
Configuring IP Services
5-18
Routing Policies
BGP accept and announce policies govern which routes are used by a router, and
which are propagated to other routers.
So that every BGP border router within an AS comes to the same decision in
constructing path attributes for an external path, route policies must be
coordinated between all of the BGP speakers within an AS. It is suggested that the
accept and announce policies on all IBGP connections accept and propagate all
routes. On external BGP connections, consistent routing policy decisions should
be made.
Calculating the BGP-4 Local Preference Attribute
BGP-4 update messages include a Local Preference attribute that allows an AS
border router to assign a preference value to a route when advertising it to IBGP
peers. The calculation of the Local Preference attribute is implementation-
specific. A higher value indicates that the route is more preferred.
You can configure a BGP-4 accept or announce policy to override the value in the
Local Preference attribute. For details and instructions, see the accept Local
Preference parameter on page 9-20 and the announce Local Preference Override
parameter on page 9-44.
The router uses the following equations to calculate a value for the Local
Preference attribute:
local preference = 8191 – origin value – AS path weight
where origin value is 0 for routes with an Origin Path attribute of IGP and 4096
otherwise, and AS path weight is a sum of weight values associated with AS
numbers listed in the route’s AS Path attribute. These weight values can be
configured and default to 8.
Note: By default, an external BGP-3 or BGP-4 speaker will neither advertise
any routes to a peer, nor inject any routes into its IGP. Route policies must be
configured to enable any route advertisement.
Note: In addition to announce and accept policies, Bay Networks supports
import and export filters for BGP-3. Import and export filters provide a subset
of the parameters provided by the policies. In a future release, support for
import and export filters will be dropped.
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