Session
Session
connector
connector
SID
SID
SID
SID
245
321
321
548
NN 1
NN 4
EN A
EN B
SID 245
SID 321
SID 548
Session Stage 1
Session Stage 2
Session Stage 3
6-8
Cisco CCIE Fundamentals: Network Design
Using Dependent Logical Unit Requester/Server
This routing algorithm is called intermediate session routing (ISR). It supports dynamic route definition and incorporates the following legacy features:
• Node-to-node error and flow control processing—This reflects the 1970s method of packet switching in which many line errors dictated error and flow control at each node. Given the
current high-quality digital facilities in many locations, this redundant processing is unnecessary and significantly reduces end-to-end throughput. End-to-end processing provides better
performance and still delivers the necessary reliability.
• Disruptive session switching around network failures—Whenever a network outage occurs, all sessions using the path fail and have to be restarted to use an alternative path.
Because these features are undesirable in most high-speed networks today, a newer routing
algorithm—High Performance Routing (HPR)—has been added to APPN that supports
nondisruptive rerouting around failures and end-to-end error control, flow control, and
segmentation. Cisco routers support both ISR and HPR.
Using Dependent Logical Unit Requester/Server
Dependent Logical Unit Requester/Server (DLUR/DLUS) is an APPN feature that allows legacy
traffic to flow on an APPN network. Prior to the introduction of this feature, the APPN architecture assumed that all nodes in a network could initiate peer-to-peer traffic (for example, sending the BIND to start the session). Many legacy terminals that are referred to as Dependent Logical Units (DLUs) cannot do this and require VTAM to notify the application, which then sends the BIND.
Getting the legacy sessions initiated requires a client-server relationship between ACF/VTAM
(Dependent LU server—DLUS) and the Cisco router (Dependent LU Requester—DLUR). A pair of
logical unit (LU) type 6.2 sessions are established between the DLUR and DLUS—one session is
established by each end point. These sessions are used to transport the legacy control messages that must flow to activate the legacy resources and initiate their logical unit to logical unit (LU-LU) sessions. An LU-LU session is the connection that is formed when the five steps described earlier in the section “Establishing APPN Sessions” are completed.
For example, an activate logical unit (ACTLU) message must be sent to the LU to activate a legacy LU. Because this message is not recognized in an APPN environment, it is carried as encapsulated
data on the LU 6.2 session. DLUR then deencapsulates it, and passes it to the legacy LU. Likewise, the DLU session request is passed to the ACF/VTAM DLUS, where it is processed as legacy traffic.
DLUS then sends a message to the application host, which is responsible for sending the BIND.
After the legacy LU-LU session is established, the legacy data flows natively with the APPN traffic, as shown in Figure 6-5.
Designing APPN Internetworks 6-9
Cisco Implementation of APPN
Figure 6-5
DLU session processing.
Application
VTAM notifies
host
application host
DLUS
Session establishment:
LU 6.2 "pipes"
APPN network
Session data
DLUR
Session data
LU 6.2 pipes
Dependent LUs
Cisco Implementation of APPN
This section provides an overview of Cisco’s implementation of APPN and discusses where APPN
resides in the Cisco IOS software. Cisco licensed the APPN source code from IBM and then ported
it to the Cisco IOS software using network services from the data-link controls (DLCs).
Applications use APPN to provide network transport. APPN runs on top of the Cisco IOS software.
APPN is a higher-layer protocol stack that requires network services from DLC. Cisco’s APPN
implementation is compliant with the APPN Architecture of record. When used with other features
in the Cisco IOS software, APPN provides the following unique features:
• APPN can use DLSw+ or RSRB as a network transport, thereby supporting APPN over a native
TCP/IP network.
• APPN can be used with downstream physical unit concentration (DSPU) to reduce the number
of downstream PUs visible to VTAM. This reduces VTAM definition and network restart times.
• In addition to COS, priority queuing, custom queuing, and weighted fair queuing can be used
with COS to ensure traffic prioritization and/or bandwidth reservation between protocols.
• Network management options are supported that include native SNA management services using
Native Service Point (NSP) in the Cisco router, and Simple Network Management Protocol
(SNMP) management using CiscoWorks Blue applications.
• Using Channel Interface Processor (CIP) or Channel Port Adapter (CPA), the Cisco APPN
network node can interface directly with ACF/VTAM across the channel. VTAM can be defined
either as an end node or network node.
6-10
Cisco CCIE Fundamentals: Network Design
Scalability Issues
Scalability Issues
As a single-network link state architecture, the network topology is updated as changes occur. This results in significant network traffic if instability occurs, and significant memory and processing to maintain the large topology databases and COS tables. Similarly, in large networks, dynamic