Web OS 10.0 Application Guide92 n Chapter 4: OSPF 212777-A, February 2002Example 4: Host RoutesThe Web OS 10.0 implementation of OSPF includes host routes. Host routes are used foradvertising network device IP addresses to external networks and allows for Server Load Bal-ancing (SLB) within OSPF. It also makes ABR load sharing and failover possible.Consider the example network in Figure 4-8. Both Web switches have access to servers withidentical content and are configured with the same virtual server IP addresses: 10.10.10.1 and10.10.10.2. Web switch #1 is given a host route with a low cost for virtual server 10.10.10.1and another host route with a high cost for virtual server 10.10.10.2. Web switch #2 is config-ured with the same hosts but with the costs reversed; one host route has a high cost for virtualserver 10.10.10.1 and another has a low cost for virtual server 10.10.10.2.All four host routes are injected into the upstream router and advertised externally. Trafficcomes in for both virtual server IP addresses (10.10.10.1 and 10.10.10.2). The upstream routersees that both addresses exist on both Web switches and uses the host route with the lowestcost for each. Traffic for 10.10.10.1 goes to Web switch #1 because its host route has the low-est cost for that address. Traffic for 10.10.10.2 goes to Web switch #2 because its host route hasthe lowest cost. This effectively shares the load among ABRs. Both Web switches then usestandard Server Load Balancing (SLB) to distribute traffic among available real servers.In addition, if one of the Web switches were to fail, the upstream routing device would forwardthe traffic to the ABR whose host route has the next lowest cost. In this example, the remainingWeb switch would assume the entire load for both virtual servers.Figure 4-8 Configuring OSPF Host RoutesABR Load Sharing Standard SLBPreferred path(lowest cost)to Host Route10.10.10.1Preferred path(lowest cost)to Host Route10.10.10.2Virtual Servers/Host Routes:10.10.10.1, Cost 110.10.10.2, Cost 100Web Switch #1Web Switch #2Virtual Servers/Host Routes:10.10.10.1, Cost 10010.10.10.2, Cost 1Area 1Stub AreaArea 0BackboneReal Serverswith IdenticalContent
