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14.3 Local Preference Attribute (Type 5) Configuration
Example
Local Preference is a well-known non-transitive attribute that influences the flow of outbound traffic by
setting the exit point of an AS. Border routers within an AS calculate Local Preference if the attribute is
not configured in a BGP accept policy.
The Local Preference attribute is local to ASs and is exchanged between iBGP peers only; e.g. it does
not have any effect on the internal IGP protocol being used.
When BGP must select the best route and there are multiple paths to the same destination, the path with
the larger preference is preferred.
In this example, we want to influence the traffic so the link from ERS8000-C to 8008 is used as the
preferred path and the link from ERS8000-D to 9001 is used for back up only. 8008 is set with a higher
Local Preference while 9001 is set for a lower local preference. We will also configure 8008 to inject a
default route with a lower OSPF metric than 9001 resulting in 8008 having a higher preference. With this
configuration, all traffic leaving AS 40 will exit via the customer AS will exit via ERS8000-C.
Local Preference can also be used to load balance outbound traffic based on CIDR or network address
groups.
Figure 13: BGP Local Preference Configuration Example
Avaya Inc. – External Distribution
83
January 2016
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