258 CHAPTER 16: IP ROUTING PROTOCOL 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 you
have checked all troubleshooting items listed above, check the following
configurations:
■If more than two areas are configured on a router, at least one area should be
configured as the backbone area.
As shown in Figure 59, RTA and RTD are each configured to belong to only one
area, whereas RTB and RTC are both configured to belong to two areas. RTB
belongs to area0, which is compliant with the requirement. However, RTC does
not belong to area0. Therefore, a virtual link must be set up between RTC and RTB
to ensure that area2 and area0 (the backbone area) are connected.
Figure59 OSPF areas
■The backbone area (area 0) cannot be configured as a STUB area and the virtual
link cannot pass through the STUB area. So if a virtual link has been set up
between 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 policies
to filter the routing information so that it can receive or distribute only the routing
information that meets specified conditions. A routing protocol, such as RIP, may
need to import routing information discovered by other protocols to enrich its routing
knowledge. While importing the routing information, it must import only the
information that meets its conditions.
To implement a routing policy, you must define a set of rules by specifying the
characteristics of the routing information to be filtered. You can set the rules based
on 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 introduce
these filters:
■Route Policy
■ACL
■IP Prefix
Route Policy
A route policy is used to match some attributes with given routing information and
the 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, and
the nodes will be matched in a sequence-number-based order. Each node comprises a
set of if-match and apply clauses. The if-match clauses define the matching rules
RTA RTB RTC RTD
area0 area1 area2
RTA RTB RTC RTD
area0 area1 area2