends the use of local acknowledgment when one or both of the following conditions exist:
The basic STUN implementation has the following limitations:
In Figure 20-5, the routers pass data over an IP WAN. The FEPs are configured for DTE, full-duplex mode, and NRZ encoding. The serial interfaces on the routers are configured for DCE.
The following commands configure basic STUN (example 1) for Router A:
stun peer-name 1.1.1.1 stun protocol-group 1 basic ! interface serial 0 no ip address encapsulation stun stun group 1 stun route all tcp 1.1.2.1 clockrate 19200 ! interface tokenring 0 ip address 1.1.4.1 255.255.255.0 ! interface serial 1 ip address 1.1.3.1 255.255.255.0 ! interface loopback 0 ip address 1.1.1.1 255.255.255.0 ! router igrp 1 network 1.0.0.0
The following commands configure basic STUN (example 1) for Router B:
stun peer-name 1.1.2.1 stun protocol-group 1 basic ! interface serial 0 no ip address encapsulation stun stun group 1 stun route all tcp 1.1.1.1 clockrate 19200 interface tokenring 0 ip address 1.1.5.1 255.255.255.0 ! interface serial 1 ip address 1.1.3.2 255.255.255.0 ! interface loopback 0 ip address 1.1.2.1 255.255.255.0 ! router igrp 1 network 1.0.0.0
In Figure 20-6, the routers transmit data over a Frame Relay WAN. The FEPs are configured for DTE, full-duplex mode, and NRZI encoding. The serial interfaces on the routers are configured for DCE.
The following commands configure basic STUN (example 2) for Router A:
stun peer-name 1.1.1.1 stun protocol-group 1 basic ! interface serial 0 no ip address encapsulation stun stun group 1 stun route all tcp 1.1.2.1 nrzi-encoding clockrate 56000 ! interface tokenring 0 ip address 1.1.4.1 255.255.255.0 ! interface serial 1 ip address 1.1.3.1 255.255.255.0 encapsulation frame-relay frame-relay map ip 1.1.3.2 40 broadcast ! interface loopback 0 ip address 1.1.1.1 255.255.255.0 ! router igrp 1 network 1.0.0.0
The following commands configure basic STUN (example 2) for Router B:
stun peer-name 1.1.2.1 stun protocol-group 1 basic ! interface serial 0 no ip address encapsulation stun stun group 1 stun route all tcp 1.1.1.1 nrzi-encoding clockrate 56000 ! interface tokenring 0 ip address 1.1.5.1 255.255.255.0 interface serial 1 ip address 1.1.3.2 255.255.255.0 encapsulation frame-relay frame-relay map ip 1.1.3.1 40 broadcast ! interface loopback 0 ip address 1.1.2.1 255.255.255.0 ! router igrp 1 network 1.0.0.0
The SDLC STUN implementation requires coordination of SDLC addresses between the router and the NCP configuration. To configure the router for SDLC STUN, the network administrator must know the relative position of the ADDRESS parameters in the NCP configuration. For details, see the earlier "ADDRESS Parameter" section. Network administrators use SDLC STUN for two purposes:
SDLC STUN is limited by its lack of support for transmission groups.
In Figure 20-7, the routers transmit data over a serial line. The FEPs are configured for DTE, full-duplex mode, and NRZ encoding. The router serial interfaces are configured as DCE devices.
The following commands configure SDLC STUN for Router A:
stun peer-name 1.1.1.1 stun protocol-group 1 sdlc ! interface serial 0 no ip address encapsulation stun sdlc address 04 stun route address 04 interface s1 stun route address ff interface s1 clockrate 19200 ! interface tokenring 0 ip address 1.1.4.1 255.255.255.0 ! interface serial 1 ip address 1.1.3.1 255.255.255.0 ! interface loopback 0 ip address 1.1.1.1 255.255.255.0 ! router igrp 1 network 1.0.0.0
The following commands configure SDLC STUN for Router B:
stun peer-name 1.1.2.1 stun protocol-group 1 sdlc ! interface serial 0 no ip address encapsulation stun sdlc address 04 stun route address 04 interface s1 stun route address ff interface s1 clockrate 19200 ! interface tokenring 0 ip address 1.1.5.1 255.255.255.0 ! interface serial 1 ip address 1.1.3.2 255.255.255.0 ! interface loopback 0 ip address 1.1.2.1 255.255.255.0 ! router igrp 1 network 1.0.0.0
SDLC-Transmission Group (TG) STUN is a complex implementation that supports enhanced NCP features.When configuring STUN-TG, many network administrators also configure the routers to take advantage of the advanced features described in the "Advanced Router Features" section earlier in this chapter. Because these features increase memory and processor use, they should be used only when necessary to support the existing network or to relieve congestion. SDLC-TG STUN forces local acknowledgment. If you do not want to configure local acknowledgment, use the basic STUN or the SDLC STUN implementation.
The SDLC-TG implementation requires coordination of SDLC addresses between the router and the NCP configuration. To configure the router for SDLC-TG, the network administrator must know the relative position of the ADDRESS parameters in the NCP configuration. For details, see the "ADDRESS Parameter" section earlier in this chapter.
Figure 20-8 illustrates a network that implements SDLC-TG STUN. The routers transmit data over an IP WAN. The FEPs are configured for DTE, full-duplex mode, and NRZ encoding. The serial interfaces on the routers are configured as DCE devices.
To the primary FEP, Router A looks like a secondary FEP. To the secondary FEP, Router B looks like a primary FEP. The following commands configure SDLC-TG STUN for Router A:
stun peer-name 1.1.1.1 stun remote-peer-keepalive stun protocol-group 1 sdlc-tg ! interface tokenring 0 ip address 1.1.4.1 255.255.255.0 ! interface serial 1 mtu 4400 hold-queue 150 in no ip address encapsulation stun stun group 1 stun sdlc-role secondary sdlc n1 35200 sdlc address 01 echo stun route address 1 tcp 1.1.2.1 local-ack tcp-queue-max 120 clockrate 56000 interface serial 2 mtu 4400 hold-queue 150 in no ip address encapsulation stun stun group 1 stun sdlc-role secondary sdlc n1 35200 sdlc address 02 echo stun route address 2 tcp 1.1.2.1 local-ack tcp-queue-max 120 clockrate 56000 ! interface serial 3 ip address 1.1.3.1 interface loopback 0 ip address 1.1.1.1 255.255.255.0 ! router igrp 1 network 1.0.0.0
The following commands configure SDLC-TG STUN for Router B:
stun peer-name 1.1.2.1 stun remote-peer-keepalive stun protocol-group 1 sdlc-tg ! interface tokenring 0 ip address 1.1.5.1 255.255.255.0 ! interface serial 1 mtu 4400 hold-queue 150 in no ip address encapsulation stun stun group 1 stun sdlc-role primary sdlc line-speed 56000 sdlc n1 35200 sdlc address 01 echo stun route address 1 tcp 1.1.1.1 local-ack tcp-queue-max 120 clockrate 56000 ! interface serial 2 mtu 4400 hold-queue 150 in no ip address encapsulation stun stun group 1 stun sdlc-role primary sdlc line-speed 56000 sdlc n1 35200 sdlc address 02 echo stun route address 2 tcp 1.1.1.1 local-ack tcp-queue-max 120 clockrate 56000 ! interface serial 3 ip address 1.1.3.2 ! interface loopback 0 ip address 1.1.2.1 255.255.255.0 ! router igrp 1 network 1.0.0.0
The stun peer-name global configuration command identifies this router as a peer to its peer group.
The stun remote-peer-keepalive global configuration command causes Router A and Router B to exchange keepalive messages on each idle line. (An idle line is a line over which no I-frames are flowing.) Keepalive messages allow a router to detect when its peer router is not longer available. A peer router might become unavailable if it goes down or if the line goes down. The routers do not send keepalive traffic to the FEPs.
Routers send keepalive messages over an idle line at a default interval of 30 seconds and waits three times that interval for a response. If the router does not receive a response, it closes the STUN session.
The stun protocol-group global configuration command establishes a protocol group that is part of an SNA transmission group. The sdlc-tg keyword can be used only when the stun route address tcp interface configuration command is used to configure local acknowledgment and TCP encapsulation. The SDLC broadcast address 0xFF is routed automatically for interfaces on which the sdlc-tg keyword is configured. The stun protocol-group global configuration command also alerts the router that it should support transmission group features, such as the following:
The mtu interface configuration command specifies a maximum transmission unit (MTU) of 4400 bytes, which is the highest recommended value, for the interface. The value of the NCP MAXDATA parameter should be no more than the MTU on the router interface. The recommended value of MAXDATA is 4096 bytes.
The hold-queue interface configuration command increases the size of the input hold queue from 75 packets (the default) to 150 packets. The specified value should be greater than the depth of the TCP output queue (as specified by the tcp-queue-max keyword of the stun route address tcp interface configuration command). Increasing the size of the input hold queue allows flow control to activate when the TCP output queue reaches a threshold of 90 percent, which occurs before the input interface throttling mechanism can activate.
The stun sdlc-role primary interface configuration command is used when the router is connected to a secondary FEP. The stun sdlc-role secondary interface configuration command is used when the router is connected to a primary FEP.
The sdlc address interface configuration command specifies an SDLC address. The specified address must be the same as the relative line number at which the ADDRESS parameter is specified in the NCP configuration of the FEP to which the router is connected. (For more information, see the "ADDRESS Parameter" section earlier in this chapter.) The echo keyword causes the router to treat nonecho (for example, 0x01) and echo (for example, 0x81) SDLC addresses as the same address. The sdlc address interface configuration command is valid only for interfaces on which the stun protocol-group command with the sdlc-tg keyword is configured. Only one sdlc address interface configuration command with echo keyword is required per interface.
The stun route address tcp interface configuration command specifies TCP encapsulation. The value of address specifies the SDLC address, which must be specified with the echo bit turned off. The local-ack keyword causes the router to perform local acknowledgment and is required when the sdlc-tg keyword appears with the stun protocol-group command. The tcp-queue-max keyword sets the maximum size of the TCP output queue for a serial line. The default is 100 packets. The recommended minimum is 70, and the recommended maximum is 500. The clockrate interface configuration command specifies the clocking speed when the serial interface is in DCE mode.
This case study presents three types of STUN implementations in SNA environments: basic STUN, SDLC STUN, and SDLC-TG STUN. Although basic STUN is the easiest to configure because it does not require the configuration of line addresses on the router, it does not support local acknowledgment. Compared to basic STUN, the SDLC STUN implementation is the most flexible because it supports, but does not require, local acknowledgment. However, the use of SDLC STUN is limited because it does not support transmission groups. SDLC-TG STUN is not as flexible as SDLC STUN because it enforces local acknowledgment. At the same time, SDLC-TG STUN is the only STUN implementation that supports transmission groups.
Posted: Wed Apr 10 10:48:07 PDT 2002
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