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Customizing BGP Services
114065 Rev. A 6-7
BGP-4 Local Preference Values
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.
The router uses the following equations to calculate a value for the Local
Preference attribute:
local preference = 8191 – origin valueAS 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.
A steep penalty is applied to routes that are advertised with an Origin attribute
other than IGP -- that is, EGP or Incomplete.
For an OSPF internal route or a direct route, the Local Preference attribute is set to
local preference = (8191 + 256 – (metric & 255))
where metric is the OSPF metric for an OSPF route or the configured cost for a
direct route.
For a RIP route, an EGP route, an OSPF ASE route, or a static route, the Local
Preference attribute is set to
local preference = (256 – metric)
where metric is the RIP metric for a RIP route, the EGP metric for an EGP route,
the OSPF metric for an OSPF ASE route, or the configured cost for a static route.
Note that Local Preference values for OSPF internal routes and direct routes are
higher than the Local Preference values calculated for BGP routes.
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