Traffic Flows

Traffic Flows

When designing any network, it is important to understand how the expected traffic from source to destination will be accommodated by the design. Figure 11-10 illustrates the expected traffic flow between Host 1 and Server 1.

Host 1 is configured to use the active HSRP address for VLAN 101 on SW-D1 as its default gateway, and Server 1 is configured to use the active HSRP address for VLAN 401 on SW-C1 as its default gateway. Assuming all the links between the core and distribution switches are of equal speed, SW-D1 will have two paths in its routing table to VLAN 401. Dynamic routing protocols, such as EIGRP, automatically load balance traffic using up to four equal cost paths by default. You can adjust this behavior to use fewer than four paths or up to eight paths using the maximum-paths command. (See Example 11-2.) Normally, the default of four maximum paths is sufficient and should not be changed.

The same situation exists on the return path from Server 1 to Host 1 because SW-C1 has equal paths in its routing table to VLAN 101 via SW-D1 and SW-D2. In this situation, traffic returning from Server 1 to Host 1 might take a different path than traffic originating from Host 1 to Server 1. This behavior is described as asymmetrical routing and is typically not an issue for most types of traffic. Because return traffic can take multiple paths, some packets may be received out of order and must rely upon the application to reassemble the packets in the correct order. If a requirement exists to limit the traffic flow to one well-known path unless a link failure occurs, administrators might adjust the parameters of dynamic routing protocols to favor one path over another. In the case of EIGRP, you can adjust the delay parameter so that one route is preferred over another. Delay, and not bandwidth, should be increased on the links that should not be used during normal traffic flow. Once adjusted, if the preferred link fails, the link with the additional delay will be used. Example 11-3 shows the default delay of 10 microseconds on a GigabitEthernet interface and the possible values for delay. Delay is adjusted on an interface-by-interface basis.