Cabletron Systems SmartSwitch Router 9032578-02 - Manual (Page 109)

SmartSwitch Router User Reference Manual 109
Chapter 7: Routing Policy Configuration Guide
Import Policies
Import policies control the importation of routes from routing protocols and their
installation in the routing databases (Routing Information Base and Forwarding
Information Base). Import Policies determine which routes received from other systems
are used by the SSR routing process. Every import policy can have up to two components:
• Import-Source
• Route-Filter
Import-Source
This component specifies the source of the imported routes. It can also specify the
preference to be associated with the routes imported from this source.
The routes to be imported can be identified by their associated attributes:
• Type of the source protocol (RIP, OSPF, BGP).
• Source interface or gateway from which the route was received.
• Source autonomous system from which the route was learned.
• AS path associated with a route. Besides autonomous system, BGP also supports
importation of routes using AS path regular expressions, and AS path options.
• If multiple communities are specified using the optional-attributes-list, only updates
carrying all of the specified communities will be matched. If the specified optional-
attributes-list has the value none for the well-known-community option, then only
updates lacking the community attribute will be matched.
In some cases, a combination of the associated attributes can be specified to identify the
routes to be imported.
Note: It is quite possible for several BGP import policies to match a given update. If
more than one policy matches, the first matching policy will be used. All later
matching policies will be ignored. For this reason, it is generally desirable to order
import policies from most to least specific. An import policy with an optional-
attributes-list will match any update with any (or no) communities.
The importation of RIP routes may be controlled by source interface and source gateway.
RIP does not support the use of preference to choose between RIP routes. That is left to the
protocol metrics.
Due to the nature of OSPF, only the importation of ASE routes may be controlled. OSPF
intra-and inter-area routes are always imported into the routing table with a preference of
10. If a tag is specified with the import policy, routes with the specified tag will only be
imported.