Bgp Troubleshooting - Edge-Core ES4624-SFP Routing Management Manual

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
SwitchA(config-router-bgp)# bgp always-compare-med
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"

1.8.5 BGP Troubleshooting

In the process of configuring and implementing BGP protocol, physical connection,
configuration false probably leads to BGP protocol doesn't work. Therefore, the customers
should give their attention to points as follow:
First of all, to ensure correct physical connection;
Secondly, to ensure interface and link protocol are UP (execute show interface instruction);
And startup BGP protocol (use router bgp command), configure affiliated IBGP and EBGP
neighbors (use neighbor remote-as command).
Notice BGP protocol itself can't detect route, needs to import other routes to create BGP
route. Only it enables these routes to announce IBGP and EBGP neighbors by importing routes.
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