Allied Telesis AT-8824 Guía de instalación descargar pdf (Pagina 153)

Filtering IP Routes 2-31

Software Version 2.7.5

C613-10454-00 REV A

Limitations of route

filtering on OSPF

As the previous diagram shows, the OSPF LSA database is a completely

separate entity to the router or switch’s RIB. The OSPF design does not allow

you to filter the contents of the database before advertising routes to

neighbouring routers. This is because OSPF shares LSAs between all the

routers in an area. The protocol assumes that all the routers in the area have

shared all the advertisements among each other, and that all agree on the state

of the complete link state database for the area. If some routers in the area are

learning, but not advertising, that breaks the OSPF model.

These limitations mean you can only filter to control:

■ which routes learned by OSPF can be imported by the router or switch

from the LSA database into the RIB

We recommend you use route maps to filter this (see “Filtering OSPF routes

when writing to the RIB” on page 2-22)

■ which static, BGP or RIP routes can be exported from the RIB into the LSA

database

We recommend you use route maps to filter static routes, IP filters to filter

BGP routes and IP route filters to filter RIP routes (see “Filtering when

copying routes to OSPF” on page 2-24)

■ which summary routes can be exported from the RIB into the LSA database

for advertising as summary LSAs

You can use IP route filters to filter this (see “Filtering when copying routes

to OSPF” on page 2-24)

Another way to filter summary LSAs is to define a “do not advertise” OSPF

range on an Area Border Router. This stops OSPF from advertising inter-

area routes into another area. To do this, use the command:

set ospf range=ipadd effect=donotadvertise [other-options]

Note that filtering cannot:

■ remove an entry from the LSA database once the entry has been added

■ prevent the router or switch from advertising an entry to interfaces in the

same area that the entry is relevant to

■ prevent updates that OSPF learns from being put into the database

■ change the properties of an entry in the database

Type-3 Summary-LSA inter-area destinations, when the

destination is an IP network

from the RIB, by Area

Border Routers

Type-4 Summary-LSA inter-area destinations, when the

destination is an Autonomous

System (AS) boundary router

by OSPF, by Area Border

Routers

Type-5 AS-external-LSA a destination outside the AS from the RIB, by AS

boundary routers

Type-7 AS-external-LSA a destination outside the AS. Used

in not-so-stubby areas

from the RIB, by AS

boundary routers

LSA Name LSA describes LSA is created

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Allied Telesis AT-8824 Guía de instalación Pagina 153

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