PLANET XGS3-24040 - Manual (Page 329)

Chapter 39 BGP
39-23
The configurations of SwitchA:
SwitchA(config)#router bgp 100
SwitchA(config-router-bgp)#neighbor 2.2.2.1 remote-as 300
SwitchA(config-router-bgp)#neighbor 3.3.3.2 remote-as 300
SwitchA(config-router-bgp)#neighbor 4.4.4.3 remote-as 400
The configurations of SwitchC:
SwitchC(config)#router bgp 300
SwitchC (config-router-bgp)#neighbor 2.2.2.2 remote-as 100
SwitchC (config-router-bgp)#neighbor 2.2.2.2 route-map set-metric out
SwitchC (config-router-bgp)#neighbor 1.1.1.2 remote-as 300
SwitchC (config-router-bgp)#exit
SwitchC (config)#route-map set-metric permit 10
SwitchC (Config-Router-RouteMap)#set metric 120
The configurations of SwitchD
SwitchD (config)#router bgp 300
SwitchD (config-router-bgp)#neighbor 3.3.3.3 remote-as 100
SwitchD (config-router-bgp)#neighbor 3.3.3.3 route-map set-metric out
SwitchD (config-router-bgp)#neighbor 1.1.1.1 remote-as 300
SwitchD (config-router-bgp)#exit
SwitchD (config)#route-map set-metric permit 10
SwitchD (Config-Router-RouteMap)#set metric 200
The configurations of SwitchB
SwitchB (config)#router bgp 400
SwitchB (config-router-bgp)#neighbor 4.4.4.4 remote-as 100
SwitchB (config-router-bgp)#neighbor 4.4.4.4 route-map set-metric out
SwitchB (config-router-bgp)#exit
SwitchB (config)#route-map set-metric permit 10
SwitchB (Config-Router-RouteMap)#set metric 50
After the configuration above, SwitchB, SwitchC and SwitchD are assumed to send a route 12.0.0.0 to
SwitchA. According to the comparison of BGP route strategy; there is an assumption that the routes sent by
the three switches above have the same attribute value before the comparison of metric attribute. At this time,
the route with lower value is the better route. But the comparison of metric attribute will only be done with the
routes from the same AS. For SwitchA, the routes passed SwitchC are preferable to the one passed SwitchD.
Because SwitchC and SwitchB are not located in the same AS, the SwitchA will not do metric comparison
between the two switches. If the metric comparison between different AS is needed, the command” bgp
always-compare-med” will be used. If this command is configured, the routes passed SwitchB are the best to
SwitchA. At this time, the following command may be added on SwitchA:
SwitchA (config-router-bgp)#bgp always-compare-med