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The local preference is a well-known
discretionary attribute (type code 5). The local preference attribute is a degree of preference given to a route to compare it
with other routes for the same destination; higher local preference values are preferred.
local preference, as indicated by the name, is
local to the AS and gets exchanged between IBGP peers only-it is not
advertised to EBGP peers.
An AS connected via BGP to multiple
other ASs will get routing updates about the same destinations from
different ASs. local preference is usually used to set the exit
point of an AS to reach a certain destination. Because this
attribute is communicated within all BGP routers inside the AS, all
BGP routers will have a common view on how to exit the AS.
Consider the environment illustrated
in Figure
. Suppose that company ANET has purchased Internet connections via
two service providers, XNET and YNET. ANET is connected to YNET via
a primary T3 link and to XNET via a backup T1 link.
It is important for ANET to decide
what path its outbound traffic is going to take. Of course ANET's
network administrators prefer to use the T3 link via YNET in normal
operation because it is a high-speed link. This is where local preference
comes into play: the Los Angeles router can give the
routes coming from YNET a local preference of 300, and the San Jose
router can give the routes coming from XNET a lower value, say 200.
Because both the Los Angeles and San Jose routers are exchanging
routing updates via IBGP, they both agree that the exit point of the
AS is going to be via YNET because of the higher local preference.
In Figure
, ANET learns route 128.213.0.0/16 via XNET and YNET. The San Jose
and Los Angeles routers will agree on using YNET as the exit point
for destination 128.213.0.0/16 because of the higher local preference
value of 300. The local preference manipulation
discussed in this case affects the traffic going out of the AS and
not traffic coming into the AS. Inbound traffic can still come via
the T1 link.
The weight parameter is similar to
the local preference in that it gives higher preference to the route
that has a higher weight. The difference is that the weight
parameter is local to the router and does not get exchanged between
routers. The weight parameter influences routes coming from
different providers to the same router (one router with multiple
connections to two or more providers). The weight parameter has a
higher precedence than any other attribute; it is used as a main
(proprietary) switch to determine route preference.
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