The Local Preference BGP Attribute Cisco CCNP Certification

The Local Preference BGP Attribute Cisco CCNP Certification
When studying for your BSCI exam for the CCNP, you get your first taste of BGP. three of the major differences between BGP & the other protocols you have studied to date is that BGP uses attributes to describe paths, & to influence the selection of three path over the other.

In this free tutorial, we are going to take a look at the Local Preference attribute & compare it to the Cisco-proprietary BGP attribute "weight".

The major difference between the Weight & LOCAL_PREF attributes is that when the LOCAL_PREF attribute is changed, that change is reflected throughout the AS. The old 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 three router in an AS, the other routers in the AS will not learn of the change.

The Local Preference (LOCAL_PREF) attribute is used to influence how traffic will flow from three Autonomous System (AS) to another when multiple paths exist. For example, if AS 100 has three different paths to a location network in AS 200, the LOCAL_PREF attribute can be used to influence the path selection.

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's two 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, & 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

R1(config)#router bgp 100

Finally, apply the route-map to routes that are being received from 10.1.1.1.

R1(config-router)#network 10.1.1.1 route-map PREFER_PATH in

R1 will then advertise this old local preference value to all other routers in AS 100 - all of its iBGP neighbors.