3com 5500-SI Manual Do Utilizador

C

HAPTER

 16: IP R

OUTING

 P

ROTOCOL

 O

PERATION

■

Ensure the backbone area connects with all other areas.

■

The virtual links cannot pass through the STUB area.

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.

Figure 59   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.

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:

■

■

■

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 

 and 

 clauses. The 

 clauses define the matching rules 

RTA

RTB

RTC

RTD

area0

area1

area2

RTA

RTB

RTC

RTD

area0

area1

area2