IBM Communications Server for AIX, V6 New Features and Implementation Scenarios

© Copyright IBM Corp. 2000 115

Chapter 7. Branch Extender

This chapter describes the functionality of Branch Extender and how it is

configured, and an example scenario illustrates the use of Branch Extender in

combination with HPR.

7.1 Overview

As the name implies, Branch Extender is designed for networks that can be

divided into distinct areas, such as separate branches of a large organization.

It works by separating out branch offices from the main APPN WAN backbone

network, for example, the organization’s headquarters network.

Each branch contains a node of the type called branch network node (BrNN),

which combines the functions of an APPN network node and an APPN end

node.

To the backbone network, the BrNN appears as an end node, connected to its

network node server (NNS) in the backbone network. Because the BrNN

appears as an end node, it does not receive topology information from the

backbone network (topology information is transmitted only between network

nodes).

The BrNN registers all resources in the branch with its NNS as though they

were located on the BrNN itself.This means that the nodes in the backbone

network can locate resources in the branch without having to be aware of the

separate nodes in the branch, thus reducing the amount of topology

information that must be stored. While a BrNN knows the topology of its local

branch, it knows nothing of the uplink topology.

To the branch network, the BrNN appears as a network node, acting as the

NNS for end nodes in the branch. Each node in the branch sees the rest of

the network as being connected through its NNS, in the same way as for a

standard NNS. When the BrNN can’t resolve a request for network services

locally, it refers the request to its NNS, which connects to other NNs using

normal CP-CP sessions. This can be thought of as default routing. That is

why a BrNN’s uplink to its NNS may be called the “default routing link”.

For directory services purposes, the BrNN appears to own all branch

resources. For route selection purposes, the BrNN appears to be the origin or

destination of all sessions involving branch LUs.

116 IBM Communications Server for AIX, V6: New Features and Implementation Scenarios

The Branch Extender feature of APPN is illustrated in Figure 73 and Figure

74.

Figure 73. Sample “branched” network 1

Chapter 7. Branch Extender 117

Figure 74. Sample “branched” network 2

Large enterprise networks can benefit greatly from advanced HPR functions

such as automatic resource discovery and route selection. However, the price

of dynamic network operation is the need to advertise the network topology or

search the network for resources. Prior to Branch Extender, any HPR node

that routed data between other systems (for example, CS/AIX connecting a

branch office to a host) was required to be a network node, and thus to send

and receive network control messages. In a small network, the overhead of

these network control messages is insignificant. But in a large network made

up of hundreds to thousands of relatively slow lines, even small overhead may

be unacceptable. See 7.2, “Topology databases” on page 118. IBM

introduced Branch Extender to enable such enterprises to gain the benefits of

HPR throughout their entire network without these concerns. It can support

end-to-end HPR connections all the way from computers in the branch to a

host system (this provides maximum flexibility in rerouting connections

around failed network components). The Branch Extender itself can also be

the endpoint of reroutable HPR connections and extend HPR reliability to

3270 sessions.