H3C S5500-EI series, Руководство по настройке

Страница: 232 / 457

215
To do… Use the command… Remarks
Configure the default MED value default med med-value
Optional
0 by default
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
Enable the comparison of MED of routes from each AS
Route learning sequence may affect optimal route selection.
Figure 86 Route selection based on MED
As shown in Figure 86 , 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.
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.
Because 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. Afterwards, 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. It then compares routes from different groups. This mechanism avoids the aforementioned