Allied Telesis AT-8824 Guía de instalación descargar pdf (Pagina 129)

Filtering IP Routes 2-7

Software Version 2.7.5

C613-10454-00 REV A

For OSPF, you can use route maps:

■ to filter routes from OSPF before adding them to the RIB

■ when importing static routes into the OSPF LSA database

When applied to OSPF routes, route maps can:

■ accept or reject particular routes on the basis of their metric, route type,

source, nexthop or tag, or the interface they are received on

■ accept or reject particular routes, by comparing the update message’s

routes with a prefix list

■ alter matching routes’ metric, type and tag.

About IP Route Filters

Description Route filters are simple filters that examine a number of aspects of each route.

When you apply filters to routing information that the router or switch

receives, the filter determines whether each route is added to the RIB. When

you apply filters to routing information that the router or switch transmits, the

filter determines whether each route is advertised.

When to use IP

route filters

“Applying Filters” on page 2-20 describes in detail how to use IP route filters,

but this section summarises the uses. The main uses of IP route filters are to

select:

■ RIP routes when adding routes to the RIB

■ RIP routes when determining which routes to advertise

■ OSPF, static or interface routes when determining which routes to

redistribute from the RIB into RIP

■ RIP routes and OSPF summary routes when determining which routes to

redistribute from the RIB into the OSPF LSA database

You can also use IP route filters to select:

■ OSPF routes to add to the RIB, but we recommend you use route maps

instead

■ static routes to redistribute from the RIB into the OSPF LSA database, but

we recommend you use route maps instead

■ BGP routes to redistribute from the RIB into the OSPF LSA database, but

we recommend you use IP filters with a filter number in the range 300 to

399 instead

IP route filters affect the interaction between the routing module and the RIB,

but IP route filters do not filter receipt of routing protocol messages by the

routing module and do not directly filter messages sent from the routing

protocol. Messages sent from the routing protocol are affected if and only if

they are derived from the RIB, which is true in most situations, including RIP,

OSPF-ext messages, and OSPF summary Link State Advertisements (LSAs).

Note that the design of OSPF prevents route filters from filtering some types of

OSPF LSAs (see “Limitations of route filtering on OSPF” on page 2-31).

IP route filters do not filter BGP-derived routes, except when determining

whether to add BGP routes to the OSPF LSA database. This means you cannot

use IP route filters to select the routes that BGP receives, copies to the RIB, or

advertises. For an overview of the filter types to use with BGP, see “Border

Gateway Protocol (BGP-4)” on page 2-29.

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