170
fulfilled:
The switch’s route must be next hop reachable. That is in the route table there is
the route that can reach the next hop.
BGP must be synchronized with IGP (unless asynchronism is configured; only
restricted to IBGP)
BGP route selecting process is based on the BGP attribute. When there are several routes
that indicate the same destination, BGP need select the best route to the destination. The
decision-making process is as the following:
1
．
Select the route with the most weight first;
2. If the weights are the same, select the route with the most local preference;
3. If the local preferences are the same, select the route generated by local switch.
4. If the local preferences are the same and there is no route generated by local switch,
select the route with the shortest AS path;
5. If the AS paths are the same, select the route with the lowest “origin” type
（
IGP<EGP<INCOMPLETE
）
;
6. If the “origin” types are the same, select the route with the lowest MED attribute. Unless
activating command “bgp always-compare-med”, this comparison is only available among the
routes from the same neighbor AS.
7. If the MED attributes are the same, EBGP is preferable to outer confederation and outer
confederation is preferable to IBGP.
8. If it’s still the same by now, BGP router ID
（
router ID
）
is used to break the balance. The
best route is the one from the least router ID.
1.8.2 BGP Configuration Task List
The BGP configuration tasks include basic and advanced tasks. Basic BGP configuration
tasks include the following:
1
．
Enable BGP Routing (required)
2
．
Configure BGP Neighbors (required)
3
．
Administrate the change of routing policy
4
．
Configure BGP Weights
5
．
Configure BGP Route Filtering policy basing on Neighbors
6
．
Configure Next-Hop of BGP
7
．
Configure Multi-Hop of EGBP
8
．
Configure BGP Session Identifier
9
．
Configure BGP Version
Advanced BGP configuration tasks include the following: