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Cisco CCNP / BSCI Certification: The Local Preference BGP
Attribute |
By:
Chris Bryant |
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When studying for your BSCI exam for the CCNP, you get your
first taste of BGP. One of the major differences between BGP and
the other protocols you've studied to date is that BGP uses
attributes to describe paths, and to influence the selection of
one path over the other.
In this free tutorial, we're going to take a look at the Local
Preference attribute and compare it to the Cisco-proprietary BGP
attribute "weight".
The Local Preference (LOCAL_PREF) attribute is used to influence
how traffic will flow from one Autonomous System (AS) to another
when multiple paths exist. For example, if AS 100 has two
different paths to a destination network in AS 200, the
LOCAL_PREF attribute can be used to influence the path selection.
The major difference between the Weight and LOCAL_PREF
attributes is that when the LOCAL_PREF attribute is changed,
that change is reflected throughout the AS. The new LOCAL_PREF
value will be advertised to all other routers in the AS, as
compared to the Weight attribute, which is locally significant
only. If you change the Weight for a path on one router in an
AS, the other routers in the AS will not learn of the change.
A route-map can be used to change a local preference value. For
example, if you want to change the local preference value to 200
for the path advertisement 10.2.2.0/24 coming in from neighbor
10.1.1.1, there are three steps involved. First, write an ACL
matching the remote network you want to change the local
preference for.
R1(config)#access-list 5 permit 10.2.2.0 0.0.0.255
Second, write a route-map setting the local preference to 200.
This will double the default value of 100, and the path with the
highest local preference will be the preferred path.
R1(config)#route-map PREFER_PATH permit 10
R1(config-route-map)#match ip address 5
R1(config-route-map)#set local-pref 200
Finally, apply the route-map to routes that are being received
from 10.1.1.1.
R1(config)#router bgp 100
R1(config-router)#network 10.1.1.1 route-map PREFER_PATH in
R1 will then advertise this new local preference value to all
other routers in AS 100 - all of its iBGP neighbors.
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Article Source: http://www.powerdirectory.net/articles/article51204.html |
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