With OSPF in the same topology, the default route is propagated to the totally stubby peer (the access layer switch in this case) when the neighbor relationship is established, regardless of the ability of the distribution node to forward traffic to the core. As a side effect, a convergence event on the uplink or on the primary distribution node affects only half as many hosts, giving a convergence event an average of 50 percent less impact (see Figure 39). Cisco introduced the hierarchical design model, which uses a layered approach to network design in 1999 (see Figure 1). Similarly to the core, the distribution layer is engineered with sufficient bandwidth and capacity so that the complete failure of one of the distribution nodes does not impact the performance of the network from a bandwidth or switching capacity perspective. This can cause unexpected and unwanted Internal Gateway Protocol (IGP) behavior. Additionally, Cisco switch operating software can now tag all native VLAN traffic. In the campus, the amount of Tx-queue space is much smaller than the amount of memory available in a WAN router. The following configuration example shows how to change the 802.1Q native VLAN to something other than 1 (the default). Figure 56 Convergence Events with Return Path Traffic. Two types of trunks are currently available: 802.1Q is the Institute of Electrical and Electronics Engineers (IEEE) standard implementation. From an STP perspective, both access layer uplinks are forwarding, so the only convergence dependencies are the default gateway and return path route selection across the distribution-to-distribution link (see Figure 58). In this figure you can see that this campus network has two buildings that are both three stories tall. It may be a single building or a group of buildings spread over an extended geographic area. This section describes why QoS is needed and discusses specific cases where QoS is most beneficial. Finally, this topology has not been widely deployed and tested over time, while the design with the L2/L3 boundary at the distribution layer has. This section describes the best way to build a topology that includes VLANs spanning access layer switches and that depend on STP/RSTP for convergence (see Figure 57). The following are the design recommendations for Layer 2 foundation services: If you are compelled by application requirements to depend on STP to resolve convergence events, use Rapid PVST+, which is far superior to 802.1d and even PVST+ (802.1d plus Cisco enhancements) from the convergence perspective. Additionally, in the event of a link failure, it is possible for traffic to transit through a neighboring access layer switch, which is not desirable. This section describes the recommended best practices for ensuring high availability in the campus network and includes the following topics: •Spanning VLANs Across Access Layers Switches, • Deploying the L2 /L3 Boundary at the Distribution Layer. As discussed previously, the OSPF SPF timer does not allow an OSPF environment to converge as quickly as EIGRP, PVST, or PVST+. •L3 in the access is an emerging and intriguing option. This is not an issue when VLANs are not present across access layer switches because the flooding occurs only to switches where the traffic would have normally been switched. The same is true for Campus vs. Data Center routers & switches. Only use L2 looped topologies if it cannot be avoided. Preemption is the desired behavior because the STP/RSTP root should be the same device as the HSRP primary for a given subnet or VLAN. Number of interfaces and the packet source and destination lower priority best-effort traffic may also be affected memory associated this! 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