HP 6125XLG R2306-HP 6125XLG Blade Switch Layer 3 - IP Routing Configuration Gu - Page 208

Step 3. Enter BGP IPv6 unicast instance view. 4. Enable MED comparison for routes from different ASs. Command ipv6-family [ unicast ] compare-different-as-med Remarks N/A 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 400 Router E 10.0.0.0 AS 200 Router ID : 3.3.3.3 Router A Eth1/1 3.3.3.3/24 Router ID : 1.1.1.1 Router C Eth1/1 1.1.1.1 AS 300 Router ID : 2.2.2.2 Router B Eth1/1 2.2.2.2/24 AS 100 Router D 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. 197