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.If one of its DLSw partners can reach the specified MAC address, the partner replies withan icanreach frame.The specific sequence includes a canureach ex (explorer) to find the resourceand a canureach cs (circuit setup) that triggers the peering routers to establish a circuit.At this point, the DLSw partners establish a circuit that consists of three connections: the twodata-link control connections between each router and the locally attached SNA end system, and theTCP connection between the DLSw partners.This circuit is uniquely identified by the source anddestination circuit IDs, which are carried in all steady state data frames in lieu of data-link controladdresses such as MAC addresses.Each circuit ID is defined by the destination and source MACaddresses, destination and source link service access points (LSAPs), and a data-link control port ID.The circuit concept simplifies management and is important in error processing and cleanup.Oncethe circuit is established, information frames can flow over the circuit.NetBIOS circuit establishment is similar, but instead of forwarding a canureach frame that specifiesa MAC address, DLSw routers send a name query (NetBIOS NAME-QUERY) frame that specifiesa NetBIOS name.Instead of an icanreach frame, there is a name recognized (NetBIOSNAME-RECOGNIZED) frame.Most DLSw implementations cache information learned as part of the explorer processing so thatsubsequent searches for the same resource do not result in the sending of additional explorer frames.Flow ControlThe DLSw standard describes adaptive pacing between DLSw routers but does not indicate how tomap this to the native data-link control flow control on the edges.The DLSw standard specifies flowcontrol on a per-circuit basis and calls for two independent, unidirectional circuit flow-controlmechanisms.Flow control is handled by a windowing mechanism that can dynamically adapt tobuffer availability, TCP transmit queue depth, and end-station flow-control mechanisms.Windowscan be incremented, decremented, halved, or reset to zero.The granted units (the number of units that the sender has permission to send) are incrementedwith a flow-control indication from the receiver (similar to classic SNA session-level pacing).Flow-control indicators can be one of the following types:" RepeatçøIncrement granted units by the current window size" IncrementçøIncrement the window size by one and increment granted units by the new windowsize" DecrementçøDecrement window size by one and increment granted units by the new windowsize" ResetçøDecrease window to zero and set granted units to zero to stop all transmission in onedirection until an increment flow-control indicator is sent" HalfçøCut the current window size in half and increment granted units by the new window sizeFlow-control indicators and flow-control acknowledgments can be piggybacked on informationframes or can be sent as independent flow-control messages, but reset indicators are always sent asindependent messages.Designing DLSw+ Internetworks 7-3 Introduction to DLSw+DLSw+ FeaturesDLSw+ is Cisco s implementation of DLSw.It goes beyond the standard to include the advancedfeatures of Cisco s current remote source-route bridging (RSRB) and provides additionalfunctionality to increase the overall scalability of DLSw.DLSw+ includes enhancements in thefollowing areas:" ScalabilityçøConstructs IBM internetworks in a way that reduces the amount of broadcast trafficand therefore enhances their scalability" AvailabilityçøDynamically finds alternative paths quickly, and optionally load-balances acrossmultiple active peers, ports, and channel gateways" Transport flexibilityçøHigher-performance transport options when there is enough bandwidth tohandle the traffic load without risk of timeouts, and the option to use lower-overhead solutionswhen bandwidth is at a premium and nondisruptive rerouting is not required" Modes of operationçøDynamically detects the capabilities of the peer router, and operatesaccording to those capabilitiesDLSw+ Improved ScalabilityOne of the most significant factors that limits the size of LAN internetworks is the amount ofexplorer traffic that traverses the WAN.There are several optimizations in DLSw+ to reduce thenumber of explorers.Peer Group ConceptPerhaps the most significant optimization in DLSw+ is a feature known as peer groups.Peer groupsare designed to address the broadcast replication that occurs in a fully meshed network.Whenany-to-any communication is required (for example, for NetBIOS or APPN environments), RSRBor standard DLSw implementations require peer connections between every pair of routers.This setup is not only difficult to configure, it results in branch access routers having to replicatesearch requests for each peer connection.This wastes bandwidth and router cycles.A better conceptis to group routers into clusters and designate a focal router to be responsible for broadcastreplication.This capability is included in DLSw+.With DLSw+, a cluster of routers in a region or a division of a company can be combined into a peergroup.Within a peer group, one or more of the routers are designated to be the border peers.Insteadof all routers peering to one another, each router within a group peers to the border peer; border peersestablish peer connections with each other (see Figure 7-1).When a DLSw+ router receives a TESTframe or NetBIOS NAME-QUERY, it sends a single explorer frame to its border peer.The borderpeer forwards the explorer on behalf of the peer group member.This setup eliminates duplicateexplorers on the access links and minimizes the processing required in access routers.7-4 Cisco CCIE Fundamentals: Network Design DLSw+ FeaturesFigure 7-1 The peer group concept can be used to simplify and scale any-to-anynetworks
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