User's Manual

453
Users Manual of CS-6306R
EGP is different from IGP with its enhanced control capability. BGP provides multiple optional methods to
control the routes.
 Use neighbor-based access-list, aspath-list and prefix-list to filter the route. Or use port-based
access-list and prefix-lis
t to filter the route or the Nexthop attribute of the route.
 Use route-map to modify BGP route's attributes such as MED, Local Preference and Weight.
 To interact with dynamic IGRPs such as ospf and rip, you can use the distribute command to
redistribute the route. The BGP routing information is thus automatically generated. The BGP route can
be generated by manually configuring network and aggregate. When the BGP route is generated, you
can use route-map to set the attribute of the route.
 To control the priority of BGP routes in the system, run the distance command to set the management
distance of the BGP route.
45.4.1.2 BGP Route Selection
The decision procedure of BGP is based on route attribute comparing. When there are multiple routes to
reach the same network, BGP selects the optimal route. The procedure of BGP selecting the optimal route is
shown as follows:
 If the next hop cannot be reached, the optimal route is considered.
 If the route is an internal one and synchronization is activated, the optimal route is not considered when
the route is not in IGP.
 The route with maximum weight is preferentially selected.
 If all routes have the same weight, the route with maximum local priority is preferentially selected.
 If all routes have the same local priority, the route generated by the local router is preferentially selected.
For example, routes may be generated when the local router runs the network command or the
aggregate command or the IGP routes are forwarded.
 If the local priority is same, or if the routes are not generated by the local router, the route with the
shortest AS path is first selected.
 If the AS paths are same, the route with the smallest Origin attribute value (IGP < EGP < INCOMPLETE)
is first selected.
 If the Origin attribute values is the same, the route with the smallest MED value is first selected. The
MED value compare is for the routes from the same neighboring AS unless bgp always-compare-med
is activated.
 If all routes have the same MED, the EBGP is first selected. All paths in the autonomous system are
taken as IBGP.
If each route has the same connection attribute, the route with the smallest router-id is first selected.