APPN only in the routers in the data center...

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If you want the routing decision to be made at the
backbone routers, place the APPN network node in the backbone routers, where alternative paths are available for routing decisions outside of the data center. In this example, this latter approach is preferred because it isolates the function at the data centers routers to channel attachment, reduces the number of hops to the second data center, and provides a path to a backup data center if
something catastrophic occurs.
Because APPN requires more memory and additional software, it is generally a more expensive
solution. The advantages of direct APPN routing and COS, however, often offset the added expense.
In this case, the added expense to add APPN to the backbone and data center routers might be
justifiable, whereas added expense at the branch might not be justifiable.
APPN at Every Branch
There are two cases for which adding an APPN network node at every branch can be cost justified:
• When COS Is Required
• When Branch-to-Branch Routing Is Required
6-4
Cisco CCIE Fundamentals: Network Design
APPN at Every Branch
When COS Is Required
COS implies that the user accesses multiple applications and must be able to prioritize traffic at an application level. Although other priority schemes, such as custom queuing, might be able to
prioritize by end user, they cannot prioritize between applications for an individual user. If this capability is critical, APPN network nodes must be placed in the individual branches to consolidate the traffic between multiple users using COS. For instance, COS can ensure that credit card
verification always gets priority over batch receipts to a retail company’s central site.
It is important to understand where COS is used in the network today. If the network is a subarea SNA network, COS is used only between front-end processors (FEPs) and ACF/VTAM on the
mainframe. Unless there is already an FEP at the branch office, they do not have traffic prioritization from the branch, although traffic can be prioritized from the FEP out. In this case, adding an APPN
network node at the branch office would prioritize the traffic destined for the data center sooner rather than waiting until it reaches the FEP—adding function over what is available today.
When Branch-to-Branch Routing Is Required
If branch-to-branch traffic is required, you can send all traffic to the central site and let those APPN
network nodes route to the appropriate branch office. This is the obvious solution when both data center and branch-to-branch traffic are required and the branch is connected to the backbone over a single link. However, if a separate direct link to another branch is cost-justifiable, routing all traffic to the data center is unacceptable. In this case, making the routing decision at the branch is necessary.
Using an APPN network node at the branch, data center traffic is sent over the data center link and branch-to-branch traffic is sent over the direct link.
In the example in Figure 6-3, each branch has two links to alternative routers at the data center. This is a case where APPN network nodes might be required at the branches so that the appropriate link can be selected. This can also be the design for branch-to-branch routing, adding a single hop rather than creating a full mesh of lines. This provides more direct routing than sending everything through the data center.
Designing APPN Internetworks 6-5
When to Use APPN Versus Alternative Methods of SNA Transport
Figure 6-3
Sample network for which branch-to-branch routing is required.
Data
center
Token
Token
Ring
Ring
Backbone
Branch
As you also learn in this chapter, scalability issues make it advantageous to keep the number of
network nodes as small as possible. Understanding where native routing and COS is needed is key
in minimizing the number of network nodes.
In summary, choosing where to implement APPN must be decided based on cost, scalability, and
where native routing and COS are needed. Implementing APPN everywhere in your network might
seem to be an obvious solution, even when not necessary. It must be understood, however, that if you were to deploy APPN everywhere in your network it probably would be a more costly solution than
necessary and may potentially lead to scalability problems. Consequently, the best solution is to deploy APPN only where it is truly needed in your network.
When to Use APPN Versus Alternative Methods of SNA Transport
APPN and boundary network node (BNN)/boundary access node (BAN) over Frame Relay using
RFC 1490 are the two methods of native SNA transport, where SNA is not encapsulated in another
protocol. BAN and BNN allow direct connection to an FEP, using the Frame Relay network to switch
messages, rather than providing direct SNA routing.
Although native might seem to be the appropriate strategy, APPN comes at the price of cost and network scalability, as indicated in the preceding section. With BNN/BAN additional cost is required to provide multiprotocol networking because the FEP does not handle multiple protocols. This
implies that additional routers are required in the data center for other protocols and separate virtual circuits are required to guarantee service delivery for the SNA or APPN traffic.
DLSw+ provides encapsulation of SNA, where the entire APPN message is carried as data inside a