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.SNA end stations generally produce an all-routes explorer packet.NetBIOS endstations produce a spanning explorer packet.Note As of Cisco IOS Software Release 10.2, auto spanning tree (AST) for SRB is supported.Theimplementation of AST in Cisco IOS Software Release10.2 is based on the IEEE 802.1 standard andis fully compatible with IBM PC bridging.New global and interface configuration commands arerequired to configure a router for AST.Once configured, AST can be enabled and disabled throughLAN Network Manager (LNM).The following discussion of spanning tree explorer packets appliesto the manual spanning tree functionality available in software releases prior to Cisco IOS SoftwareRelease 10.2.To pass a spanning explorer packet on a router, the configuration for the router s Token Ringinterface must have the source-bridge spanning interface configuration command for the specificring.If this interface command is not included, spanning explorer packets are discarded.In contrast, an all-routes explorer packet can find any valid SRB ring.No specific routerconfiguration other than specification of SRB is required to pass all-routes explorer packets.Explorer packet processing works as illustrated in Figure 4-5.If End station X sends an all-routesexplorer packet, Bridge B1 and Bridge B2 both forward the explorer packet.End station Y receivestwo all-routes explorer packets in this configuration.End station Y responds to each of the all-routesexplorer packets by sending a directed, nonbroadcast packet.In the example illustrated in Figure 4-5,four packets are generated:" Two all-routes explorer packets inbound (to End station Y)" Two nonbroadcast packets outbound (from End station Y)Figure 4-5 Explorer packet processing (all-routes broadcast).Bridge B1All-routes explorer packetsent by End station XToken TokenRing 1 Ring 2End station X End station YDirected, nonbroadcast packetsent by End station YBridge B2Figure 4-6 illustrates an end station sending a spanning explorer packet.Bridge B1 and Bridge B2make their respective forwarding decisions based on whether or not spanning is enabled.AssumeBridge B1 has spanning enabled and Bridge B2 does not have spanning enabled.Bridge B1 forwardsthe spanning explorer packet, and Bridge B2 does not.End station Y receives one spanning explorerpacket and returns an all-routes explorer packet for each single route received.As before, Bridge B1and Bridge B2 forward the all-routes explorer packet.In this example, the following packets aregenerated:" One spanning explorer packet inbound (to End station Y)" Two all-routes explorer packets outbound (to End station X)4-6 Cisco CCIE Fundamentals: Network Design Explorer Packets and PropagationFigure 4-6 Explorer packet processing (spanning explorer broadcast).Bridge B1Spanning explorer packetsent by End station XToken TokenRing 1 Ring 2End station X End station YAll-routes explorer packetsent by End station YBridge B2If spanning were enabled on Bridge B2, it would also forward the spanning explorer packet.Thefollowing packets would be generated:" Two spanning explorer packets inbound (to End station Y)" Four all-routes explorer packets outbound (to End station X)Note In general, there should be only a single path through the network for spanning explorerpackets.If redundancy is required, a trade-off should be made between automatic redundancy andtolerance for additional explorer packet traffic.When redundancy is required, AST should be used.Redundancy can be achieved in many instances within the router-based cloud as a result ofencapsulation in either TCP or IP, the latter called Fast Sequenced Transport (FST).To contrastredundancy provided by a pure SRB environment and an internetwork that combines routingcapabilities with SRBs, consider the networks illustrated in Figure 4-7, Figure 4-8, and Figure 4-9.Figure 4-7 illustrates a pure bridged network.Figure 4-8 and Figure 4-9 illustrate an SRB networkrunning over routers.Designing SRB Internetworks 4-7 SRB Technology Overview and Implementation IssuesFigure 4-7 Redundancy in a pure SRB network.TokenRing 30Bridge B2 Bridge B1Split Bridge B2 Split Bridge B1over serial link over serial linkBridge B2 Bridge B1Token TokenRing 10 Ring 20Client ServerBridge B3 Bridge B3Split Bridge B3over serial linkIn Figure 4-7, there are two SRB paths between Token Ring 10 and Token Ring 20:" Token Ring 10 to split Bridge B3 to Token Ring 20" Token Ring 10 to split Bridge B2 to Token Ring 30 to split Bridge B1 to Token Ring 20If spanning is enabled on both paths, the traffic resulting from a spanning explorer broadcast fromthe server is as follows:" Two spanning explorer packets inbound (to the server)" Four all-routes explorer packets outbound (to the client)In router-based networks, the same type of redundancy is achieved in a different, more efficientmanner, as illustrated in Figure 4-8 and Figure 4-9.Figure 4-8 Redundancy in a router-based SRB network (physical router connectivity).TokenRing 30Router BRouter A Router CToken TokenRing 10 Ring 20ClientServer4-8 Cisco CCIE Fundamentals: Network Design Explorer Packets and PropagationFigure 4-9 Redundancy in a router-based SRB network (logical SRB connectivity).TokenRing 30Bridge CBridge A Bridge BVirtual ring100TokenTokenRing 20Ring 10Client ServerIn Figure 4-9, there is only one SRB path between Token Ring 10 and Token Ring 20.The path isToken Ring 10 to Bridge A to Virtual ring 100 to Bridge B to Token Ring 20.When the client sendsa spanning explorer packet, the following occurs:" One spanning explorer packet goes inbound (to the server)." Two all-routes broadcasts go outbound one to the client on Token Ring 10 and one to TokenRing 30
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