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This chapter addresses some of the special requirements for implementing routing technology within IBM System Network Architecture (SNA) environments. Internetworking within an SNA environment often involves making special accommodations for devices that were not originally designed for connection to meshed internetworks.
This chapter describes three techniques designed to enable internetworking capabilities for SNA-based network architectures:
The sections that describe serial tunneling (STUN), Synchronous Data Link Control (SDLC) over the Logical Link Control, type 2 (LLC) protocol (SDLLC), and Qualified Logical Link Control (QLLC) focus on the following topics:
SDLC via serial tunneling (STUN) encapsulates SDLC frames into Internet Protocol (IP) packets and routes the encapsulated packets over IP-supported network media. The SDLC frame is transmitted without modification, and the information within the frame is transparent to the network. All SNA physical unit (PU) types are supported. This section focuses on the SDLC data-link protocol and its various configurations and then explains how to implement STUN.
Figure 5-1 illustrates elements of STUN configuration in an environment that includes front-end processors (FEPs) and cluster controllers.
SDLC is the synchronous, bit-oriented protocol used by the SNA data-link control layer. As formally defined by IBM, SDLC is a line discipline for managing synchronous, code-transparent, serially transmitted bit information over a data link. Transmission exchanges can be full duplex or half duplex and can occur over switched or nonswitched links. The configuration of the link connection can be point-to-point, multidrop, or loop.
Common physical link-layer implementations are V.24 (EIA/TIA-232, formerly RS-232), V.35, and X.21. This section describes SDLC as it applies to STUN.
The SDLC data link allows a reliable exchange of information over a communication facility between SNA devices. The protocol synchronizes receivers and transmitters and detects transmission errors. It accomplishes these functions by acknowledging frame receipt and by performing a cyclic redundancy check (CRC) on the data.
This section provides information related to router-specific hardware implementation. Table 5-1 provides a matrix of SDLC support for V.24.
| Product Type | NRZ/NRZI | DTE/DCE | Full Duplex | Half Duplex | Maximum MTU |
|---|---|---|---|---|---|
| Cisco 7000 | Both | Both | Yes | Yes | 4 KB |
| Cisco 7010 | Both | Both | Yes | Yes | 4 KB |
| AGS+ | Both | Both | Yes | Yes | 4 KB |
| MGS | Both | Both | Yes | Yes | 4 KB |
| Cisco 2500 | Both | Both | Yes | Yes | 8 KB |
| Cisco 4000 | Both | Both | Yes | 4T card only | 8 KB |
| Cisco 4500 | Both | Both | Yes | 4T card only | 8 KB |
| Cisco 3104 | Both | Both | Yes | Dual serial card only | 8 KB |
| Cisco 3204 | Both | Both | Yes | Dual serial card only | 8 KB |
The following notes apply to the entries in Table 5-1:
The SDLC frame format is illustrated in Figure 5-2.
The Flag field starts and ends the frame and initiates and terminates error checking. When the link is idle, the router sends streaming flags to maintain link synchronization, but this is not necessary to keep the link up.
The Addr field contains the SDLC address of the secondary station regardless of whether the frame is coming from the primary or secondary station. The Addr field can contain a specific address, a group address, or a broadcast address. Routers support specific addresses and support broadcast addressing on a limited basis.
The Control field is a 1-byte field (for modulo 8 frames) or a 2-byte field (for modulo 128 frames). The extra 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 in
Table 5-2.
| Hex Equivalent | |||||
|---|---|---|---|---|---|
Format | Binary Configuration | [P/F off) | (P/F on) | Command Name | Acronym |
| Unnumbered | 000 P1/F2 0011 000 F 0111 000 P 0111 000 F 1111 010 F 0011 010 P 0111 011 F 0011 100 P 0011 110 P 1111 100 F 0111 101 P/F 1111 111 P/F 0011 | 03 07 07 0F 43 43 63 83 CF 87 AF E3 | 13 17 17 1F 53 53 73 93 DF 97 BF F3 | Unnumbered Info Request Initialization Mode Set Initialization Mode Disconnect Mode Request Disconnect Disconnect Unnumbered Ack Set Normal Response. Set Normal Response. Mode Ex. Frame REJECT Exchange ID Test | UI RIM SIM DM RD DISC UA SNRM SNRME FRMR XID TEST |
| Supervisory | RRR3 P/F 0001 RRR P/F 0101 RRR P/F 1101 | x14 x5 x9 | x1 x5 x9 | Receive Ready Receive Not Ready Reject | RR RNR REJ |
| Information | RRR P/F SSS05 | xx | xx | Numbered Info Present | Transfer |
The Info field is a variable-length field containing a path information unit (PIU) or exchange identification (XID) information. Table 5-3 lists supported PIUs.
The frame check sequence (FCS) field is a 2-byte field that, for transmitted data, contains the result of a CRC performed on the first bit after the Flag field through the last bit of the Info field. If the frame format is unnumbered or supervisory, the CRC is performed through the last bit of the Control field. As the remote device receives the data, it performs the same CRC computation and compares the result with the contents of the FCS field. If the comparison fails to find a match, the frame is discarded and recovery procedures take place.
The following sections provide design and implementation information for a variety of STUN-related configuration topics.
Local termination of SDLC sessions allows frames to be locally acknowledged by the receiving router. By locally terminating SDLC sessions, acknowledgment and keepalive traffic is prevented from traversing the backbone, and SNA sessions are preserved if the network fails.
Local acknowledgment locally terminates supervisory frames, which include receiver-ready, receiver-not-ready, and reject frames. Figure 5-3 illustrates the operation of SDLC local acknowledgment.
Virtual multidrop exploits SDLC address mapping to allow an FEP to communicate with multiple cluster controllers. With a virtual multidrop configuration, the address of each SDLC frame is checked individually. Only addresses that match the configuration are forwarded to the specified destination, which allows an FEP to communicate with multiple 3174s from a single serial link--a multidrop link. You can also use SDLC address mapping as a security feature to restrict access based on SDLC address, as shown in Figure 5-4.
The following steps are required to establish the network configuration illustrated in Figure 5-4:
Step 1 Include a LINE definition in the Network Control Program (NCP) running in the FEP, followed by PU definitions for SDLC address 7, 9, and A. The NCP interprets these definitions as a multidrop link.
Step 2 Use the stun route address tcp global configuration command to specify how to forward frames.
Step 3 Determine if priority queuing is required. If so, local acknowledgment is required.
The SDLC broadcast feature (introduced in Cisco IOS Software Release 10.2) allows SDLC broadcast address 0xFF to be replicated for each of the STUN peers, so that each end station receives the broadcast frame.
In Figure 5-5, the FEP views the end stations as if they were on an SDLC multidrop link. Router A duplicates any broadcast frames sent by the FEP and sends them to any downstream routers (in this example, routers B and C).
The sdlc virtual-multidrop interface configuration command enables SDLC broadcast and should only be used on the router that is configured as the secondary station on the SDLC link. In addition, the stun route address tcp command for SDLC address 0xFF must be configured on the secondary station (in this example, Router A) for each STUN peer. A sample configuration follows:
stun peernamexxx.xxx.xxx.xxxstun protocol-group 1 sdlc ! interface serial 1 encapsulation stun stun group 1 stun sdlc-role secondary sdlc virtual-multidrop sdlc address 01 sdlc address 02 sdlc address 03 stun route address 01 tcpyyy.yyy.yyy.yyylocal-ack stun route address 02 tcpzzz.zzz.zzz.zzzlocal-ack stun route address 03 tcpzzz.zzz.zzz.zzzlocal-ack stun route address FF tcpyyy.yyy.yyy.yyystun route address FF tcpzzz.zzz.zzz.zzz
For STUN prioritization of SDLC addresses over simple serial transport connections, use the priority-list or the queue-list global configuration command and the priority-group interface configuration command or the custom-queue-list interface configuration command, respectively, on the interface that connects to the remote router (the output interface).
For STUN prioritization of SDLC addresses over TCP/IP transport connections, you must configure the priority-list global configuration command and use the priority-group interface configuration command on the interfaces that connect to the end devices (the input interfaces). Also, you must specify the local-ack and priority keywords of the stun route address tcp global configuration command.
Two-way simultaneous mode (introduced in Cisco IOS Software Release 10.2) allows a router that is configured as a primary SDLC station to utilize a full-duplex serial line more efficiently. When two-way simultaneous mode is enabled in a multidrop environment, the router can poll a secondary station and receive data from that station while it sends data to or receives data from a different secondary station on the same serial line. (See Figure 5-6.)
The sdlc simultaneous command enables two-way simultaneous mode in a multidrop environment.
When two-way simultaneous mode is enabled for a point-to-point connection to a secondary station, the router can send data to the secondary station even if there is an outstanding poll, as long as the window size limit is not reached. The sdlc simultaneous command with the single keyword enables two-way simultaneous mode in a point-to-point link environment.
To prioritize logical units, use the locaddr-priority-list global configuration command on each router. For example:
locaddr-priority-list 1 02 high locaddr-priority-list 1 03 high locaddr-priority-list 1 04 low
You must also assign a priority list to the STUN priority ports using the priority-list global command. For example:
priority-list 1 protocol ip high tcp 1994 priority-list 1 protocol ip medium tcp 1990 priority-list 1 protocol ip low tcp 1991
The serial interfaces attached to the end systems (input interfaces) must be associated with priority lists using the locaddr-priority and priority-group interface configuration commands. The locaddr-priority command links the interface to a local LU priority list (specified with the
locaddr-priority-list global configuration command). The priority-group command links the interface to a TCP priority list (specified with a priority-list global configuration command). For example:
interface serial 1 locaddr-priority 1 priority-group 1
In addition, you must specify the local-ack and priority keyword options of the stun route address tcp global configuration command.
The LU address prioritization feature has been enhanced to allow you to specify the PU on which a LU resides. This enhancement is important because there might be multiple PUs on a multidropped SDLC line that have the same LU address. For example, in Figure 5-7, LU02 on 3174-2 is a 3287 printer, and LU02 on 3174-1 is a 3278 terminal. Do not assign the same priority to the printer and the terminal.
As of Software Release 9.1(9), LU address prioritization for both remote source-route bridging (RSRB) and STUN solved this problem. In addition to the LU address, you can specify the SDLC address to identify a PU in a multidropped SDLC line.
The syntax of the locaddr-priority global configuration command follows:
locaddr-priority list lu-address sdlc secondaryThe keyword sdlc indicates the next byte (in hexadecimal), and secondary is the secondary SDLC address.
SDLC-level flow control is also offered with local termination. When the router detects that the TCP queue is 90 percent full, it blocks further SDLC frames until the TCP queue recedes to 80 percent full. This is accomplished by transmitting receiver-not-ready frames.
There is also a flow control protocol between STUN peers. When SDLC output queues become congested, a router can request the remotely attached router to exert back-pressure on the SDLC link.
This section describes the transmission group and class of service (COS) support that NCP-to-NCP communications provide, including the following topics:
In a typical NCP-to-NCP communications arrangement, a host is channel-attached to an FEP acting as an NCP. In Figure 5-8, NCP A is the primary SDLC station and NCP B (remote NCP) is a secondary SDLC station. The NCPs dynamically determine their relationship; the NCP with the higher subarea number becomes the primary SDLC station. NCP V5R4 and later allows you to determine which NCP is the primary and which NCP is the secondary station.
A transmission group is defined as one or more parallel SDLC links connecting adjacent PU Type 4 (NCP) nodes. Transmission groups are used to increase the reliability of the logical link connection between NCPs and to provide additional bandwidth capacity. When one link fails or is congested, data is routed on one of the other links in the group. The transmission group function is implemented at the path control (PC) layer of the NCP architectural model. The PC layer encapsulates request/response units in PIUs and sends them to the data-link control (DLC) layer for transmission.
The PC layer uses the transmission header of the PIU to route messages through the network. SNA defines different transmission header formats and identifies the different formats by Format Identification (FID) type. A transmission header of type FID 4 is used to route data between type 4 nodes that support explicit and virtual routes.
The NCP assigns a sequence order number to each link in a transmission group. In later versions of NCP, you can specify the sequence order in which an NCP should use the transmission group links; otherwise, this order is determined by the order of link activation. Deactivation and reactivation of a link cause it to become the last activated link in the transmission group, and PIU traffic will be sent on the last activated link only if all other links fail or are busy.
Traditionally, the PC layer communicates directly with the DLC layer to transmit PIUs. When sending PIUs over a multilink transmission group, a transmission group layer exists between the PC and DLC layers. The transmission group layer contains a transmit queue. When the transmission group layer gets a frame to send, it checks for the availability of a link in the transmission group in priority (activation default) order. If the transmission group layer finds an available link (that is, a link that is not down and is not busy), it assigns the PIU the next NCP sequence number and sends the frame on that link. NCP sequence numbers range from 0 to 4095 and wrap on overflow.
When all links in the transmission group are busy, PIUs are placed in the transmission group transmit queue to await transmission. PIUs accumulate in the queue until a link becomes available. When an SDLC link becomes available, a COS algorithm is performed on the PIUs in the transmit queue. The PIU with the highest priority is dequeued, assigned the next NCP sequence number, and sent on the available link. Sequence numbering must occur when PIUs are removed from the transmit queue because PIUs can overtake each other on the transmit queue when COS priority processing is performed. PIUs are never preempted by other PIUs on the same SDLC link queue.
There are several reasons why PIUs might arrive at the receiving NCP out of transmission group sequence: links that operate at different speeds, PIUs on different links that have different lengths, and SDLC link transmission errors that cause retransmission. Because PIUs can arrive out of order, the receiving FEP performs resequencing by queuing incoming PIUs if their sequence number is larger than the next expected sequence number. The algorithm is not important, but the rule is that the receiving FEP propagates PIUs by sequence number order. PIUs with a lower sequence number than expected are considered duplicates and are discarded.
Later versions of NCP deviate from the SDLC standard in their use of SDLC echo addressing. The SDLC secondary NCP sets the high-order bit of the SDLC address when sending a response. For example, the primary NCP sends frames with address 01, and the secondary NCP sends frames with address 81. This addressing scheme limits the range of SDLC addresses from 01 to 7F. Additionally, SDLC address FF is used on frames preceding and during the NCP's XID exchange. The XID and DM frames use the broadcast address.
Another deviation from the SDLC standard occurs in NCP-to-NCP communication when a host NCP loads a remote NCP. Normally, numbered information frames can be sent only after an SNRM (or SNRME), which resets the station counts to ensure that the two stations start with consistent counts. When a host NCP loads a remote NCP, the host sends a Set Initialization Mode (SIM) and the remote NCP responds with a Request Initialization Mode (RIM), which allows numbered information frames to flow. NCPs are allowed to violate the SDLC standard because these violations (echo addressing, broadcast addressing, and sending numbered information frames before SNMRE) occur only when an NCP communicates with another NCP.
There are several reasons for routers to carry NCP-to-NCP traffic. Routers allow other protocols to share high-cost bandwidth (such as leased lines) that is set aside for SNA traffic. In the traditional NCP-to-NCP communications, a leased line is required for each line in a transmission group. However, routers enable a number of leased lines to be collapsed into one leased line. In addition, routing capabilities enable routers to dynamically determine wide-area network paths, which can result in a more reliable network.
Figure 5-9 illustrates NCP-to-NCP communications in a router-based internetwork.
Changing from STUN pass-through to STUN local acknowledgment has the following benefits:
Consider the situation illustrated in Figure 5-9. Notice that the router attached to the SDLC primary NCP (Router A) acts as an SDLC secondary station, and vice versa for the other router (Router B). For this discussion, assume that all serial links between NCPs and routers are in the same transmission group. This means that the NCP considers the lines to be in transmission group X, and the router considers the lines to be in transmission group Y. There is no relationship between X and Y. X is used in the NCP system generation, and Y is used in the router configuration.
The following features facilitate the support of transmission groups and COS in Cisco routers:
The following guidelines and notes should be considered when implementing transmission groups and COS:
If you are using Software Release 9.1 and an SNA WAN as a multiprotocol backbone, give SNA traffic the 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 line bandwidth so that your SNA traffic does not monopolize the WAN.
Table 5-4 lists equivalent commands for configuring priority queuing and custom queuing.
| Pause Statement Parameter | IBM Guideline |
|---|---|
| NCP primary PAUSE | Specifies the time the NCP will wait between sending polls if it has no data to send. (Default is 0.2 seconds; 0 is recommended) |
| NCP secondary PAUSE | Specifies the time that the secondary NCP will wait before returning 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 that the two SDLC sessions do not preserve the original characteristics of the original NCP-to-NCP SDLC session. To adapt a secondary NCP to the router environment, change its system generation PAUSE statement to a value between 0.0 and 0.1 seconds, inclusive.
When designing STUN-based internetworks featuring routers and IBM SNA entities, you must carefully consider the configuration of SNA nodes and routing nodes. Appendix D, "SNA Host Configuration for SDLC Networks," provides examples of SNA host configurations that focus on two specific SNA devices:
Before implementing a serial tunneling (STUN) internetwork, make sure you are familiar with the information in the Router Products Configuration Guide and the Router Products Command Reference publications that deals with Synchronous Data Link control (SDLC). Depending on your implementation, you may need to review the "SDLC via STUN" section earlier in this chapter.
Use the following steps as a checklist when implementing SDLC STUN in your internetwork:
Step 1 Evaluate your current environment by answering the following questions:
Step 2 Determine current host configurations. Important information includes the following:
Step 3 Determine what router-based IBM features will best suit your requirements:
Step 4 Determine what FEP-to-NCP conversion changes are required:
Step 5 Determine how end-station controllers are configured and, if possible, configure the router to accommodate them:
The SDLLC function allows serial-attached devices using the SDLC protocol to communicate with LAN-attached devices using the LLC2 protocol. The basic purpose of the SDLLC function is to consolidate the traditionally disparate SNA/SDLC networks onto a LAN-based, multiprotocol, multimedia backbone network.
Routers use the SDLLC feature to terminate SDLC sessions, to translate SDLC to the LLC2 protocol, and to forward the LLC2 traffic through remote source-route bridging (RSRB) over a point-to-point or IP network. Because a router-based IP network can use any arbitrary media such as FDDI, Frame Relay, X.25, or leased lines, routers support SDLLC over all such media through IP encapsulation. Figure 5-10 illustrates a general SDLLC media translation internetwork arrangement.
The following sections provide design and implementation information for the following SDLLC-related configuration topics:
Local acknowledgment of LLC2 sessions allows frames to be locally terminated by the Token Ring-attached router, which guarantees delivery to the ultimate destination through the reliable transport services of TCP/IP. Locally terminating LLC2 sessions enables packet reception to be locally acknowledged, prevents acknowledgment and keepalive traffic from traversing the backbone, and preserves SNA sessions if the network fails. The router that is performing the media translation always acknowledges the SDLC session in an SDLLC environment.
Local acknowledgment locally terminates supervisory frames, which include receiver-ready, receiver-not-ready, and reject frames.
Figure 5-11 illustrates the operation of local acknowledgment.
There are two ways to configure multidrop operation for the SDLC link in an SDLLC environment. The first way is to use a line-sharing device or a modem-sharing device (MSD) to connect multiple controllers at a single site to a single SDLC port on the router. The second way is to connect multiple controllers at different sites through a multidrop service provided by a telephone company. For more information about multidrop connections, refer to Appendix B, "IBM Serial Link Implementation Notes."
Consider line speed, link utilization, and the number of controllers that will share a single line when designing a multidrop environment. In addition, consider the number of attached LUs associated with individual PUs, and determine if these LUs are being heavily used. If so, increase the bandwidth of the attached serial line. When implementing multidrop environments featuring large numbers of PUs and LUs, contact your technical support representative for specific capabilities.
To configure a router for SDLLC, you need certain virtual telecommunications access method (VTAM) and NCP definition statements. Figure 5-12 illustrates the required configuration information.
Consider an example of two routers that implement the SDLLC functionality in an environment that interconnects a remote site to a host channel attached to a 3174 Token Ring gateway, as shown in Figure 5-13.
The following conditions apply to the sample network illustrated in Figure 5-13:
The 3174R must be defined in the configuration of the 3174L using the virtual Token Ring MAC address. This address is created in the router configuration; it includes the SDLC address as the last byte. This virtual MAC address is mapped to a host subchannel address. One host subchannel address is assigned for each downstream physical unit at host system generation time. PU and LU functions are defined to VTAM within the switched major node function.
The following configuration commands are required on Router 1:
sdllc xid c1 00000000
Cisco routers provide several types of encapsulation solutions. Because encapsulation always incurs a certain amount of overhead, you need to assess the advantages and performance trade-offs of each encapsulation solution within the constraints of your environment.
TCP/IP encapsulation is recommended most frequently because it is very robust, provides a high quality of service, and is media independent. If SDLLC local acknowledgment is required,
TCP/IP encapsulation is required. If SDLLC local acknowledgment is not required, Fast-Sequenced Transport (FST) encapsulation is highly recommended because it is less CPU intensive.
Direct High-Level Data Link Control (HDLC) encapsulation can only be used in point-to-point environments. FST and direct HDLC encapsulation are comparable in performance, but FST has more overhead, which may be an issue on low-speed serial links. TCP/IP encapsulation has the most overhead in terms of processor utilization and the WAN connection. If TCP/IP encapsulation with header compression is a requirement, use it only on link speeds of 64 kbps or less.
Table 5-6 outlines encapsulation overhead for SDLLC and RSRB implementations.
TCP/IP | FST | TCP/IP with Header Compression | HDLC |
|---|---|---|---|
| CPU intensive | Less CPU intensive | Very CPU intensive | Least CPU intensive |
| 4 bytes for HDLC
20 bytes for IP 20-24 bytes for TCP 16 bytes for virtual ring | 4 bytes for HDLC
20 bytes for IP 16 bytes for virtual ring | 4 bytes for HDLC
3-8 bytes for TCP/IP 16 bytes for virtual ring | 4 bytes for HDLC
16 bytes for virtual ring |
| Total: 60-64 bytes | Total: 40 bytes | Total: 23-28 bytes | Total: 20 bytes |
The following suggestions can help improve resource response time and network performance:
The following case study shows how an internetwork can evolve from a SNA-specific SDLC environment featuring 3x74 controllers and 3270 terminals to a network of PCs with client/server applications. The most important requirement for this evolution is the protection of existing SNA investment.
Assume that the original network consisted of hundreds of SDLC 3x74 controllers connected to a number of 37x5 FEPs in the data center. A disaster recovery center maintains the "mirror-image" of the data center. Many 3x74s are multidrop-connected to the host via 9.6- or 19.2-kbps leased lines. The challenges facing the corporate MIS organization for this internetwork include the following:
A phased strategy aimed at addressing these challenges would consist of three phases. Each of these phases is discussed in the following implementation examples.
Build a redundant backbone network with routers and high-speed E1 or T1 links in each regional office, as shown in Figure 5-14. Connect multiple SDLC devices to a router via SDLC transport with virtual multidrop. The resulting network increases reliability and minimizes leased-line charges.
Implement a fault-tolerant host FEP Token Ring, as shown in Figure 5-15. Connecting existing SDLC devices to the host Token Ring via SDLLC improves response time. Because SDLC devices appear as Token Ring-attached devices to the host, you do not need to regenerate NCP and reload when you are adding or changing PUs and LUs. This can be done dynamically through VTAM-switched major nodes. This implementation also reduces FEP CPU utilization.
Implement LAN (both Token Ring and Ethernet) internetworks in selected locations along with alternative WAN technologies such as Frame Relay, as shown in Figure 5-16. Connect LAN-based and remote SDLC devices to host FEP Token Ring via SDLLC, RSRB, and translational bridging, and to host FEP SDLC via reverse SDLLC (SDLC side primary). SNA session integrity is maintained through local termination of both LLC2 and SDLC traffic. These solutions provide needed support of LAN-based applications and improve availability and up time for SNA network devices.
Before implementing an SDLLC-based internetwork, make sure you are familiar with the information that deals with SDLC in the Router Products Configuration Guide and Router Products Command Reference publications. Depending on your implementation, you might need to review the "SDLLC Configuration" and "SDLLC Implementation Scenarios" sections earlier in this chapter.
In general, the following guidelines help you create a working, manageable network:
Strive to create a network that has predictable behavior early in the development cycle. This strategy can prevent problems as more devices are brought on line.
The following is a specific SDLLC implementation checklist that you can use to identify technologies, implementations, and possible solutions for your internetwork:
Step 1 Evaluate the customer requirements for SDLLC support:
Step 2 Determine current configuration. Important information includes the following:
Step 3 Determine the SDLLC features that best suit your requirements.
Confirm that devices to be attached are SDLC PU type 2 devices. Select specific feature requirements, such as local acknowledgment and virtual multidrop.
Step 4 Determine what host conversion changes are required:
QLLC is a data-link protocol defined by IBM that allows SNA data to be transported across X.25 networks. With QLLC, each SDLC physical link is replaced by a single virtual circuit. Figure 5-17 illustrates a typical QLLC topology. In this topology, both ends of the connection over the X.25 network must be configured for QLLC.
QLLC conversion is a feature of Cisco IOS Software Release 10.2 that causes the router to perform all of the translation required to send SNA data over an X.25 network so that IBM devices that are connected to a router do not have to be configured for QLLC.
QLLC conversion allows a device (typically a FEP or an AS/400) that is attached either directly to the router or through a Token Ring to communicate with a device (typically a 3174 terminal controller) that is attached to an X.25 network, as shown in Figure 5-18. In this example, only the terminal controller must be configured for QLLC and must have an X.25 interface.
In some topologies, one router interface uses SDLC to communicate with the terminal controller, and another router interface uses X.25 to communicate with the remote device over the X.25 network. In Figure 5-19, the router, configured for QLLC conversion, handles SNA traffic between the terminal controller and the FEP.
QLLC conversion also supports multiple SDLC connections coming through an MSD, as shown in Figure 5-20.
The router that is configured for QLLC conversion does not need to be on the same Token Ring as the FEP. In Figure 5-21, Router A is configured for QLLC and remote source-route bridging (RSRB), and Router B is configured for RSRB only. RSRB allows the FEP to connect to Router A. If a Token Ring connected to the X.25 network communicates with the Token Ring attached to the FEP by a protocol other than SRB, RSRB can provide connectivity.
Figure 5-21 shows an example using local acknowledgment, which causes the LLC2 session from the Token Ring-attached SNA device (the FEP) to be terminated at the adjacent router (Router B). A TCP session transports the data from Router B to the router attached to the X.25 network (Router A). Only Router A is configured for QLLC conversion. When enabled, local acknowledgment applies to all QLLC connections. The source-bridge qllc-local-ack global configuration command enables local acknowledgment and applies to all QLLC connections.
In pass-through mode, local acknowledgment is not used. Instead, the LLC2 session from the Token Ring-attached SNA device (the FEP) is terminated at the router connected to the X.25 network (Router A).
QLLC conversion also supports a configuration in which SNA end stations (3174 or equivalent) that are connected to a Token Ring reach the FEP through an X.25 connection, as shown in Figure 5-22. In this case, IBM Network Packet Switching Interface (NPSI) software is installed on the FEP.
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