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.A site with two WAN linksand a Token Ring can require several routers if it is the main FEP site.Hierarchical Virtual RingsUsing hierarchical virtual rings, you can use physical token rings and virtual rings to create a hierarchyof virtual rings.Figure 4-35 illustrates such a hierarchy.Figure 4-35: Hierarchical virtual ring topology.http://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2004.htm (44 of 51) [9/16/2000 5:05:09 PM] Designing SRB InternetworksBecause the destination of all explorer packets is to the core virtual ring, you can use filters to eliminateexplorer packet traffic crossing between local-access virtual rings at the point where rings meet.Thefilters would also filter out FEP traffic.As an alternative, you can use the same virtual ring number foreach virtual ring to filter out FEP traffic that might otherwise traverse the local-access virtual rings.This design is limited in that the hop count might limit Token Ring SRB connectivity.Because theconnectivity from access point to FEP uses four hops, additional local bridges at the access points or atthe central site might not be reachable from the entire network.Combined DesignsNetworks can be built from hierarchical designs and from scalable partially meshed designs as long asyou prevent explorer packet traffic from reexploring access points.To fulfill this requirement, writeaccess lists to prevent explorer packet traffic from entering a ring if that traffic did not originate from thering that has an FEP.Hierarchical Design for NetBIOS EnvironmentsThe challenge of NetBIOS is that applications might require unrestricted ring-to-ring connectivity.TheSRB protocol was not designed to scale, and network designers often demand that routers help scalebeyond the original design of the protocol.Limitations on the maximum number of peers mandates that your only topological option for a NetBIOSSRB network is the hierarchical environment illustrated in Figure 4-35.This design poses certainchallenges because of increased explorer packet traffic.It is imperative that you create a single-stream,spanning connection through the network to minimize explorer packets.To succeed, a hierarchicalNetBIOS network needs three elements:http://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2004.htm (45 of 51) [9/16/2000 5:05:09 PM] Designing SRB InternetworksProxy explorerAggressive explorer packet cachingNetBIOS name cachingThese features allow switching of valid NetBIOS traffic even under the worst conditions of high explorerpacket load.You might not be able to use the partially meshed network design if you have to maintainunrestricted ring-to-ring connectivity.Contact your technical support representative to determine anyspecific limitations for your network topology and design implementation.Refer to the "Explorer Packetsand Propagation" and "NetBIOS Broadcast Handling" sections earlier in this chapter for additionaldetails concerning these topics.Queuing and Prioritization SchemesThe following information focuses on current prioritization mechanisms.Prioritization tools discussedinclude:Priority QueuingCustom QueuingSAP PrioritizationEnhanced LU Address PrioritizationSAP Filters on WAN LinksNote The queuing and prioritization schemes described in this section rely on process switching.If therouter is configured for fast switching or for autonomous switching, the configuration of a queuing orprioritization scheme will increase processor utilization.However, increased processor utilization isusually not a problem when the router is sending traffic over low-speed WAN links.Priority QueuingPriority queuing (introduced in Software Release 9.1) allows packets to be prioritized.When priorityqueuing is enabled on an interface, the router maintains up to four output queues for that interface.During congestion, the packets are placed in one of the four queues according to their priority.The routerservices all packets on the highest priority queue before moving on to the next highest priority queue.Inother words, the queuing delay of a packet on a lower priority queue is nondeterministic: An RSRBsession set to normal priority might time out if, for example, IPX packet traffic is heavy and isconfigured for the highest priority queue.This scheme introduces a problem in that packets configured for lower priority queues might not beserviced in a timely manner, or at all, depending on the bandwidth used by packets sent from the higherpriority queues.Priority queuing does not provide bandwidth allocation.Priority queuing can be used when there is sufficient bandwidth to accommodate all packets destined fora particular interface, but where packets from certain protocols such as file transfers cause otherprotocols such as Telnet sessions to suffer from poor response.http://www.cisco.com/cpress/cc/td/cpress/ccie/ndcs798/nd2004.htm (46 of 51) [9/16/2000 5:05:09 PM] Designing SRB InternetworksIf there is insufficient bandwidth on an output interface to pass data from various sources, priorityqueuing cannot solve the limited bandwidth condition.If there is not enough bandwidth to pass all of thedata destined for an interface, protocols assigned to the lower priority queues suffer packet loss.Priority queuing introduces processor overhead that might be acceptable for slow interfaces, but might beunacceptable for higher speed interfaces such as Ethernet, Token Ring, or FDDI.If you are currently fastswitching packets, be aware that priority queuing requires that these packets be process switched, whichwould negatively impact performance.Use the priority-list global configuration command to define priority lists and the priority-groupinterface command to assign a list to an interface.Priority queuing can be configured instead of, but notin addition to, custom queuing
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