258 CHAPTER 16: IP R OUTING P ROTOCOL OPERATION■ Ensure the backbone area connects with all other areas.■ The virtual links cannot pass through the STUB area.Troubleshooting globally: If OSPF cannot discover the remote routes and youhave checked all troubleshooting items listed above, check the followingconfigurations:■ If more than two areas are configured on a router, at least one area should beconfigured as the backbone area.As shown in Figure 59, RTA and RTD are each configured to belong to only onearea, whereas RTB and RTC are both configured to belong to two areas. RTBbelongs to area0, which is compliant with the requirement. However, RTC doesnot belong to area0. Therefore, a virtual link must be set up between RTC and RTBto ensure that area2 and area0 (the backbone area) are connected.Figure 59 OSPF areas■ The backbone area (area 0) cannot be configured as a STUB area and the virtuallink cannot pass through the STUB area. So if a virtual link has been set upbetween RTB and RTC, neither area1 nor area0 can be configured as a STUB area.In Figure 59, only area 2 can be configured as stub area.■ Routers in the STUB area cannot redistribute the external routes.■ The backbone area must guarantee the connectivity of all nodes.IP Routing Policy When a router distributes or receives routing information, it must implement policiesto filter the routing information so that it can receive or distribute only the routinginformation that meets specified conditions. A routing protocol, such as RIP, mayneed to import routing information discovered by other protocols to enrich its routingknowledge. While importing the routing information, it must import only theinformation that meets its conditions.To implement a routing policy, you must define a set of rules by specifying thecharacteristics of the routing information to be filtered. You can set the rules basedon such attributes as the destination address and source address of the information.The rules can be set in advance and then used in the routing policy to advertise,receive and import the route information.The Switch 5500 supports three kinds of filters. The following sections introducethese filters:■ Route Policy■ ACL■ IP PrefixRoute PolicyA route policy is used to match some attributes with given routing information andthe attributes of the information will be set if the conditions are satisfied.A route policy can comprise multiple nodes. Each node is a unit for match testing, andthe nodes will be matched in a sequence-number-based order. Each node comprises aset of if-match and apply clauses. The if-match clauses define the matching rulesRTA RTB RTC RTDarea0 area1 area2RTA RTB RTC RTDarea0 area1 area2
