3Com S7906E - Switch, Configuration Manual

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Figure 1-16 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.
Network NextHop MED LocPrf PrefVal Path/Ogn
*>i 10.0.0.0 3.3.3.3 50 0 200e
* i 10.0.0.0 2.2.2.2 50 0 300e
* i 1.1.1.1 60 0 200e
Note that, in this case, BGP load balancing cannot be implemented because load balanced routes must
have the same AS-path attribute.
Follow these steps to enable the comparison of MED of routes from each AS:
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 each AS
bestroute compare-med
Optional
Not enabled by default