H3C Technologies H3C S7500E Series Switches - Manual (Page 232)

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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 6-16 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 therefore 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.
In this case, 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.