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Figure 62 Route selection based on MED (in an IPv4 network)
AS 400
AS 200
Router A
Loop0
3.3.3.3/32
GE1/0/1
13.1.1.2/24
AS 100
As shown in
Figure
62, Router D establishes indirect EBGP peer relationships with Router A, Router
B, and Router C, and learns addresses 1.1.1.1/32, 2.2.2.2/32, and 3.3.3.3/32 through OSPF. The
following output shows the routing information on Router D.
Destination/Mask
1.1.1.1/32
2.2.2.2/32
3.3.3.3/32
Router D learns network 10.0.0.0 from both Router A and Router B. Because the route learned from
Router B has a smaller IGP metric, the route is optimal. The following output shows the BGP routing
table on Router D.
Network
*>e
10.0.0.0
* e
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. The route from Router C has a smaller IGP metric than the route from
Router B, so the route from Router C becomes optimal. The following output shows the BGP routing
table on Router D.
Network
*>e
10.0.0.0
* e
10.0.0.0
* e
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. Network 10.0.0.0 learned from Router B is the optimal route. The
following output shows the BGP routing table on Router D.
Network
*>e
10.0.0.0
* e
Router E
Router C
Loop0
1.1.1.1/32
GE1/0/1
11.1.1.2/24
GE1/0/1
GE1/0/3
Router D
Proto
Pre Cost
O_INTRA 10
10
O_INTRA 10
20
O_INTRA 10
30
NextHop
2.2.2.2
3.3.3.3
NextHop
1.1.1.1
2.2.2.2
3.3.3.3
NextHop
2.2.2.2
3.3.3.3
259
10.0.0.0
Router B
GE1/0/1
12.1.1.2/24
GE1/0/2
NextHop
11.1.1.2
12.1.1.2
13.1.1.2
MED
LocPrf
50
50
MED
LocPrf
60
50
50
MED
LocPrf
50
50
AS 300
Loop0
2.2.2.2/32
Interface
GE1/0/1
GE1/0/2
GE1/0/3
PrefVal Path/Ogn
0
300 400e
0
200 400e
PrefVal Path/Ogn
0
200 400e
0
300 400e
0
200 400e
PrefVal Path/Ogn
0
300 400e
0
200 400e
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