R3166-R3206-HP High-End Firewalls Network Management Configuration Guide-6PW101

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To do… Use the command… Remarks
Configure the default MED value default med med-value
Optional
0 by default
2. Enable the comparison of MED of routes from different ASs
Follow these steps to enable the comparison of MED of routes from different ASs:
To do… Use the command…
Remarks
Enter system view system-view
—
Enter BGP view bgp as-number
—
Enable the comparison of MED of routes
from different ASs
compare-different-as-med
Required
Not enabled by default
3. Enable the comparison of MED of routes from each AS
Route learning sequence may affect optimal route selection.
Figure 179 Route selection based on MED
As shown in the figure above, Router D learns network 10.0.0.0 from both Router A and Router B.
Because Router B has a smaller router ID, the route learned from it is optimal.
Network NextHop MED LocPrf PrefVal Path/Ogn
*>i 10.0.0.0 2.2.2.2 50 0 300e
* i 3.3.3.3 50 0 200e
When Router D learns network 10.0.0.0 from Router C which has a smaller router ID than Router B, the
route from Router C becomes optimal, as shown below.
Network NextHop MED LocPrf PrefVal Path/Ogn
*>i 10.0.0.0 1.1.1.1 60 0 200e
* i 10.0.0.0 2.2.2.2 50 0 300e
* i 3.3.3.3 50 0 200e
However, Router C and Router B reside in the same AS, and BGP will compare the MEDs of them. Since
Router C has a greater MED, network 10.0.0.0 learned from it is not optimal.
You can configure the bestroute compare-med command on Router D. After that, Router D will put routes
received from the same AS into a group. For the same group, the route with the lowest MED is selected.
Then, it compares routes from different groups. This mechanism avoids the above-mentioned problem.
The following output is the BGP routing table on Router D after the comparison of MED of routes from
each AS is enabled. Network 10.0.0.0 learned from Router C is the optimal route.