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This chapter describes Frame Relay Access Support (FRAS) for Systems Network Architecture (SNA) devices. It also explains how to configure FRAS and how to use a FRAS host to connect Cisco Frame Relay Access Devices (FRADs) to channel-attached mainframes, LAN-attached FEPs, and LAN-attached AS/400s through a Cisco router.
For a complete description of the FRAS commands in this chapter, refer to the "SNA Frame Relay Access Support Commands" chapter of the Bridging and IBM Networking Command Reference. To locate documentation of specific commands, use the command reference master index or search online.
To configure FRAS, perform the tasks described in the following sections:
To configure the FRAS host, see the section "Configure FRAS Host." For configuration examples, see the section "FRAS and FRAS Host Configuration Examples" at the end of this chapter.
| Command | Purpose |
|---|---|
fras map llc mac-address lan-lsap lan-rsap serial port frame-relay dlci fr-lsap fr-rsap [pfid2 | afid2 | fid4] | Associate an LLC connection with a Frame Relay DLCI. |
fras map sdlc sdlc-address serial port frame-relay dlci fr-lsap fr-rsap [pfid2 | afid2 | fid4] | Associate an SDLC link with a Frame Relay DLCI. |
In this implementation, you configure and define each end station MAC and SAP address pair statically.
Because 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) accepts all LLC2 interface configuration commands.
| Command | Purpose |
|---|---|
fras map llc lan-lsap serial interface frame-relay dlci dlci fr-rsap | Associate an LLC connection with a Frame Relay DLCI. |
fras map sdlc sdlc-address serial port frame-relay dlci fr-lsap fr-rsap [pfid2 | afid2 | fid4] | Associate an SDLC link with a Frame Relay DLCI. |
When you associate an LLC connection with a Frame Relay DLCI, the router "learns" the MAC/SAP information as it forwards packets to the host. The FRAS BNN feature provides seamless processing at the router regardless of end station changes. End stations can be added or deleted without reconfiguring the router.
When you associate an SDLC link with a Frame Relay DLCI, you configure and define each end station MAC and SAP address pair statically.
Because 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.
| Command | Purpose |
|---|---|
fras ban local-ring bridge-number ring-group ban-dlci-mac dlci dlci#1 [dlci#2 . . . dlci#5] [bni mac-addr] | Associate a bridge to the Frame Relay BAN. |
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.
To configure SRB over Frame Relay, use the following commands in interface configuration mode:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| interface serial number | Specify the serial port. | ||
| encapsulation frame-relay | Enable Frame Relay encapsulation. | ||
| interface serial slot/port.subinterface-number point-to-point | Configure a Frame Relay point-to-point subinterface. | ||
| frame-relay interface-dlci dlci ietf | Configure a DLCI number for the point-to-point subinterface. | ||
| source-bridge source-ring-number bridge-number target-ring-number conserve-ring | Assign a ring number to the Frame Relay permanent virtual circuit. |
Cisco IOS software offers the ability to encapsulate source-route bridging traffic using RFC 1490 Bridged 802.5 encapsulation. This provides SRB over Frame Relay functionality. This SRB over Frame Relay feature is interoperable with other vendors' implementations of SRB over Frame Relay and with some vendors' implementations of FRAS BAN.
SRB over Frame Relay does not support the following Cisco IOS software functions:
To configure congestion management, use the following commands in interface configuration mode:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| llc2 local-window packet-count | Specify the maximum window size for each logical connection. | ||
| llc2 dynwind [nw nw-number] [dwc dwc-number] | Enable the dynamic window flow-control mechanism. |
You can enable the dynamic window mechanism only if you are using Frame Relay IETF encapsulation.
To configure FRAS DLCI backup, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
fras ddr-backup interface interface dlci-number | Specify an interface to be used for the backup connection and indicate the DLCI number of the session. |
FRAS DLCI backup is an enhancement to Cisco's FRAS implementation that lets you configure a secondary path to the host to be used when the Frame Relay network becomes unavailable. When the primary Frame Relay link to the Frame Relay WAN fails, the FRAS DLCI backup feature causes the router to reroute all sessions from the main Frame Relay interface to the secondary interface. The secondary interface can be either serial or ISDN and must have a data link connection identifier (DLCI) configured.
Figure 136 illustrates Frame Relay backup over an ISDN connection.
To configure RSRB dial backup, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
fras backup rsrb vmacaddr local-ring-number target-ring-number host-mac-address | Activate Frame Relay RSRB dial backup. |
The FRAS dial backup over DLSw+ feature provides a secondary path that is used when the Frame Relay network becomes unavailable. If preconfigured properly, when the primary link to the Frame Relay WAN fails, FRAS dial backup over DLSw+ feature moves existing sessions to the alternate link automatically. When the primary link is restored, existing sessions are kept on the backup connection so they can be moved non-disruptively to the primary link at the user's discretion.
To enable FRAS dial backup over DLSw+, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
fras backup dlsw virtual-mac-address target-ring-number host-mac-address [retry number] | Configure an auxiliary (backup) route between the end stations and the host for use when the DLCI connection to the Frame Relay network is lost. |
Figure 137 shows a Frame Relay network with FRAS dial backup over DLSw+.
Figure 138 shows the active FRAS dial backup over DLSw+ when the Frame Relay connection to the NCP is lost.
To display information about the state of FRAS, use the following command in privileged EXEC mode:
| Command | Purpose |
|---|---|
Display the mapping and connection state of the FRAS. |
The FRAS Host provides a scalable and efficient solution for SNA FRAD access to channel-attached hosts and to LAN-attached hosts. The FRAS Host function operates in two modes, which are documented in the following sections:
The FRAS Host LLC passsthru feature combines with a CIP-attached Cisco router's high-speed channel access to provide FEP-class performance at a fraction of what it would cost to achieve similar functionality using a FEP. If the CIP SNA feature is used to interface with the mainframe, then FRAS Host LLC2 passthru mode is the recommended solution. In this topology the LLC2 passthru solution to the CIP-SNA LLC2 stack provides better performance, is more robust, and responds well to different types of congestion.
To prevent LLC2 session timeout, LLC2 characteristics (windows and timers) may be tuned on the CIP internal LAN adapter. The CIP/SNA LLC2 stack reacts to congestion by dynamically adjusting its LLC2 transmit window for that LLC2 session in response to dropped frames.
With the FRAS Host LLC passthru feature, you gain performance benefits of a channel attachment without FEP upgrades such as the addition of a Frame Relay interface, an upgrade to NCP (with its associated increase in monthly charges), and a possible increase in system memory.
Figure 139 illustrates Cisco FRAD access to a mainframe through a channel-attached Cisco router.
If the FRAS Host feature is used to allow remote FRADs to communicate with a LAN-attached IBM 3745 or AS/400, then LLC2 termination via DLSw+ local switching is the recommended solution. With this approach, the LLC2 sessions are terminated at the Route Processor. To prevent LLC2 session timeout, LLC2 characteristics (windows and timers) may be tuned on the virtual Token Ring interface. If the dynamic window algorithm is enabled on the virtual Token Ring interface, LLC2 local termination will react to congestion by dynamically adjusting its LLC2 transmit window in response to occurrence of Frame Relay BECN.
When you use the FRAS Host LLC2 local termination feature on a Token Ring-attached FEP, the FRAS Host Cisco router shields the FEP from having to manage the interface to the Frame Relay network. This avoids interface, memory, and NCP upgrades. The FRAS Host Cisco router simply provides LLC2 sessions to the FEP over the LAN.
If used in an environment with AS/400s, FRAS Host LLC2 local termination provides an even more valuable function. The Cisco FRAS Host router offloads the management of the Frame Relay connections from the AS/400. This reduces AS/400 system hardware requirements and frees AS/400 CPU cycles for user applications.
Figure 140 illustrates Cisco FRAD access to a LAN-attached SNA host through a Cisco router.
Both passthru and local acknowledgment environments support frame discard eligibility (DE) for additional congestion management. In both environments, you can further tune the interface to the Frame Relay network by taking advantage of the Cisco IOS Frame Relay features. Taken together, these features increase overall throughput dramatically by comparison to generic FRADs, which typically cannot use the network with the same degree of efficiency.
To configure the FRAS Host migration feature, perform the tasks in the following sections:
To configure a virtual Token Ring interface, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
interface virtual-tokenring number | Configure a virtual Token Ring interface. |
To configure SRB on the Token Ring interface, use the following commands, beginning in global configuration mode:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| source-bridge ring-group ring-group virtual-mac-address | Enable local source-route bridging. | ||
| source-bridge local-ring bridge-number target-ring | Enable FRAS Host traffic to access the SRB domain. |
LLC2 passthru is the default operational mode for all FRAS Host connections that use a virtual Token Ring interface. You do not need to perform any configuration to accept the default LLC2 passthru mode.
To enable LLC2 local termination for FRAS Host connections using the virtual Token Ring, use the following commands, beginning in global configuration mode
| Step | Command | Purpose | ||
|---|---|---|---|---|
| dlsw local-peer | Enable data link local switching. | ||
| fras-host dlsw-local-ack | Enable LLC2 local termination for FRAS Host connections. |
To enable the FRAS Host for BAN or BNN, use the following commands in interface configuration mode:
| Step | Command | Purpose | ||
|---|---|---|---|---|
| fras-host bnn (sub)interface fr-lsap sap vmac virt-mac hmac hmac [hsap hsap] | Configure the FRAS host for BNN. | ||
| fras-host ban (sub)interface hmac hmac [bni bni-mac] | Configure the FRAS host for BAN. |
To display the status of LLC2 sessions using FRAS Host, use the following command in privileged EXEC mode:
| Command | Purpose |
|---|---|
show fras-host [(sub)interface] [dlci dlci-num] [detail] | Display the status of LLC2 sessions using FRAS Host. |
The following sections provide both FRAS and FRAS Host configuration examples:
Figure 141 illustrates the configuration of SNA devices attached to a LAN.
The configuration for the network shown in Figure 141 is as follows:
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 142 illustrates the configuration of SDLC-attached SNA devices.
The configuration file for the network shown in Figure 142 is as follows:
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
FRAS BNN transports SNA traffic across different media through a Cisco router and then through a Frame Relay link to the host. SNA PU 2.0 and PU 2.1 devices may be attached to the remote router through Token Ring, SDLC, or Ethernet to access the Frame Relay network. The FRAS BNN topology is illustrated in Figure 143.
The original Frame Relay BNN feature transports traffic from multiple PUs over a single DLCI. This function is called SAP multiplexing. The router uses a unique SAP address (fr-lsap) for each downstream PU when communicating with the host. In this implementation, each end station's MAC/SAP address pair must be statically defined to the router. Consequently, the router must be re-configured each time an end station is moved, added, or deleted. The configuration overhead for this implementation can be high.
The FRAS BNN feature, where the router "learns" the MAC/SAP information as it forwards packets to the host, offers several advantages over the original FRAS BNN implementation. The BNN enhancement alleviates the need to reconfigure the router when end stations are moved, added, or deleted. The configuration is simple: one map definition in the router is sufficient for multiple downstream devices. The router "learns" the addresses of the downstream devices in the normal course of communication (as shown in Figure 144).
Figure 144 illustrates the Frame Relay BNN configuration for both the original implementation and the enhanced implementation.
If the end station initiates the LLC session, the router acquires the Token Ring address and the SAP value of the end station from the incoming frame. Instead of mapping the end station's MAC/SAP address pair (as was done in the original FRAS BNN implementation), the destination MAC/SAP address pair of the incoming frame is mapped to the Frame Relay DLCI. If the destination SAP specified by the end station is equal to the lan-lsap address, the router associates the LLC (LAN) connection with the Frame Relay DLCI. The MAC address and the SAP address of the end station are no longer required in the router configuration. Thus, in the enhanced FRAS BNN implementation one configuration command achieves the same result for the end stations as did multiple configuration commands in the original FRAS BNN implementation.
The following configuration example enables the FRAS BNN feature. The topology is illustrated in Figure 145.
interface Serial0 no ip address encapsulation frame-relay IETF frame-relay lmi-type ansi frame-relay map llc2 16 ! interface TokenRing0 no ip address ring-speed 16 fras map llc 0800.5aab.0856 04 04 Serial 0 frame-relay 16 04 04 fras map llc 04 Serial 0 frame-relay dlci 16 04
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 as follows:
! interface Serial1 description SDLC line PU2.0 mtu 265 no ip address encapsulation sdlc no keepalive clockrate 9600 sdlc role primary sdlc vmac 4000.0000.0000 sdlc address C2 sdlc xid C2 05D01502 sdlc partner 4000.0000.2345 C2 sdlc address C8 sdlc xid C8 05D01508 sdlc partner 4000.0000.2345 C8 sdlc address C9 sdlc xid C9 05D01509 sdlc partner 4000.0000.2345 C9 fras ban frame-relay Serial0 4000.0000.2345 dlci 16 ! interface Serial2 description SDLC line PU2.1 no ip address encapsulation sdlc no keepalive clockrate 19200 sdlc role prim-xid-poll sdlc vmac 2000.0000.0000 sdlc address C6 sdlc partner 1000.2000.3000 C6 fras ban frame-relay serial0 1000.2000.3000 dlci 16
Figure 146 illustrates the interoperability provided by SRB over Frame Relay. FRADs B and C forward frames from their locally attached Token Rings over the Frame Relay network using SRB.
Figure 146 illustrates a network with the following characteristics:
In this example we configure a new option, conserve-ring, on the source-bridge interface configuration command. When this option is configured, the SRB software does not add the ring number associated with the Frame Relay PVC to outbound explorer frames. This option is permitted for Frame Relay subinterfaces only.
The router configures the partner FRAD's virtual ring number as the ring number for the PVC.
This approach does not require a separate ring number per DLCI. The router configures the partner FRAD's virtual ring number as the ring number for the PVC.
FRAD B configures its virtual ring as 200 and the ring for the PVC as 100. FRAD C configures its virtual ring as 300 and the ring for the PVC as 100.
The following example shows a configuration for FRAS DLCI backup over a serial interface:
interface serial0 mtu 3000 no ip address encapsulation frame-relay IETF bandwidth 56 keepalive 11 frame-relay map llc2 277 frame-relay map llc2 278 frame-relay lmi-type ansi fras ddr-backup interface serial1 188 ! interface serial1 mtu 3000 no ip address encapsulation frame-relay IETF no cdp enable frame-relay map llc2 188 frame-relay lmi-type ansi ! interface serial2 no ip address encapsulation sdlc no keepalive clock rate 19200 sdlc role prim-xid-poll sdlc address D6 fras map sdlc D6 s0 frame-relay 277 8 4 ! interface tokenring0 no ip address ring-speed 16 fras map llc 0000.f63a.2f70 4 4 serial0 frame-relay 277 4 4
source-bridge ring-group 100 ! interface Serial1 encapsulation frame-relay ! interface Serial1.1 point-to-point frame-relay interface-dlci 30 ietf source-bridge 200 1 100 conserve-ring source-bridge spanning ! interface Serial1.2 point-to-point frame-relay interface-dlci 31 ietf source-bridge 300 1 100 conserve-ring source-bridge spanning ! interface TokenRing0 source-bridge 500 1 100
The following examples show configurations for FRAS dial backup over DLSw+:
source-bridge ring-group 200 dlsw local-peer peer-id 10.8.8.8 dlsw remote-peer 0 tcp 10.8.8.7 dynamic interface ethernet0 ip address 10.8.8.8 255.255.255.0 ! interface serial0 no ip address encapsulation frame-relay IETF frame-relay lmi-type ansi ! interface Serial0.1 point-to-point description fras backup dlsw+ listening on dlci 16 configuration example no ip address frame-relay interface-dlci 16 fras backup dlsw 4000.1000.2000 200 1000.5aed.1f53 ! interface TokenRing0 no ip address ring-speed 16 fras map llc 0000.f63a.2f50 4 4 Serial0.1 frame-relay 16 4 4
source-bridge ring-group 200 dlsw local-peer peer-id 10.8.8.8 dlsw remote-peer 0 tcp 10.8.8.7 dynamic interface ethernet0 ip address 10.8.8.8 255.255.255.0 ! interface serial0 no ip address encapsulation frame-relay IETF frame-relay lmi-type ansi frame-relay map llc2 16 fras backup dlsw 4000.1000.2000 200 1000.5aed.1f53 ! interface Serial1 ip address 10.8.8.8 ! interface tokening0 no ip address ring-speed 16 fras map llc 0000.f63a.2f50 4 4 Serial0 frame-relay 16 4 4
This section provides the following configuration examples ( see Figure 147)
interface Serial0 encapsulation frame-relay IETF frame-relay map llc2 16 ! interface TokenRing0 fras map llc 4001.2222.0000 4 4 Serial0 frame-relay 16 4 4
source-bridge ring-group 200 ! interface Serial0 encapsulation frame-relay IETF frame-relay map llc2 37 fras ban 10 1 200 4000.3745.0000 dlci 37 ! interface TokenRing0 source-bridge 20 1 200
source-bridge ring-group 400 ! interface Serial0 encapsulation frame-relay IETF frame-relay map llc2 46 fras ban 50 1 400 4000.3745.0220 dlci 46 bni 4001.3745.1088 ! interface TokenRing0 source-bridge 60 1 400
The following example shows the configuration for the network shown in Figure 148.
source-bridge ring-group 100 ! interface Serial0/1 encapsulation frame-relay IETF frame-relay map llc2 16 frame-relay map llc2 46 ! interface Serial0/2 encapsulation frame-relay IETF ! interface Serial0/2.37 point-to-point frame-relay interface-dlci 37 ! interface Channel4/0 no keepalive ! interface Channel4/1 no keepalive lan TokenRing 0 source-bridge 104 1 100 adapter 0 4001.3745.1008 ! interface Virtual-TokenRing0 source-bridge 47 1 100 source-bridge spanning fras-host bnn Serial 0/1 fr-lsap 04 vmac 4005.3003.0000 hmac 4001.3745.1088 fras-host ban Serial 0/1 hmac 4001.3745.1088 bni 4001.3745.1088 fras-host ban Serial 0/2.37 hmac 4001.3745.1088
The configuration example in this section is shown in Figure 148.
source-bridge ring-group 226 dlsw local-peer dlsw bridge-group 1 ! interface Ethernet0 bridge-group 1 ! interface Serial2 encapsulation frame-relay IETF frame-relay map llc2 502 frame-relay lmi-type ansi ! interface Virtual-TokenRing0 no ip address ring-speed 16 source-bridge 1009 1 226 fras-host dlsw-local-ack fras-host bnn Serial2 fr-lsap 04 vmac 4000.1226.0000 hmac 0800.5ae1.151d
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