214 | Border Gateway Protocol IPv4 (BGPv4)w w w . d e l l . c o m | s u p p o r t . d e l l . c o m WeightThe Weight attribute is local to the router and is not advertised to neighboring routers. If the router learnsabout more than one route to the same destination, the route with the highest weight will be preferred. Theroute with the highest weight is installed in the IP routing table.Local PreferenceLocal Preference (LOCAL_PREF) represents the degree of preference within the entire AS. The higher thenumber, the greater the preference for the route.The Local Preference (LOCAL_PREF) is one of the criteria used to determine the best path, so keep inmind that other criteria may impact selection, as shown in Figure 10-4. For this example, assume thatLOCAL_PREF is the only attribute applied. In Figure 10-5, AS100 has two possible paths to AS 200.Although the path through the Router A is shorter (one hop instead of two) the LOCAL_PREF settingshave the preferred path go through Router B and AS300. This is advertised to all routers within AS100causing all BGP speakers to prefer the path through Router B.Figure 10-5. LOCAL_PREF ExampleMulti-Exit Discriminators (MEDs)If two Autonomous Systems (AS) connect in more than one place, a Multi-Exit Discriminator (MED) canbe used to assign a preference to a preferred path. The MED is one of the criteria used to determine the bestpath, so keep in mind that other criteria may impact selection, as shown in Figure 10-4.Router ARouter BAS 100Router CT1 LinkOC3 LinkAS 200Router DRouter EAS 300Router FRouter ESet Local Preference to 200Set Local Preference to 100
