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.Figure 6-13 APPN with DLSw+ using a channel-attached router.ENNNTokenRingENTokenRouter RouterRingDLSw+ DLSw+ EN6-16 Cisco CCIE Fundamentals: Network DesignLOCATE Search ReductionDLSw+ also brings nondisruptive rerouting in the event of a WAN failure.Using DLSw+ as atransport reduces the number of network nodes in the network.A disadvantage is that remoteend-node workstations require WAN connections for NNS services.Another disadvantage is thatwithout APPN in the routers, APPN transmission priority is lost when traffic enters the DLSw+network.For detailed information on DLSw and DLSw+, refer to Chapter 7,  Designing DLSw+Internetworks.APPN over FRAS BNN/BANIf the APPN network is based on a Frame Relay network, one option is to use the FRAS/BNN or theFrame Relay BAN function for host access.Both BNN and BAN allow a Cisco router to attachdirectly to an FEP.When you use FRAS/BNN, you are assuming that the Frame Relay network isperforming the switching and that native routing is not used within the Frame Relay network.For anexample of how APPN with FRAS BNN/BAN can be used in your network design, see the section Example of APPN with FRAS BNN later in this chapter.APPN over RSRBUsing RSRB, the SNA traffic can be bridged from a remote site to a data center.The use of RSRBsignificantly reduces the total number of network nodes in the network, thus reducing the number ofTDU flows in the network.Another advantage of using RSRB is that it provides nondisruptiverouting in the event of a link failure.For more information on using RSRB, refer to Chapter 4, Designing SRB Internetworks.LOCATE Search ReductionThis section describes the broadcast traffic in an APPN network and how LOCATE searches canbecome a scalability issue in an APPN network.The impact of LOCATE searches in an APPNnetwork varies from one network to the other.This section first identifies some of the causes of anexcessive number of LOCATE searches, and then discusses the following four techniques you canuse to minimize them:" Safe-Store of Directory Cache" Partial Directory Entries" Central Directory Server (CDS)/Client" Central Resource RegistrationAn APPN network node provides dynamic location of network resources.Every network nodemaintains dynamic knowledge of the resources in its own directory database.The distributeddirectory database contains a list of all the resources in the network.The LOCATE search requestallows one network node to search the directory database of all other network nodes in the network.When an end-node resource requests a session with a target resource that it has no knowledge of, ituses the distributed search capabilities of its network-node server to locate the target resource.If thenetwork node does not have any knowledge of the target resource, the network node forwards thelocate search request to all its adjacent network nodes requesting these nodes to assist thenetwork-node server to locate the resource.These adjacent network nodes propagate these locatesearch requests to their adjacent network nodes.This search process is known as broadcast search.Designing APPN Internetworks 6-17Scalability IssuesAlthough several mechanisms are put into place to reduce the LOCATE broadcast searches (forexample, resource registration, and resource caching), there might still be an excessive amount ofLOCATE flows in a network for such reasons as the network resources no longer exist, there is amixture of subarea networks and APPN networks, or the resources are temporarily unavailable.Safe-Store of Directory CacheThe first technique that you can use to minimize the LOCATE flows in your APPN network is theSafe-Store of Directory Cache, which is supported by the Cisco network-node implementation.Cache entries in a network node s directory database can be periodically written to a permanentstorage medium: a tftp host.This speeds recovery after a network-node outage or initial power loss.Resources do not have to be relearned through a LOCATE broadcast search after a router failure.This reduces spikes of broadcasts that might otherwise occur when the APPN network is restarted.Partial Directory EntriesThe second technique that you can use to minimize the LOCATE flows in your APPN network is todefine the resources in the local directory database by identifying the end node or network nodewhere the particular resource is located.The following is a sample configuration:appn partner-lu-location CISCO.LU21owning-cp CISCO.CP2completeThe preceding example defines the location of an LU named CISCO.LU21 that is located with endnode or network node CISCO.CP2.This command improves network performance by allowingdirected Locate, instead of a broadcast.The disadvantage is that definitions must be created.Toalleviate this definition problem, it may be possible to use partially specified names to definemultiple resources.The following is a sample configuration:Sample configuration:appn partner-lu-location CISCO.LUowning-cp CISCO.CP2wildcardcompleteThe preceding example defines the location of all the LUs prefixed with the characters LU.Obviously, a naming convention is essential to the success of this type of node definition.Central Directory Server (CDS)/ClientThe third technique that you can use to minimize the LOCATE flows in your APPN network is touse the CDS/client function.The APPN architecture specifies a CDS that allows a designatednetwork node to act as a focal point for locating network resources.In current APPN networks, everynetwork node can potentially perform a broadcast search for a resource.This is because the directoryservices database is not replicated on every network node.The CDS function allows a network node, with central directory client support, to send a directedLOCATE search to a CDS [ Pobierz całość w formacie PDF ]