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This chapter describes Frame Relay access support for Systems Network Architecture (SNA) devices. For a complete description of the commands mentioned in this chapter, refer to the "SNA Frame Relay Access Support Commands" chapter of the Bridging and IBM Networking Command Reference.
To configure Frame Relay access support, perform the tasks described in the following sections:
See the end of this chapter for "Frame Relay Access Support Configuration Examples."
To configure Frame Relay access support (FRAS), perform one of the following tasks in interface configuration mode:
Since Frame Relay itself does not provide a reliable transport as required by SNA, the RFC 1490 support of SNA uses LLC2 as part of the encapsulation to provide link-level sequencing, acknowledgment, and flow control. The serial interface configured for Internet Engineering Task Force (IETF) encapsulation (that is, RFC 1490) can take all LLC2 interface configuration commands.
To configure Frame Relay boundary access node (BAN), perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Associate a bridge to the Frame Relay BAN. | fras ban local-ring bridge-number ring-group ban-dlci-mac dlci dlci#1 [dlci#2 . . . dlci#5] [bni mac-addr] |
BAN simplifies router configuration when multiple LLC sessions are multiplexed over the same DLCI. By comparison, SAP multiplexing requires static definitions and maintenance overhead. By using BAN, the Token Ring MAC address is included in every frame to uniquely identify the LLC session. Downstream devices can be dynamically added and deleted with no configuration changes required on the router.
Frame Relay access support provides a congestion control mechanism based on the interaction between congestion notification bits in the Frame Relay packet and the dynamic adjustment of the LLC2 send window. This window is the number of frames the Cisco IOS software can send before waiting for an acknowledgment. The window size decreases with the occurrence of backward explicit congestion notification (BECN) and increases when no BECN frames are received.
To configure congestion management, perform the following tasks in interface configuration mode:
| Task | Command |
|---|---|
| Specify the maximum window size for each logical connection. | llc2 local-window packet-count1 |
| Enable the dynamic window flow-control mechanism. | llc2 dynwind [nw nw-number] [dwc dwc-number] |
You can enable the dynamic window mechanism only if you are using Frame Relay IETF encapsulation.
When the Frame Relay network is down, the Cisco IOS software checks whether the dial backup feature is configured for the particular DLCI number. If it is configured, the software removes the FRAS to the downstream device connection and establishes the RSRB to this downstream device connection.
To configure RSRB dial backup, perform the following task in interface configuration mode:
| Task | Command |
|---|---|
| Activate Frame Relay RSRB dial backup. | fras backup rsrb vmacaddr local-ring-number target-ring-number host-mac-address |
To display information about the state of Frame Relay access support, perform the following command in privileged EXEC mode:
| Task | Command |
|---|---|
| Display the mapping and connection state of the Frame Relay access support. | show fras map |
The following sections provide Frame Relay access support configuration examples:
Figure 98 illustrates the configuration of SNA devices attached to a local-area network (LAN).
The following is the configuration for the network shown in Figure 98:
interface tokenring 0 no ip address no keepalive ring-speed 16 fras map llc 0800.5a8f.8802 4 4 serial 0 frame-relay 200 4 4 ! interface serial 0 mtu 2500 no ip address encapsulation frame-relay IETF keepalive 12 frame-relay lmi-type ansi frame-relay map llc2 200
Figure 99 illustrates the configuration of SDLC-attached SNA devices.
The following is the configuration file for the network shown in Figure 99:
interface serial 1 no ip address encapsulation sdlc no keepalive clockrate 56000 sdlc address C1 sdlc xid C1 05D01501 sdlc role primary fras map sdlc C1 serial 0 frame-relay 200 4 4 ! interface serial 0 mtu 2500 no ip address encapsulation frame-relay ietf keepalive 12 frame-relay lmi-type ansi frame-relay map llc2 200
The following configuration shows FRAS BAN support for Token Ring and serial interfaces. You must specify the source-bridge ring-group global command before you configure the fras ban interface command. When Token Ring is configured, the source-bridge interface command includes the local-ring, bridge-number, and the target-ring values. The source-bridge command enables local source-route bridging on a Token Ring interface.
source-bridge ring-group 200 ! interface serial 0 mtu 4000 encapsulation frame-relay ietf frame-relay lmi-type ansi frame-relay map llc2 16 frame-relay map llc2 17 fras ban 120 1 200 4000.1000.2000 dlci 16 17 ! interface tokenring 0 source-bridge 100 5 200
For SDLC connections, you must include SDLC configuration commands:
interface serial 0 encapsulation sdlc sdlc vmac 4000.1000.1000 110 5 200 sdlc address C1 sdlc partner 4000.4444.1000 C1
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