Information Technology Reference
In-Depth Information
10
CHAPTER
Troubleshooting IBM
This chapter focuses on connectivity and performance problems associated with bridging and routing in
IBM-based networks. When troubleshooting IBM-based networks, it is important to have a knowledge
of Synchronous Data Link Control (SDLC) and source-route bridging (SRB), as well as data-link
switching (DLSw). The following sections provide an overview of DLSw, SDLC, and SRB.
DLSw
Data-link switching was developed to provide support for SNA and NetBIOS in multiprotocol routers.
SNA and NetBIOS are basically connection-oriented protocols, so the data link control procedure that
they use on the LAN is IEEE 802.2 Logical Link Control (LLC) Type 2. Data-link switching also
accommodates SNA protocols over WAN links via the SDLC protocol. For more information about
DLSw, refer to RFC 1795, which defines the protocol.
For more information about troubleshooting DLSw problems, refer to the online “DLSw
Troubleshooting Guide” at www.cisco.com/warp/customer/697/dlswts1.html.
SDLC
IBM developed the SDLC protocol in the mid-1970s for use in Systems Network Architecture (SNA)
environments. SDLC was the first of an important new breed of link-layer protocols based on
synchronous, bit-oriented operation. Compared to synchronous character-oriented (for example, Bisync,
from IBM) and synchronous byte count-oriented protocols (for example, Digital Data Communications
Message Protocol [DDCMP], from Digital Equipment Corporation), bit-oriented synchronous protocols
are more efficient, more flexible, and often faster.
After developing SDLC, IBM submitted it to various standards committees. The International
Organization for Standardization (ISO) modified SDLC to create the High-Level Data Link Control
(HDLC) protocol. The International Telecommunications Union-Telecommunications Standards
Section (ITU-T, formerly CCITT) subsequently modified HDLC to create Link Access Procedure (LAP)
and then Link Access Procedure, Balanced (LAPB). The Institute of Electrical and Electronic Engineers
(IEEE) modified HDLC to create IEEE 802.2. Each of these protocols has become important in its own
domain. SDLC remains the SNA primary link-layer protocol for wide-area network (WAN) links.
