Avaya Configuring IP Services Bedienungsanleitung PDF herunterladen (Seite 276)

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Configuring IP Services

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BGP-4 Local Preference Value

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 specific to each

implementation. A higher value indicates that the route is more preferred.

The router uses the following equation 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 or 4096 for

other routes, 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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