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The Logical Link Control, type 2 (LLC2) and Synchronous Data Link Control (SDLC) protocols provide data link-level support for higher-level network protocols and features such as SDLLC and remote source-route bridging (RSRB) with local acknowledgment. The features that are affected by LLC2 parameter settings are listed in the next section, "LLC2." The features that require SDLC configuration and use SDLC parameters are listed in the section "SDLC" later in this chapter.
LLC2 and SDLC package data in frames. LLC2 and SDLC stations require acknowledgments from receiving stations after a set amount of frames have been sent before sending further data. The tasks described in this chapter modify default settings regarding the control field of the data frames. By modifying the control field parameters, you can determine the amount of acknowledgments sent for frames received and the level of polling used to determine available stations. In this manner, you can set the amount of resources used for frame checking and optimize the network load.
SDLC is used as the primary Systems Network Architecture (SNA) link-layer protocol for wide-area network (WAN) links. SDLC defines two types of network nodes: primary and secondary. Primary nodes poll secondary nodes in a predetermined order. Secondaries then transmit if they have outgoing data. When configured as primary and secondary nodes, our routers are established as SDLC stations.
You do not need to configure LLC2 because it is already enabled on Token Ring interfaces. You can change the default settings of LLC2 parameters as needed. To support SDLC, you need to configure the router as a primary or secondary SLDC station. You also can change default settings on any of the SDLC parameters listed. Configuration examples for both LLC2 and SDLC are given at the end of the chapter.
For a complete description of the commands mentioned in this chapter, refer to the "LLC2 and SDLC Commands" chapter in the Router Products Command Reference publication. For historical background and a technical overview of LLC2 and SDLC, see the Internetworking Technology Overview publication.
The following features use LLC services:
Since LLC2 is already enabled on Token Rings, you do not need to enable it on the router. However, you can complete the tasks in the following sections to enhance LLC2 performance:
To determine which LLC2 parameters need adjustment, you can perform the following task:
See the end of this chapter for an LLC2 configuration example
You can control the number of information-frames (I-frames) and acknowledgments sent on the LLC2 network by completing the following tasks described in this section.
You can reduce overhead on the network by increasing the maximum number of frames the router can receive at once before it must send the sender an acknowledgment. To do so, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set maximum number of I-frames the router can receive before it sends an acknowledgment. | llc2 ack-max packet-count |
You can ensure timely receipt of acknowledgments so that transmission of data is not delayed. Even if the maximum amount of frames has not been reached, you can set a timer forcing the router to send an acknowledgment and reset the maximum amount counter to 0.
To set the maximum delay time, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set the I-frame acknowledgment time. | llc2 ack-delay-time milliseconds |
You can set the maximum number of I-frames that the router sends to an LLC2 station before the router requires an acknowledgment from the receiving end. A higher value reduces overhead on the network. Ensure that the receiving LLC2 station can handle the number of frames set by this value.
To set this value, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set the maximum number of I-frames the router sends before it requires an acknowledgment. | llc2 local-window packet-count |
You can set the number of times the router will resend a frame when the receiving station does not acknowledge the frame. Once this value is reached, the session is dropped. This value also is used to determine how often the router will retry polling a busy station. This task should be done in conjunction with the task for setting the time for resending I-frames (described next). Performing them together ensures that frame transmission is monitored at a reasonable level, while limiting the number of unsuccessful repeated tries.
To set the number of retries, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Establish number of times the router will resend unacknowledged frames or try polling a busy station. | llc2 n2 retry-count |
You can set the amount of time the router will wait before resending unacknowledged I-frames. This interval is called the T1 time. Perform this task in conjunction with the tasks of setting the number of retries and setting the transit poll-frame timer. Performing these tasks in conjunction with each other provides a balance of network monitoring and performance.
To set the T1 time, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Control how long the router waits for an acknowledgment of transmitted I-frames. | llc2 t1-time milliseconds |
You can set the amount of time that the router will wait for an expected frame before sending a reject command (REJ). Typically, when an LLC2 station sends an I-frame, a sequence number is included in the frame. The LLC2 station that receives these frames will expect to receive them in order. If it does not, it can reject a frame and indicate which frame it is expecting to receive instead. If the correct frame is not sent to the router before the reject timer expires, the router sends a REJ to the remote station and disconnects the LLC2 session.
To set the reject timer, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set the time a router waits for a resend of a rejected frame before sending a reject command to the remote station. | llc2 trej-time milliseconds |
You can control the amount of polling that occurs on the LLC2 network by completing the following tasks described in this section:
You can set the optimum interval of time after which the router sends Receiver Readys, or frames that tell other LLC2 stations that the router is available. These polls occur during periods of idle time on the network.
To set polling frequency, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Control the polling frequency during idle traffic. | llc2 idle-time milliseconds |
The amount of time the router waits until repolling a busy station can also be set. Perform this task in conjunction with setting the number of retries. Typically, you do not need to perform this task unless an LLC2 station has unusually long busy periods before clearing the busy state. In this case, you should increase the value so that the station does not time out.
To set the polling interval, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set the amount of time the router will wait before repolling a busy station. | llc2 tbusy-time milliseconds |
When sending a command that must receive a response, a poll bit is sent in the frame. Once the router sends the poll bit, it cannot send any other frame with the poll bit set until the receiver replies to that poll frame with a frame containing a final bit set. When the timer expires, the router assumes that it can send another frame with a poll bit.
Set the transmit-poll-frame timer to reduce problems with receiving stations that are faulty and cannot send the frame with the final bit set by performing the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set the amount of time the router waits for a final response to a poll frame before the resending it. | llc2 tpf-time milliseconds |
This value should be larger than the T1 time. The T1 time determines how long the router waits for receipt of an acknowledgment before sending the next set of frames. See the section "Set the Time for Resending I-Frames" earlier in this chapter for more information.
You can control the number of frames used for identification on the LLC2 network by completing the following tasks described in this section:
Exchange of identification (XID) frames identify LLC2 stations at a higher level than the MAC address and contain information about the configuration of the stations.You can set how often the router sends an XID frame by performing the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set the frequency of XID transmissions. | llc2 xid-neg-val-time milliseconds |
 | Caution Do not change the value unless requested by your technical support representative. |
You can set the amount of time the router waits for a reply to the XID frames it sends to remote stations. The value should be larger than the T1 time, which indicates how long the router waits for an acknowledgment before dropping the session.
To set the time for XID retries, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Set how long the router waits for a reply to the XID frames it sends to remote stations. | llc2 xid-retry-time milliseconds |
You can display the configuration of LLC2 stations to determine which LLC2 parameters need adjustment. Perform the following task in EXEC mode:
| Task | Command |
|---|---|
| Display the configuration of LLC2 stations. | show llc2 |
The SDLC tasks described in this section configure the router as an SDLC station. This is in contrast to a router configured for SDLC Transport where the router is not an SDLC station, but passes SDLC frames between two SDLC stations across a mixed-media, multiprotocol environment.
The tasks described in this section support the following features:
You must complete the tasks in the first two sections, following, to enable the router as an SDLC station. The remainder of the sections describe tasks to enhance SDLC performance. Defaults should normally suffice.
To determine which SDLC parameters need adjustment, you can perform the following task:
See the end of this chapter for an SDLC configuration example.
SDLC defines two types of network nodes: primary and secondary. Primary nodes poll secondary nodes in a predetermined order. Secondaries then transmit if they have outgoing data. When configured as primary and secondary nodes, our routers are established as SDLC stations.
To establish routers as SDLC stations, complete tasks in the order listed in the following table. One serial interface can have two or more secondary stations attached to it through a modem sharing device (MSD). Each secondary station address must be assigned to the primary station. You must perform the following tasks in interface configuration mode for the serial interface:
Use the show interfaces command to list the configuration of the SDLC serial lines. Use the no sdlc address command to remove a secondary address assignment. Addresses are hexadecimal (base 16).
SDLC two-way simultaneous mode allows a primary SDLC link station to achieve better utilization of a full-duplex serial line. With two-way simultaneous mode, the primary link station can send data to one secondary link station while there is a poll outstanding. Two-way simultaneous mode works on the SDLC primary side only. On a secondary link station, it responds to a poll from the primary station.
SDLC two-way simultaneous mode operates in either a multidrop link environment or point-to-point link environment.
In a multidrop link environment, a two-way simultaneous primary station is able to poll a secondary station and receive data from the station, and send data (I-frames) to other secondary stations.
In a point-to-point link environment, a two-way simultaneous primary station can send data (I-frames) to the secondary station although there is a poll outstanding, as long as the window limit is not reached.
To enable two-way simultaneous mode, perform either of the following tasks in interface configuration mode:
You can specify that a secondary station does not send Frame Rejects (FRMRs), or reject commands indicating frame errors. If you do so, the router will drop an SDLC connection if it receives an error from the secondary station. To determine handling of FRMRs, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Specify that this secondary station does not support Frame Rejects (FRMRs). | sdlc frmr-disable |
To specify that the secondary station does support FRMRs, use the no sdlc frmr-disable command.
When an SDLC station sends a frame, it waits for an acknowledgment from the receiver saying that this frame has been received. You can modify the time the router allows for an acknowledgment before resending the frame. You can also determine the number of times that a router resends a frame before terminating the SDLC session. By controlling these values, you can reduce network overhead while continuing to check transmission of frames.
To set the SDLC timer and retry counts, perform one or both of the following tasks in interface configuration mode:
| Task | Command |
|---|---|
| Control the amount of time the router waits for a reply. | sdlc t1 milliseconds |
| Set the number of times a router will retry an operation that has timed out. | sdlc n2 retry-count |
You can set the maximum size of an incoming frame and set the maximum number of I-frames (or window size) a router will receive before sending an acknowledgment to the sender. By using higher values, you can reduce network overhead.
To set SDLC frame and window sizes, perform any of the following tasks in interface configuration mode:
You can control the router's buffer size. The buffer holds data that is pending transmission to a remote SDLC station. This task is particularly useful in the case of the SDLLC media translator, which allows an LLC2-speaking SNA station on a Token Ring to communicate with an SDLC-speaking SNA station on a serial link. The frame sizes and window sizes on Token Rings are often much larger than those acceptable for serial links, and serial links are often slower than Token Rings.
To control backlogs that can occur during periods of high data transfer from the Token Ring to the serial line, perform the following task in interface configuration mode on a per-address basis:
| Task | Command |
|---|---|
| Set the maximum number of packets held in queue before transmitting. | sdlc holdq address queue-size |
You can control the intervals at which the router polls secondary stations, the length of time a primary station can send data to a secondary station, and how often the router polls one secondary station before moving on to the next station.
Keep the following points in mind when performing these tasks:
To control polling of secondary stations, perform one or more of the following tasks in interface configuration mode:
To retrieve default polling values for these operations, use the no forms of these commands.
By default, SDLC interfaces operate in full-duplex mode. To configure an SDLC interface for half-duplex mode, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Configure an SDLC interface for half-duplex mode. | sdlc hdx |
On an interface that is in half-duplex mode and that has been configured for DCE, you can adjust the delay between the detection of Request To Send (RTS) and the assertion of Clear To Send (CTS). To do so, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Delay the assertion of a CTS. | sdlc cts-delay unit |
On an interface that is in half-duplex mode and that has been configured for DTE, you can adjust the time the interface waits for the DCE to assert CTS before dropping an RTS. To do so, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Adjust the amount of time before interface drops an RTS. | sdlc rts-timeout unit |
You can monitor the configuration of SDLC stations to determine which SDLC parameters need adjustment by performing the following task in EXEC mode:
| Task | Command |
|---|---|
| Display SDLC station configuration information. | show interfaces |
The following sections provide LLC2 and SDLC configuration examples:
You can configure the number of LLC2 frames received before an acknowledgment. For this example, assume that at time 0, two I-frames are received. The maximum amount of three has not been reached, so no acknowledgment for these frames is sent. If a third frame, which would force the router to send an acknowledgment, is not received in 800 milliseconds, an acknowledgment will be sent anyway, because the delay timer alarm will have gone off.
interface tokenring 0
llc2 ack-max 3
llc2 ack-delay-time 800
At this point, because all frames are acknowledged, the counter for the maximum amount of I-frames will be reset to zero.
The following configuration defines serial interface 0 as the primary SDLC station with two SDLC secondary stations, C1 and C2, attached to it through a modem sharing device. Two-way simultaneous mode is enabled.
interface serial 0
encapsulation sdlc-primary
sdlc address c1
sdlc address c2
sdlc simultaneous full-datmode
The network for this configuration is shown in Figure 24-1. 
In the following example, an SDLC interface has been configured for half-duplex mode.
encapsulation sdlc-primary
sdlc hdx
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