HP 6125XLG - Page 208

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Step Command Remarks
3. Enter BGP IPv6 unicast
instance view.
ipv6-family [ unicast ] N/A
4. Enable MED comparison for
routes from different ASs.
compare-different-as-med By default, this feature is disabled.
Enabling MED comparison for routes on a per-AS basis
This task enables BGP to compare the MEDs of routes from an AS.
Figure 53 Route selection based on MED (in an IPv4 network)
As shown in Figure 53, 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.
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. This mechanism
avoids the above-mentioned problem. 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.
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