Discussion

13.5 Adjusting Local Preference Values
Problem
You want to change the value of the BGP local preference attribute to control which
routes the router uses.
Solution
There are two ways to change the local preference value. The first method changes
the value for all routes distributed into IBGP from the router:
[edit protocols bgp]
aviva@RouterF# set group internal-within-AS65500 local-preference 140
The second method allows you to change the preference of specific routes:
[edit policy-options policy-statement set-local-pref]
aviva@RouterF# set from route-filter 192.168.14.1/32 exact
aviva@RouterF# set then local-preference 140
aviva@RouterF# set then accept
[edit protocols bgp group session-to-AS65505]
aviva@RouterF# set import set-local-pref
Discussion
When IBGP routers exchange prefix information, one of the attributes associated
with each prefix is its local preference (LOCAL_PREF) value. This attribute is not
advertised to EBGP peers. IBGP routers use the local preference value as a metric to
decide which routes should exit the AS, choosing the route with the highest local
preference value. The default local preference value is 100. BGP includes the local
preference value only when advertising prefixes to IBGP peers. It is not advertised to
EBGP peers.
When the router is determining the active route to a destination (see the Introduction
to Chapter 8), one of the first things it considers is the BGP local preference, so
changing the local preference is a useful way to manipulate route selection. By selecting
from multiple routes to a destination, the local preference is the first BGP path
attribute checked, even before the AS path length, the origin, and the MED.
Don’t confuse the BGP local preference with the JUNOS software routing-protocol
preference (see Table 8-2). The JUNOS routing preference is local to each router,
and the software uses it to choose the active route when there are a number of paths
to the same prefix. The BGP local preference is used only by BGP, and only by IBGP
routers within an AS. Also, the choice is between two different types of preference
values that are the reverse of each other. For the JUNOS routing-protocol preference,
the route with the lowest value is chosen, but with the BGP local preference,
the route with the highest value is chosen.
440 | Chapter 13: BGP
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