Avaya Configuring IP Services Bedienungsanleitung PDF herunterladen (Seite 221)

Configuring BGP Services

5-17

How BGP Selects the Best Path

A BGP speaker must, at times, evaluate and compare different paths to a

destination network to determine the best path. Because all border routers must

provide the same view of the AS to external ASs, having a selection strategy that

is consistent in the router, and that can be consistent across all border routers, is

very important in BGP. To select the best available path, BGP uses AS weights

and classes and IP policies. To compare IBGP routes, BGP-4 can also calculate

and use a Local Preference value. These mechanisms are described in the

following sections.

AS Weight and Class Values

You can assign a weight class to any AS number and a weight value to a weight

class.

AS weight classes allow a network administrator to assign multiple weight values

to the same AS. This feature allows the administrator to consider an AS path

differently for different networks. For example, consider a situation in which two

networks — 192.32.1.0 and 192.32.2.0 — are both reachable by two paths. The

ﬁrst path to each network shares a common AS — AS 5. The second path to each

network also shares a common AS — AS 10. If the administrator for some reason

wants to favor AS 5 in the path to 192.32.1.0 and AS 10 in the path to 192.32.2.0,

he or she can assign one weight class to the AS as a component in the path to

network 192.32.1.0 and another class to the AS component in the path to

192.32.2.0.

An assigned weight can range from 1 to 15 plus an inﬁnity value. Weights provide

a way either to prefer or to avoid routes that pass through certain ASs. The weights

of each AS in a path are added, and the path with the smallest total weight is the

preferred path. Any path containing an AS weight of inﬁnity will be avoided.

When a BGP router receives a new route, it is evaluated against any existing

accept policies. If after this evaluation, the path still is to be used, the total weight

of the path is calculated.

AS weights should be conﬁgured the same on all BGP routers in an AS.

For instructions on assigning weight and class values to an autonomous system, see

“Conﬁguring BGP AS Weights and Weight Classes” on page 5-47.

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Avaya Configuring IP Services Bedienungsanleitung Seite 221