R3303-HP 6600/HSR6600 Routers Layer 3 - IP Routing Configuration Guide

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As shown in Figure 69, 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.
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 will not compare the MEDs
of the two routes. Router C has a smaller router ID than Router B, 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 is not optimal.
To resolve this issue, configure the bestroute compare-med command on Router D. After that, Router D
puts routes received from the same AS into a group. Router D then selects the route with the lowest MED
from the same group, and compares routes from different groups.
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 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
BGP load balancing cannot be implemented because load balanced routes must have the same AS-path
attribute.
To enable the comparison of MED of routes from each AS:
Step Command Remarks
1. Enter system view.
system-view N/A
2. Enter BGP view or BGP-VPN
instance view.
• Enter BGP view:
bgp as-number
• Enter BGP-VPN instance view:
a. bgp as-number
b. ipv4-family vpn-instance
vpn-instance-name
Use either method.
3. Enable the comparison of
MEDs for routes on a per-AS
basis.
bestroute compare-med
Optional.
Not enabled by default.
Enabling the comparison of MED of routes from confederation peers
The MED attributes of routes from confederation peers are not compared if their AS-path attributes
contain AS numbers that do not belong to the confederation, such as these three routes: AS-path
attributes of them are 65006 65009, 65007 65009, and 65008 65009, and MED values of them are
2, 3, and 1. Because the third route contains an AS number that does not belong to the confederation,
the first route becomes the optimal route.