H3C Technologies H3C S5560 Series Switches - Manual (Page 248)

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Figure 57 Route selection based on MED (in an IPv4 network)
As shown in Figure 57 , 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 Router B 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, it compares the route with the optimal route in
its routing table. Because Router C and Router B reside in different ASs, BGP does not compare the MEDs
of the two routes. Router C has a smaller router ID than Router B so 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 A reside in the same AS, and Router C has a greater MED, so network
10.0.0.0 learned from Router C should not be optimal.
To avoid this problem, you can configure the bestroute compare-med command to enable MED
comparison for routes from the same AS on Router D. After that, Router D puts the routes received from
each AS into a group, selects the route with the lowest MED from each group, and compares routes from
different groups. The following output shows the BGP routing table on Router D after this feature is
enabled. Network 10.0.0.0 learned from Router B is the optimal route.
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
* i 1.1.1.1 60 0 200e
To enable MED comparison for routes on a per-AS basis (IPv4):
Step Command Remarks
1. Enter system view. system-view N/A
AS 300
AS 100
AS 200
Router A
Router D
Router ID : 3.3.3.3 Router ID : 1.1.1.1
Router C
Eth1/1
1.1.1.1 Eth1/1
3.3.3.3/24
Router B
Router ID : 2.2.2.2
Eth1/1
2.2.2.2/24
AS 400
Router E
10.0.0.0