HP 6125G Configuration Manual page 153

Figure 58 Route selection based on MED
As shown in Figure 58, 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
*>i 10.0.0.0
* i
When Router D learns network 10.0.0.0 from Router C, it compares the route with the optimal route in its
routing table. Because Router C and Router B reside in different ASs, BGP will not compare the MEDs of
the two routes. Router C has a smaller router ID than Router B, the route from Router C becomes optimal.
Network
*>i 10.0.0.0
* i 10.0.0.0
* i
However, Router C and Router A reside in the same AS, and Router C has a greater MED, so network
10.0.0.0 learned from Router C cannot be optimal.
You can configure the bestroute compare-med command on Router D. After that, Router D puts routes
received from the same AS into a group. Router D then selects the route with the lowest MED from the
same group, and 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 B is the optimal route.
Network
*>i 10.0.0.0
* i 10.0.0.0
* i
BGP load balancing cannot be implemented because load balanced routes must have the same
AS_PATH attribute.
To enable the comparison of MED of routes from each AS:
Step
1. Enter system view.
2. Enter BGP view.
3. Enable the comparison of MED of
routes from each AS.
NextHop MED
2.2.2.2 50
3.3.3.3 50
NextHop MED
1.1.1.1 60
2.2.2.2 50
3.3.3.3 50
NextHop MED
2.2.2.2 50
3.3.3.3 50
1.1.1.1 60
Command
system-view
bgp as-number
bestroute compare-med
143
LocPrf PrefVal Path/Ogn
0
0
LocPrf PrefVal Path/Ogn
0
0
0
LocPrf PrefVal Path/Ogn
0
0
0
Remarks
N/A
N/A
Optional.
Not enabled by default.
300e
200e
200e
300e
200e
300e
200e
200e