3Com SuperStack 4 Configuration Manual page 143

IP Routing Policy
If more than two areas are configured on a router, at least one area should be
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configured as the backbone area.
As shown in Figure
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
area0 area0
RTA RTA
The backbone area (area 0) cannot be configured as a STUB area and the
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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.
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The backbone area must guarantee the connectivity of all nodes.
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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
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ACL
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IP Prefix
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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
matching rules and the matching objects are attributes of routing information. The
relationship of
if-match
statements. As a result, a match is found unless all the matching conditions
specified by the
if-match
35, RTA and RTD are each configured to belong to only one
area1 area1
RTB RTB RTC RTC
and clauses. The
apply
clauses for a node uses a series of Boolean "AND"
clauses are satisfied. The
IP Routing Policy
area2 area2
RTD RTD
clauses define the
if-match
clause specifies the
apply
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