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Using SDLC Relay

This chapter describes the OpenROUTE Networks implementation of Synchronous Data Link Control Relay (SRLY). It includes the following sections:

About SDLC Relay

How SDLC Relay Works

Setting Up SDLC Relay

Sample SDLC Relay Configuration

About SDLC Relay

 Like DLSw, SRLY is a method for consolidating SDLC traffic onto the corporate multiprotocol backbone.

Unlike DLSw, SDLC Relay does not terminate the SDLC data link to reduce the likelihood of session timeouts, and does nothing to help reduce congestion on the WAN link. What SRLY provides is a serviceable method for shipping HDLC-formatted frames across WAN links in situations when it is not possible to use data link switching (OpenROUTE Networks DLSw product).

For more information on OpenROUTE Networks DLSw product, see Using the DLSw Protocol.

How SDLC Relay Works

Despite its name, the SDLC Relay protocol (SRLY) is designed to handle other protocols besides SDLC. The protocol works by encapsulating SDLC or any bit-oriented protocol (HDLC, LAPB) in UDP packets, and transmitting them through the IP cloud on a point-to-point connection to another SRLY device.

These connections are established by matching SRLY traffic to specific ports and groups. During configuration, each group has a unique group number assigned, and exactly two ports: one SDLC primary port, and one SDLC secondary. Matching SRLY traffic to group numbers and ports ensures that attached end stations can only send packets to the end stations for which they are intended.

Once the router receives packets, it strips them of their UDP/IP header and transmits them to their destination address in their original protocol format.

Figure 1 SDLC Primary/Secondary Stations and Local/Remote Ports

Encapsulation in UDP/IP packets allows for SDLC frames to be handled via IP routing techniques. And since each SDLC frame is encapsulated unchanged, SRLY is transparent to sending and receiving stations. This transparency allows SRLY to support all SNA PU Types.

SDLC Primary and Secondary Stations

When configuring SRLY, you must connect a router's primary port to its primary end station, and connect its secondary port to its secondary end station. Within the primary-secondary communication process, the primary end station is responsible for initiating, scheduling, and terminating the session. The secondary station does not initiate communication, but responds to commands from its primary partner.

When running balanced protocols like LAP-B or HDLC, you can assign roles arbitrarily as long as one device is Primary, and its connected counterpart is Secondary.

When to Use SDLC Relay

Generally, you would use SRLY instead of DLSw when you need to exchange any bit-oriented protocol, such as LAPB, HDLC, or SDLC primary over the wide area, between SNA or non-SNA devices.

DLSw can be configured for Host 1 and end station C in Figure 2: other wide-area connections would have to be accomplished using SDLC Relay.

Figure 2 SDLC Relay Configurations

Setting Up SDLC Relay

Configuring SDLC Relay (SRLY) involves performing these steps on each of two routers.

1. Set the data link on the serial line using the set data-link command and the appropriate interface number.

2. Assign a group number using the add group command. The group number must be the same on each SRLY router. Group number 1 is the default.

3. At the SDLC Config> prompt, add a local port with the add local command. Be sure you add this port to the group defined in Step 2.

4. This port's data link type must be SDLC Relay (SRLY). Use the set data link command at the Config> prompt to set the data link type for the port.

5. At the SDLC Config> prompt, add a remote port with the add remote command. The IP address of the remote port is that of the cooperating SRLY router.

6. Repeat these steps for the second SRLY router. When prompted for the IP address of the remote port, provide the address of the first router.

SRLY Interface Requirements

When configuring SRLY, you must consider the following requirements.

Depending on the type of serial card that you are using, dual-port or quad-port, the SRLY interface may be either DTE or DCE. If you are using a dual-port, the SRLY interface is DTE. For DTE interfaces, you must use modems or modem eliminators between the primary and secondary devices and the router, if connecting to another DTE device (or, you can connect directly to a DCE device). The modem eliminator must be compatible with the interface, such as RS-232 or V.35. The interface may be DCE or DTE if you are using the quad-port and you are not using modem eliminators.

The serial line supports NRZ. The Quad Serial line supports NRZ and NRZI.

Sample SDLC Relay Configuration

Following is a complete SDLC Relay configuration. The example assumes that the router has not been configured for any other protocols or data links.

Context Diagram

The example is based on the information shown in Figure 3. The IP connection between the two routers is over PPP.

Configuring R1 for SDLC Relay requires all of the information shown. This information includes the following:

The example indicates where this information is provided in the course of the configuration procedure.

Figure 3 Context Diagram for SRLY Configuration

This example explains how to configure two routers for SRLY traffic. Router 1 (R1) is connected to a PU Type 2.0 node. Router 2 (R2) is connected to a front end processor (FEP).

Configuring SDLC Relay

On R1, set the data link of interface 2 to an SDLC Relay device. Use the set data-link command shown here.

Config>set data srly 2
You can list the devices to confirm that an SDLC Relay device has been added.

Config>list dev
Ifc 0 (Token Ring): CSR 6000000, vector 28
Ifc 1 (WAN PPP): CSR  81620, CSR2  80D00, vector 93
Ifc 2 (WAN SDLC Relay): CSR  81640, CSR2  80E00, vector 92

Set Serial Line Parameters

 Next, set the line speed and clocking type parameters for the SRLY line. Note that the prompt for the SRLY configuration module is SRLY # Config>, where # is the number of the SRLY interface.

Config>network 2
SDLC relay interface user configuration

SRLY 2 Config>set clock internal
Must also SET SPEED for internal or mixed clocking 
SLC Config>set speed
Internal Clock Speed [0]? 56000
For RBX Series routers, you must also set the cable type.

SRLY 2 Config>set cable rs-232 dce
After setting the line speed, clocking, and cable type, you can check the configuration with the list command.

SRLY 2 Config>list
Synchronous serial line interface configuration 
Maximum frame size in bytes = 2048
Encoding: NRZ
Idle State: Flag
Clocking: Internal
Cable type: RS-232 DCE
Internal Clock Speed: 56000
Transmit Delay Counter: 0
SLC Config>exit

Configuring the SDLC Relay Protocol

 Configure the SDLC Relay protocol as shown: 

Config>protocol sdlc
SDLC relay protocol user configuration
SDLC config>
As this example shows, the prompt for the SDLC Relay (SRLY) area is SDLC config>. Commands entered at this prompt only affect the SDLC Relay protocol. They have nothing to do with, and do not affect, SDLC data links or devices.

Exit the SDLC Relay configuration procedure at any time by entering exit.

Assign a Group Number

First, assign a group number with the add group command. This number is assigned to the primary and secondary ports on the router you are configuring for SRLY. The group number you designate must be the same for each router.

SDLC config>add group
Group number: [1]?
Notice that the list group command shows that no ports have yet been configured for group 1.

SDLC config>list group
Group number: [1]? 1
                         SDLC Relay Configuration
Group Number   Port Status    Net     SDLC Station  IP Address 
                              Number  address(hex)               
----------     ----------     -----   -----------   -----------
No ports configured for group  1

Add a Local Port

 Next, add a local port to group 1. The port you add is the SRLY line defined earlier.

SDLC config>add local
Group number: [1]? 
Interface number: [0]? 2
(P)rimary or (S)econdary: [P]?
Notice that the list all command shows that a local secondary port is configured for group 1.

SDLC config>list all
                            SDLC Relay Configuration
Group Number   Port Status      Net     SDLC Station  IP Address 
                                Number  address(hex)               
----------     ----------       -----   -----------   -----------
1 (E)          Local PRIMRY (E)   2
The (E) shown within the Port Status column stands for Enabled. By default, SRLY ports are enabled; SRLY ports must remain enabled in order to use the feature.

Add a Remote Port

Next add a remote port for group 1. This is the port that leads to the IP cloud. Each group must consist of a pair of ports, one primary, the other secondary. The remote port added here must be secondary since the local port attached to it is primary.

The IP address provided is that of the router on the other side of the IP cloud, R2.

SDLC config>add remote
Group number: [1]? 
IP address of remote router: [0.0.0.0]? 10.2.50.30
(P)rimary or (S)econdary: [S]? p
SDLC config>list all
                     SDLC Relay Configuration

Group Number   Port Status    Net     SDLC Station  IP Address 
                              Number  address(hex)               
----------     ----------     -----   -----------   -----------
1 (E)          Local PRMRY (E) 2                      10.2.50.30
1 (E)           Remote SCNDRY (E)

Configuring the Neighbor Router

 Up to this point, this example has shown how to configure R1 in Figure 3. SRLY requires two routers, one on either side of the IP cloud. You must configure SRLY on each of them.

Set Data Link, Add Group, and Add Port

First, set up an SRLY data link for R2. Do this in the same manner as shown earlier for R1.

Next, add a group for R2, assigning the same group number (1, in this case) as that assigned on R1. Add a local port for the assigned group. This is the SRLY line you have already defined. In this case, the port type is secondary since a front end processor (FEP) (which, like a host, is always primary) is on the line.

SDLC config>add local
Group number: [1]? 
Interface number: [0]? 
(P)rimary or (S)econdary: [S]?
SDLC config>list all
                     SDLC Relay Configuration

Group Number   Port Status    Net     SDLC Station  IP Address 
                              Number  address(hex)               
----------     ----------     -----   -----------   -----------
1 (E)          Local SCNDRY (E) 0                      10.2.50.30

Add a Remote Port

 Finally, add a remote port for group 1. This is the port that leads to the IP cloud. Since the FEP is primary, this port is secondary. As mentioned earlier, each group must consist of a primary and secondary station.

Since we are configuring R2, the IP address of the remote router belongs to R1. See Figure 3 for the addresses of R1 and R2, and their roles in the overall SRLY configuration.

SDLC config>add remote
Group number: [1]? 
IP address of remote router: [0.0.0.0]? 10.1.50.7
(P)rimary or (S)econdary: [S]? p
SDLC config>list all
                          SDLC Relay Configuration

Group Number   Port Status    Net     SDLC Station  IP Address 
                              Number  address(hex)               
----------     ----------     -----   -----------   -----------
1 (E)          Remote PRMRY (E)                       10.1.50.7
1 (E)           Local SCNDRY (E)   0

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