3Com Switch 7700 Configuration Manual page 170

162 C 5: IP R
HAPTER OUTING
P O
ROTOCOL PERATION
Configuring Local Preferences
Different local preferences can be configured to affect BGP routing. When a router
running BGP gets routes with the same destination address but different next hops
through different internal peers, it will select the route with the highest local
preference.
Perform the following configurations in BGP view..
Table 119 Configuring the Local Preferences
Operation
Configure the local preference
Restore the default local preference
The local preference is transmitted only when the IBGP peers exchange the update
packets and it will not be transmitted beyond the local AS.
By default, the local preference is 100.
Configuring MED for AS
The Multi-Exit Discriminators (MED) attribute is the external metric for a route. It is
exchanged between ASs. However, it will not be transmitted beyond an AS once it
is imported into the AS.
AS uses the local preference to select the route to the outside and MED to
determine the optimum route for entering the AS. When a router running BGP
receives routes with the same destination address but different next hops through
different external peers, it will select the route of the smallest MED as the
optimum route, provided that all the other conditions are the same.
Perform the following configurations in BGP view..
Table 120 Configuring a MED Value for the System
Operation
Configure a MED value for the system
Restore the default MED value of the system
The router configured above only compares the route MED metrics of different
EBGP peers in the same AS. Using the compare-different-as-med command,
you can compare the route MED metrics of the peers in different ASs.
By default, MED metric is 0.
Comparing the MED Routing Metrics from Peers in Different ASs
Comparison of MED routing metrics is performed to select the best route. The
route with smaller MED value will be selected.
Perform the following configurations in BGP view..
Table 121 Comparing the MED Routing Metrics from Peers in Different ASs
Operation
Compare the MED routing metrics from peers
in different ASs
Command
default local-preference value
undo default local-preference
Command
default med med-value
undo default med
Command
compare-different-as-med