IBM BC-201 manual RFC 1490 Bridged Format for LLC2 BAN, BC-225

Overview of IBM Networking

SNA FRAS

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 PVC. In the reverse direction, the software is responsible for de-encapsulating the data from the Frame Relay PVC, and for generating and sending the appropriate local data-link control frames to the downstream devices.

RFC 1490 Bridged Format for LLC2 (BAN)

BAN provides functionality similar to BNN except that it uses a bridged frame format, as illustrated in Figure 100.

Figure 100 RFC 1490 Bridged Frame Format

	Q.922 address
Control	0x03	pad	0x00
NLPID	SNAP 0x80	OUI	00x0
	OUI 0x80-C2 (bridged)
	PID 0x00-09
	pad 0x00	Frame control
	Destination/source MAC (12 bytes)
	DSAP	SSAP
	Control
	SNA data
				51912
	PCS

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 front-end processors at the data center to enhance overall availability. BAN works for devices attached by either Token Ring or Ethernet.

	Cisco IOS Bridging and IBM Networking Configuration Guide
78-11737-02		BC-225