HP 4800G Series Configuration Manual page 587

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Enter system view

Enter BGP view

Enable the comparison of MED of

routes from different ASs

Enable the comparison of MED of routes from each AS

Route learning sequence may affect optimal route selection.

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.

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.

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.

Use the command...

bgp as-number

compare-different-as-med

Not enabled by default

PrefVal Path/Ogn

PrefVal Path/Ogn

PrefVal Path/Ogn

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