IP Routing Policy 155
■ 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 35, 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.
Figure 35 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 35, 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 5500G-EI 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 and the matching objects are attributes of routing information. The
relationship of
if-match clauses for a node uses a series of Boolean “AND”
statements. As a result, a match is found unless all the matching conditions
specified by the
if-match clauses are satisfied. The apply clause specifies the
RTA RTB RTC RTD
area0 area1 area2
RTA RTB RTC RTD
area0 area1 area2
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