Dell Force10 E300, Configuration Manual

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214 | Border Gateway Protocol IPv4 (BGPv4)
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Weight
The Weight attribute is local to the router and is not advertised to neighboring routers. If the router learns
about more than one route to the same destination, the route with the highest weight will be preferred. The
route with the highest weight is installed in the IP routing table.
Local Preference
Local Preference (LOCAL_PREF) represents the degree of preference within the entire AS. The higher the
number, 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 in
mind that other criteria may impact selection, as shown in
Figure 10-4 . For this example, assume that
LOCAL_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 settings
have the preferred path go through Router B and AS300. This is advertised to all routers within AS100
causing all BGP speakers to prefer the path through Router B.
Figure 10-5. LOCAL_PREF Example
Multi-Exit Discriminators (MEDs)
If two Autonomous Systems (AS) connect in more than one place, a Multi-Exit Discriminator (MED) can
be used to assign a preference to a preferred path. The MED is one of the criteria used to determine the best
path, so keep in mind that other criteria may impact selection, as shown in
Figure 10-4 .
Router A
Router B
AS 100
Router C
T1 Link
OC3 Link
AS 200
Router D
Router E
AS 300
Router F
Router E
Set Local Preference to 200
Set Local Preference to 100