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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 Front-End Processors (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 Cisco IOS Bridging and IBM Networking Command Reference, Volume I. To locate documentation of specific commands, use the command reference master index or search online.
This chapter contains the following sections:
FRAS, the Cisco IOS software allows branch SNA devices to connect directly to a central site FEP over a Frame Relay network. FRAS converts LAN or Synchronous Data-Link Control (SDLC) protocols to a Frame Relay format understood by the Network Control Program (NCP) that runs in an FEP. The
Cisco IOS software and the NCP support two frame formats:
Management service point support in FRAS allows the SNA network management application, NetView, to manage Cisco routers over the Frame Relay network as if it were an SNA downstream PU.
FRAS provides dial backup over RSRB in case the Frame Relay network is down. While the backup Public Switched Telephone Network (PSTN) is being used, the Frame Relay connection is tried periodically. As soon as the Frame Relay network is up, it will be used.
The Frame Relay encapsulation method is based on the RFC 1490 frame format for "user-defined" protocols using Q.933 NLPID, as illustrated in Figure 183.
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Note The protocol ID for SNA subarea FID4 is 0x81. The protocol ID for SNA subarea FID2 is 0x82. The protocol ID for APPN FID2 is 0x83. |
FRAS allows the router acting as a FRAD to take advantage of the SNA BNN support for Frame Relay provided by ACF/NCP 7.1 and OS/400 V2R3. Downstream PU 2.0 and PU 2.1 devices can be attached to the router through SDLC, Token Ring, or Ethernet links. The router acting as a FRAD is connected to the Network Control Program (NCP) or AS/400 through a public or private Frame Relay network, as illustrated in Figure 184.
The frame format that communicates across the Frame Relay BNN link is defined in RFC 1490 for routed SNA traffic. From the perspective of the SNA host (for example an NCP or AS/400), the Frame Relay connection is defined as a switched resource similar to a Token Ring BNN link. Because the frame format does not include link addresses to allow the NCP to distinguish among SNA devices on the same permanent virtual circuit, Cisco supports SAP multiplexing, which allows you to configure unique LLC2 SAPs for each downstream SNA device so that they can share a single permanent virtual circuit to an FEP.
The Cisco IOS software is responsible for terminating the local data-link control frames (such as SDLC and Token Ring frames) and for modifying the data-link control frames to 802.2 compliant LLC frames. The LLC provides a reliable connection-oriented link layer transport required by SNA. (For example, 802.2 LLC is used to provide link layer acknowledgment, sequencing, and flow control.)
The Cisco IOS software encapsulates these 802.2 LLC frames according to the RFC 1490 format for SNA traffic. The frames are then forwarded to the SNA host on a Frame Relay permanent virtual circuit (PVC). In the reverse direction, the software is responsible for de-encapsulating the data from the Frame Relay PVC, and for generating and transmitting the appropriate local data-link control frames to the downstream devices.
BAN provides functionality similar to BNN except that it uses a bridged frame format, as illustrated in Figure 185.
Because it includes the MAC header information in every frame, BAN supports multiple SNA devices sharing a single permanent virtual circuit without requiring SAP multiplexing. BAN also supports load balancing across duplicate data-link connection identifiers to the same or different FEPs at the data center to enhance overall availability. BAN works for devices attached by either Token Ring or Ethernet.
To configure FRAS, perform the tasks described in the following sections:
To configure the FRAS host, see the "Configuring FRAS Host" section. For configuration examples, see the "FRAS and FRAS Host Configuration Examples" section.
To configure FRAS BNN statically, use one of the following commands in interface configuration mode:
| Command | Purpose |
|---|---|
fra s map llc mac-address lan-lsap lan-rsap serial port frame-relay dlci fr-lsap fr-rsap [pfid2 | afid2 | fid4] | Associates an LLC connection with a Frame Relay DLCI. |
fra s map sdlc sdlc-address serial port frame-relay dlci fr-lsap fr-rsap [pfid2 | afid2 | fid4] | Associates 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 (RFC 1490) accepts all LLC2 interface configuration commands.
To configure FRAS BNN dynamically, use one of the following commands in interface configuration mode:
| Command | Purpose |
|---|---|
fras map llc lan-lsap serial interface frame-relay dlci dlci fr-rsap | Associates an LLC connection with a Frame Relay DLCI. |
fra s map sdlc sdlc-address serial port frame-relay dlci fr-lsap fr-rsap [pfid2 | afid2 | fid4] | Associates 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 IETF encapsulation (RFC1490) can take all LLC2 interface configuration commands.
To configure Frame Relay BAN, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
fras ban local-ring bridge-number ring-group ban-dlci-mac dlci dlci#1 [dlci#2 ... dlci#5] [bni mac-addr] | Associates 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:
| Command | Purpose | |
|---|---|---|
Step1 | interface serial number | Specifies the serial port. |
Step2 | encapsulation frame-relay | Enables Frame Relay encapsulation. |
Step3 | interface serial slot/port.subinterface-number point-to-point | Configures a Frame Relay point-to-point subinterface. |
Step4 | frame-relay interface-dlci dlci ietf | Configures a DLCI number for the point-to-point subinterface. |
Step5 | source-bridge source-ring-number bridge-number target-ring-number conserve-ring | Assigns 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:
| Command | Purpose | |
|---|---|---|
Step1 | llc2 local-window packet-count | Specifies the maximum window size for each logical connection. |
Step2 | llc2 dynwind [nw nw-number] [dwc dwc-number] | Enables 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 | Specifies 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 186 illustrates Frame Relay backup over an ISDN connection.
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NoteThis feature provides backup for the local end of the Frame Relay connection, not the complete end-to-end 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 | Activates 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 | Configures 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 187 shows a Frame Relay network with FRAS dial backup over DLSw+.
Figure 188 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 |
|---|---|
show fras | Displays 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:
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 189 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 IBM3745 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 190 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 | Configures a virtual Token Ring interface. |
To configure SRB on the Token Ring interface, use the following commands beginning in global configuration mode:
| Command | Purpose | |
|---|---|---|
Step1 | source-bridge ring-group ring-group virtual-mac-address | Enables local SRB. |
Step2 | source-bridge local-ring bridge-number target-ring | Enables FRAS Host traffic to access the SRB domain. |
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NoteIf you are using LLC2 passthru with an Ethernet-attached host, you must configure the Cisco source-route translational bridging (SR/TLB) feature. |
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
| Command | Purpose | |
|---|---|---|
Step1 | dlsw local-peer | Enables data-link local switching. |
Step2 | fras-host dlsw-local-ack | Enables LLC2 local termination for FRAS Host connections. |
To enable the FRAS Host for BAN or BNN, use the following commands in interface configuration mode:
| Command | Purpose | |
|---|---|---|
Step1 | fras-host bnn (sub)interface fr-lsap sap vmac virt-mac hmac hmac [hsap hsap] | Configures the FRAS host for BNN. |
Step2 | fras-host ban (sub)interface hmac hmac [bni bni-mac] | Configures 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] [dlcidlci-num] [detail] | Displays the status of LLC2 sessions using FRAS Host. |
The following sections provide both FRAS and FRAS Host configuration examples:
Figure 191 illustrates the configuration of SNA devices attached to a LAN.
The configuration for the network shown in Figure 191 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 192 illustrates the configuration of SDLC-attached SNA devices.
The configuration file for the network shown in Figure 192 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 193.
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 reconfigured 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 194).
Figure 194 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.
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NoteThe new FRAS BNN feature, which provides seamless processing at the router regardless of end-station changes, is designed to coexist with the original FRAS BNN feature. In Cisco IOS Release 11.2, only LLC2 traffic will be supported. SDLC must be configured using the original BNN implementation |
The following configuration example enables the FRAS BNN feature. The topology is illustrated in Figure 195.
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
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NoteIn this configuration example, the second to last line describes the old configuration for workstation A. The last line describes the configuration for the new workstations B and C. |
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 196 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 196 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
Router A
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+:
FRAS Dial Backup on a Subinterface
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
FRAS Dial Backup on a Main Interface
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 197):
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 198.
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 198.
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
Posted: Thu Jul 20 10:26:52 PDT 2000
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