Cisco 11.0 BT Bedienungsanleitung PDF herunterladen (Seite 20)

2-20 TN3270 Design and Implementation Guide

Using the Director Response Protocol (DRP), a simple User Datagram Protocol (UDP)-based application

developed by Cisco, the Director can query properly conﬁgured Cisco routers in the ﬁeld for Exterior Gateway

Protocol (EGP) and Internal Gateway Protocol (IGP) topological “distance” metrics. With this information and

other conﬁguration metrics, the Director can assign an optimal distributed server to each client. Asa result, users

can be transparently and automatically assigned a distributed server anywhere on the Internet.

DistributedDirector monitors the status of the TN3270 Server by opening a Telnet port. If DistributedDirector

cannot open the port it marks the server as unavailable.

DistributedDirector works in conjunction with a DNS server. More than one DistributedDirector can exist in the

network. For redundancy, we suggest that you install one DistributedDirector or every major DNS server.

DNS

The use of either LocalDirector or DistributedDirector isnot the only option toprovide redundancy. DNS isalso

a valid option. To provide redundancy using DNS, you must install more than one TN3270 Server. For each

TN3270 Server, assign all the PUs to the same IP listening address. Conﬁgure the DNS server to include a DNS

entry for all the deﬁned IP listening addresses. The result is that the client requests a server and the request is

processed using a round-robin method. The disadvantage of this method is that a client still may be sent to an

inactiveTN3270 Serverbecause theDNSserver doesnot havethe capabilityto monitorthestatus ofthe TN3270

Servers. Also, because some clients ignore IP addresses returned by a DNS server after the initial response, this

solution may not work for all clients.

Satisfying Expanding Network Address Requirements

For VTAM to control routing in a network, it must know the location of its resources (LUs, PUs, and other

SSCPs). VTAM uses element addresses in conjunction with the subarea address to identify the location of

resources (also known as network addressable units[NAUs]). The subarea address indicates where the resource

is located; the element address indicates the unique address within the subarea.

Minor nodes, such as an application program or LU, require a single element address. Some minor nodes, such

as local non-SNA devices and application programs that use parallel sessions, require more than one element

address. This requirement increases the number of element addresses that are used in a subarea.

These element addresses are available up to various rangesdependent on the level of VTAM. The TN3270 Server

supports the following methods of addressing:

• Pre-extended network addressing—Used by releases prior to VTAM V3R1. Prior to extended network

addressing, you could deﬁne 255 subareas and 254 elements.

• Extended network addressing—Used by VTAM V3R1 and releases prior to VTAM V3R2. Extended network

addressing supports 255 subarea address and extends element addressing to 32,768 elements.

• Extended subarea addressing—Used by VTAM V3R2 (with compatibility PTF) and later. Extended subarea

addressing increased the size of the subarea addresses to 65535 and the number of explicit routes for each

destination subarea to 16.

Use the MAXSUBA start option or the MAXSUBA operand in NCP to enable communication between subareas

with different addressing structures. The MAXSUBA start option speciﬁes the highest subarea value used in the

network. A MAXSUBA of 63, for example, deﬁnes a network with up to 63 subareas and 1024 elements in each

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Cisco 11.0 BT Bedienungsanleitung Seite 20